Hubble finds first galaxy in the local universe without dark matter(spacetelescope.org)
spacetelescope.org
Hubble finds first galaxy in the local universe without dark matter
http://www.spacetelescope.org/news/heic1806/
291 comments
> a bunch of off-the-shelf Canon telephoto lenses stuck together on a metal frame
That's an interesting remark. It's hard to beat mass production. As number produced skyrocket to the millions, it's difficult that a custom device has the resources that a mass marketed device has.
That's why millionaires have the same phone that you, the military use Xbox 360 controllers in their submarine's periscopes and COTS supercomputers are common.
https://en.wikipedia.org/wiki/Beowulf_cluster
https://www.quora.com/What-smartphone-do-tech-billionaires-u...
"US Navy submarines are getting Xbox 360 controllers to control their periscopes" https://www.theverge.com/2017/9/19/16333376/us-navy-military...
That's an interesting remark. It's hard to beat mass production. As number produced skyrocket to the millions, it's difficult that a custom device has the resources that a mass marketed device has.
That's why millionaires have the same phone that you, the military use Xbox 360 controllers in their submarine's periscopes and COTS supercomputers are common.
https://en.wikipedia.org/wiki/Beowulf_cluster
https://www.quora.com/What-smartphone-do-tech-billionaires-u...
"US Navy submarines are getting Xbox 360 controllers to control their periscopes" https://www.theverge.com/2017/9/19/16333376/us-navy-military...
400mm f2.8 lenses aren't made in the millions. Tens of thousands probably. The lenses used in this telescope cost $8k retail and they use 8 per. Not exactly cheap but surely much lower cost than typical astronomical big iron. Astronomy RAID :)
In fact, a whole generation of HPC students and scientists write codes for consumer graphics cards (GPGPU). All of this only because they are so cheap.
I think that the most powerful thing is that they offer a low point of entry. You can start for pretty near free, but for less than £100k you can build a differentiating capability and start contributing to knowledge... vs traditional instruments where the onramp starts at £millions and goes from there.
Thank you! Never heard of the term "Beowulf cluster". Sad to see the Wikipedia page never mention good old Linda, which was developed precisely to allow collaboration in heterogeneous systems.
https://www.nytimes.com/1992/01/19/business/david-gelernter-...
https://www.nytimes.com/1992/01/19/business/david-gelernter-...
It's really hard for me to figure out if you were sarcastic or not with your "never heard of the term"? You must be either young or never visited slashdot for it not to be sarcasm. It was a running joke in slashdot on every comment section to have a Beowulf cluster of whatever was mentioned in the article.
We're just getting old - https://trends.google.com/trends/explore?date=all&q=beowulf%...
Not sarcastic at all, just a young-ish person who never used slashdot that much.
[deleted]
> Now if the controls break, “I can go to any video game store and procure an Xbox controller anywhere in the world, so it makes a very easy replacement,” Senior Chief Mark Eichenlaub told The Virginian-Pilot.
I get his point but that sounds hilarious. "Hey guys, could we surface for a moment, I need to visit the North Korean/Russian/Antarctic Gamestop for a moment"
I get his point but that sounds hilarious. "Hey guys, could we surface for a moment, I need to visit the North Korean/Russian/Antarctic Gamestop for a moment"
"Guys, if you head west now, we can intercept our shipment from PlayAsia by 0700."
I think your link is actually the more relevant one for this story on hn.
Improves the bokeh as well ;)
That's very cool! Although wouldn't the UV filter coating on commercial lenses be counter productive? Assuming they are capturing this stuff full (IR to UV) spectrum...
UV filters are generally not incorporated into lenses. Typically you have to use a screw-on filter for this, and this is only needed for film photography. On digital sensors, the sensor contains the filter, usually some sort of Bayer filter that also blocks UV.
The fluorite lens elements that Canon uses are desirable not just for their low refractive index, but also precisely because they are highly transparent in the IR and UV range. The 400mm used in this particular array uses two fluorite elements, and is one the main reasons it's suitable for this application.
The fluorite lens elements that Canon uses are desirable not just for their low refractive index, but also precisely because they are highly transparent in the IR and UV range. The 400mm used in this particular array uses two fluorite elements, and is one the main reasons it's suitable for this application.
[deleted]
"To me, the coolest thing about this story is the telescope that was used to find this galaxy."
Uh, we just found out a new principle on the large scale organization of matter in the known universe. The method is surely inspirational, but I really have to question your statement that the method of discovery credits more merit than the discovery itself.
Uh, we just found out a new principle on the large scale organization of matter in the known universe. The method is surely inspirational, but I really have to question your statement that the method of discovery credits more merit than the discovery itself.
Relax. It's just, like, my opinion man.
Sure, sorry, you did state "to me". English is not my native language and nuances sometimes escape me. The jury rigged ground based telescope is cool.
No worries. Just so you know, I'll take any opportunity to deliver a Big Lebowski reference: https://www.youtube.com/watch?v=QsogswrH6ck
Consider yourself warned.
Consider yourself warned.
> we just found out a new principle on the large scale organization of matter in the known universe.
Not really, because we've already knew that and observed it before in the bullet galaxy. This is just a 2nd observation (although it is interesting in the ways it's different).
Not really, because we've already knew that and observed it before in the bullet galaxy. This is just a 2nd observation (although it is interesting in the ways it's different).
I didn’t even know that dark matter was anything more than theoretical.
That depends on how ready you are to dismiss the existing observational evidence for it. While it's by no means proven to exist, it's not something predicted by theory, so it's unfair to call it that. Supersymmetry, for example, is purely theoretical; whereas dark matter is a response to observations that contradict theory.
That is an engineering mindset. I was confused when I first read a review of the "Apollo 13" movie that said engineers think the real heroes of the crises were the engineers on the ground - who else would someone think was a hero? Oh yeah, the astronauts who were risking their lives.
This makes me wonder what the normal error bars are for how much dark matter is in galaxies. Do we often see galaxies with 75-125% as much dark matter as our models expect? Or maybe 99-101%, or even 10-1000%? How far off of the expected value does it take to become noteworthy?
That said, that this is a headline at all tells me it's probably closer to the 99-101 case than the 10-1000 case. Given that, this seems to be the tipping to some breakthroughs, because my sense has always been that we have really little idea what dark matter is.
Edit: Through a bit of link hopping, I found this paper [1]. Based on Figure 11 in that paper, it seems that there's a relatively small error bar around the expected ratios of dark matter to baryonic matter. At the very least, the error bar is narrow enough that this new galaxy would be well outside it.
But I don't have any formal astrophysics training, so I'd love to be corrected.
[1] https://academic.oup.com/mnras/article/410/1/210/1033029
That said, that this is a headline at all tells me it's probably closer to the 99-101 case than the 10-1000 case. Given that, this seems to be the tipping to some breakthroughs, because my sense has always been that we have really little idea what dark matter is.
Edit: Through a bit of link hopping, I found this paper [1]. Based on Figure 11 in that paper, it seems that there's a relatively small error bar around the expected ratios of dark matter to baryonic matter. At the very least, the error bar is narrow enough that this new galaxy would be well outside it.
But I don't have any formal astrophysics training, so I'd love to be corrected.
[1] https://academic.oup.com/mnras/article/410/1/210/1033029
There seems to be much more "dark matter" in our own galaxy than "normal matter", so "no dark matter at all" is an extremely surprising discovery.
The galaxy in the article is a clear anomaly, which by definition puts it outside of our expected range. We have observed millions of galaxies and they all have the same general density, so it is genuinely surprising to see one that is so widely dispersed.
We don't have an accurate way to identify the amount of dark matter in a galaxy. That is why the outliers are so important.
...but what is potentially more important is the cause of these outliers. Assuming there's no smooth distribution of dark matter correlations with regular matter, then some unfathomable EVENT caused this galaxy to be different.
I'm not saying it's aliens, but...
...but what is potentially more important is the cause of these outliers. Assuming there's no smooth distribution of dark matter correlations with regular matter, then some unfathomable EVENT caused this galaxy to be different.
I'm not saying it's aliens, but...
I know (hope?) you're joking, but we know so little that allowing for that possibility means we can't rule out the opposite. Maybe "aliens" (or some unfathomable event) are responsible for populating all other galaxies with dark matter but missed one.
That's a fun thought. Assuming it's true, make me wonder: what are the odds that humans would even notice/observe the missed one?
Side effect of a war with the dark matter entities?
speaking of outliers
https://en.wikipedia.org/wiki/Messier_94
that also is said to have little dark matter, but that was an indirect observation and unconfirmed afaik
https://en.wikipedia.org/wiki/Messier_94
that also is said to have little dark matter, but that was an indirect observation and unconfirmed afaik
I’m a lay person, but...
Isn’t dark matter just matter in other dimensions? Not visible in our three, but obviously having an effect? Like the body of an arch to flatlanders?
Isn’t dark matter just matter in other dimensions? Not visible in our three, but obviously having an effect? Like the body of an arch to flatlanders?
Not sure why an honest question is getting downvotes.
1) We do not yet have any reason to believe there are more than 3 dimensions plus time [1] 2) It depends on what you mean by "visible". Take a look at this picture full screen: https://en.wikipedia.org/wiki/Dark_matter#/media/File:Gravit...
That is an image of dark matter bending the light from distant galaxies - take note of the circularly shaped bent galaxies radiating around the middle. This effect is called "gravitational lensing", and it is why we know that there actually is some matter in this picture. We also know that it is invisible because otherwise the "dark matter" would be blocking view of the galaxies you're looking at.
3) I highly recommend reading the summary on the Wikipedia article on dark matter [2]. The whole article is really long, but the summary paragraphs at the top give a great high-level explanation of "what we know" about dark matter.
[1] https://physics.stackexchange.com/questions/4079/experimenta... [2] https://en.wikipedia.org/wiki/Dark_matter
1) We do not yet have any reason to believe there are more than 3 dimensions plus time [1] 2) It depends on what you mean by "visible". Take a look at this picture full screen: https://en.wikipedia.org/wiki/Dark_matter#/media/File:Gravit...
That is an image of dark matter bending the light from distant galaxies - take note of the circularly shaped bent galaxies radiating around the middle. This effect is called "gravitational lensing", and it is why we know that there actually is some matter in this picture. We also know that it is invisible because otherwise the "dark matter" would be blocking view of the galaxies you're looking at.
3) I highly recommend reading the summary on the Wikipedia article on dark matter [2]. The whole article is really long, but the summary paragraphs at the top give a great high-level explanation of "what we know" about dark matter.
[1] https://physics.stackexchange.com/questions/4079/experimenta... [2] https://en.wikipedia.org/wiki/Dark_matter
Humans have a long history of misunderstanding the geometry of our world. I don't know jack about astrophysics, but the explanations for dark matter and dark energy seem so complicated that my bullshit meter goes off. Reminds me of the people trying to model the walk of the planets in an Earth-centric universe. I'd put money on another Newton or Einstein (or singularity AI) giving us a much more elegant solution in the next 50-100 years.
I feel similarly dissatisfied with the explanations but, given the current framework, they're actually the simplest possible models that could work. The rest of physics and the many observations that need to be consistent with it are just that elaborate.
Unfortunately, there's not enough hard evidence to support the claim that dark matter resides in other dimensions. All we know for sure is that, based on our current understanding of physics, the observable universe behaves as if it has more mass than what we are capable of detecting.
You might want to check out a very interesting non-linear spinor theory[1] which contains a chapter on dark matter and dark energy analytically explaining the formation and inner structure, as well as the ratio of dark matter and dark energy throughout the development phase of the universe.
[1] http://norbert-winter.com/wp-content/uploads/2018/02/2017-03...
[1] http://norbert-winter.com/wp-content/uploads/2018/02/2017-03...
Not quite.
No one knows what dark matter is. Astrophysicists know it by how it behaves. It seems to behave just like a normal gas, or perhaps dust or even moderate chunky stuff, except that it appears to be essentially completely transparent and simply passes through normal matter.
(There’s one candidate theory of dark matter called, IIRC, “MACHO”s, which is the idea that dark matter is just a bunch of small, very dense objects dotting the universe. We don’t see them because they’re compact and don’t take up enough space to be noticed.)
No one knows what dark matter is. Astrophysicists know it by how it behaves. It seems to behave just like a normal gas, or perhaps dust or even moderate chunky stuff, except that it appears to be essentially completely transparent and simply passes through normal matter.
(There’s one candidate theory of dark matter called, IIRC, “MACHO”s, which is the idea that dark matter is just a bunch of small, very dense objects dotting the universe. We don’t see them because they’re compact and don’t take up enough space to be noticed.)
How would we possibly know it passes through other matter?
I believe it's also reasoned to be a consequence of the apparent non-interaction with the electromagnetic force that dark matter has. Since it doesn't block light in any way we can detect (absorption, changes in polarization, changes in direction, etc.) it's also not likely to actually "touch" normal matter in the way you'd intuitively expect. That's because that touch is mediated via the electromagnetic force, the electron clouds and the charges they have repelling each other over the vast empty spaces of normal matter.
That’s one reason. As another example, a lot of the dust and gas in our solar system ended up in the sun, presumably because it collided with other stuff that formed the sun, thus losing speed and getting trapped. But there isn’t a bunch of dark matter in the sun. So either there wasn’t a bunch of diffuse dark matter in the stuff that made up the early solar system or the dark matter was somehow immune from getting trapped along with all the normal matter.
Strictly speaking we don't, but we have reasons to believe that on a galactic scale it doesn't. There was that famous example a few years back where we had observed two galaxies hitting each other, and it "looked" like the regular ol' matter (eg, interstellar gas clouds) in each galaxy had "hit" each other and stopped while the dark matter in each had passed through the other galaxy unimpeded.
Which would seem to indicate that either dark matter is sparse and heavy (and just doesn't hit things very often) or diffuse and not-very-interacting with other matter.
Which would seem to indicate that either dark matter is sparse and heavy (and just doesn't hit things very often) or diffuse and not-very-interacting with other matter.
I am not convinced. What can possibly be said about two galaxies "hitting" each other if we observe only a sliver of time? Secondly, the stopping would happen because of gravity, and that would affect dark matter too.
Gas clouds also interact electromagnetically: https://en.wikipedia.org/wiki/Bullet_Cluster
You shouldn't be convinced by one sentence saying that a conclusion was reached a few years back. But you also have no basis for confidence in the negation of that conclusion on the basis of that almost total lack of data or theory.
You can't honestly believe that teams of people with physics Ph.D.s just didn't address these questions - how do we infer from observational data, what do we infer from that data - so go read the paper.
You can't honestly believe that teams of people with physics Ph.D.s just didn't address these questions - how do we infer from observational data, what do we infer from that data - so go read the paper.
I think they mean we only observe other matter interacting with other matter, and not dark matter interacting with other matter.
I guess the options aere: (1) dark matters is a property of physics. IE, it is the effects of physics that we don't understand yet, but has consequences to matter and the properties/rules of matter that we do understand. (2) Dark matter is stuff, actual matter that we just can't "see".
Matter in other dimensions is I guess a combination of the two. Actual normal matter, "hidden" by properties of physics that we don't quite understand.
Matter in other dimensions is I guess a combination of the two. Actual normal matter, "hidden" by properties of physics that we don't quite understand.
I believe the "octane" of normal vs. dark matter explains in part the difference shape / sizes of galaxies.
I was talking with some physicists from Fermilab last week about dark matter (I'm a physics enthusiast but certainly no physicist) and one thing they were discussing was how current dark matter experiments are constraining possible dark matter candidates significantly as they get more sensitive without finding any. This has lead to increased interest by physicists in alternative theories, including MOND. This galaxy discovery, though, appears to be strong evidence against MOND or other modified-gravity theories since the dark matter is missing in this case. It provides more evidence for dark matter's existence, even as the strongest candidate, Weakly Interacting Massive Particles (WIMPs) are becoming more constrained by lack of evidence.
Why am I not surprised. As a engineering-physics major who works in software, I always correct people when they ask me "what do you think dark matter is?" I think the more appropriate question is, and always has been: Does dark matter even exist? Thank you for casting light on MOND. My money has always been on modified gravity. I think we missed a few terms in the equation.
The comment that you are responding to says that this weakens the case for MOND.
I saw that, it was more so a response to Fermi lab Physicist are taking MOND more seriously. MOND has its problems also.
Its my opinion that there is something about the black hole at the center of these galaxies that's causing discrepancies that we attribute to "dark matter". The fact that this new galaxy is so sparse and distributed and shows no sign of dark matter seems to re-affirm this. I'd be curious to know if it even has a central black hole.
Its my opinion that there is something about the black hole at the center of these galaxies that's causing discrepancies that we attribute to "dark matter". The fact that this new galaxy is so sparse and distributed and shows no sign of dark matter seems to re-affirm this. I'd be curious to know if it even has a central black hole.
To clarify, no one's taking MOND nearly as seriously as WIMPs in terms of funding for experiments to test it. That said, the lack of evidence has lead to increased interest in alternative theories that could explain the phenomenon. Some of these are alternative dark matter particle candidates though, just not WIMPs.
Wimps have gotten lots of funding. But we are probably on the last generation of wimp detector
It's a pointless conversation the argue over which unproven theory is taken most seriously.
The reality is that we don't know.
The reality is that we don't know.
The supermassive black holes at the centers of galaxies really can't have anything to do with dark matter or any possible alternatives. A few light years away from the SMBH the black hole behaves like any other ordinary astrophysical object. Moreover, at least in our galaxy, the stars in the central bulge outweigh the SMBH by a factor of about 10,000.
> The supermassive black holes at the centers of galaxies really can't have anything to do with dark matter or any possible alternatives.
What makes you so sure? I think limiting our thinking too much into the confines of regular gravitation is a mistake. Maybe SMB are the source of these theoretical heavy neutrino's we have yet to detect that are the #1 candidate for dark matter/WIMPS. Or maybe the gravitational disturbance of a singularity is far more complex that we imagine?
Take a look at the video on the Galaxy Rotation Curve video in this wiki:
https://en.wikipedia.org/wiki/Galaxy_rotation_curve#Descript...
I stare at it and cant help but wonder is something about that central singularity is unique. We still have no idea what truly happens at the center.
What makes you so sure? I think limiting our thinking too much into the confines of regular gravitation is a mistake. Maybe SMB are the source of these theoretical heavy neutrino's we have yet to detect that are the #1 candidate for dark matter/WIMPS. Or maybe the gravitational disturbance of a singularity is far more complex that we imagine?
Take a look at the video on the Galaxy Rotation Curve video in this wiki:
https://en.wikipedia.org/wiki/Galaxy_rotation_curve#Descript...
I stare at it and cant help but wonder is something about that central singularity is unique. We still have no idea what truly happens at the center.
We have observed visible objects (stars) that are close enough to be orbiting a supermassive black hole and they follow the expected orbits.
If supermassive black holes do something new and strange with gravity--something big enough to affect an entire surrounding galaxy--it seems likely it should somewhat affect the trajectories of the nearest objects too.
That's not to rule out something strange about black hole gravitation, but the observations do constrain some versions of how strange things can be.
If supermassive black holes do something new and strange with gravity--something big enough to affect an entire surrounding galaxy--it seems likely it should somewhat affect the trajectories of the nearest objects too.
That's not to rule out something strange about black hole gravitation, but the observations do constrain some versions of how strange things can be.
How does your theory fit with flat rotation curves?
My "theory" is just a hunch. Flat rotation curves IS the primary foundation for the existence of dark matter:
https://en.wikipedia.org/wiki/Galaxy_rotation_curve
I'm simply saying its more likely an unknown effect of the black hole at the center. Possibly some sort of gravitational modification effect that only occures in the presents of singularities. The fact that a sparse galaxy with possible no black hole center (I assume) shows no sign of dark matter is compelling. It may not prove the specific equations/explanations set forth in MOND, but it may hint to some other alternative gravitational theory that has yet to be flushed out, rather than relying on some invisible particles which may not exist.
https://en.wikipedia.org/wiki/Galaxy_rotation_curve
I'm simply saying its more likely an unknown effect of the black hole at the center. Possibly some sort of gravitational modification effect that only occures in the presents of singularities. The fact that a sparse galaxy with possible no black hole center (I assume) shows no sign of dark matter is compelling. It may not prove the specific equations/explanations set forth in MOND, but it may hint to some other alternative gravitational theory that has yet to be flushed out, rather than relying on some invisible particles which may not exist.
While rotation curves were one of the earlier indications of dark matter, we have perhaps stronger evidence now from CMB measurements, gravitational lensing studies, and structure formation considerations.
Got any links for cosmology laymen? I'd be curious to see if my "theory" is negated.
Not sure if this is the right level or not, but one place with nice animations is http://background.uchicago.edu/~whu/animbut/anim2.html
I'm not a physicist, but that was my thought exactly.
Black holes are weird objects, they are enormous, have huge mass and density, almost no radiation, have great gravitational effect on surroundings and they are hard to detect etc.
They seem like a perfect candidate for 'dark matter'.
Could anyone explain why it is wrong?
Black holes are weird objects, they are enormous, have huge mass and density, almost no radiation, have great gravitational effect on surroundings and they are hard to detect etc.
They seem like a perfect candidate for 'dark matter'.
Could anyone explain why it is wrong?
Black Holes of reasonable can be detected by gravitational lensing. If Black Holes / Brown Dwarfs / etc. were primarily responsible for the dark matter locally, lensing searches would have detected more. See e.g. https://arxiv.org/abs/astro-ph/0002058 for an overview.
So according to wikipedia [1] they have been looked into, they would fall under the category of MACHOs
1. https://en.wikipedia.org/wiki/Dark_matter#Baryonic_matter
1. https://en.wikipedia.org/wiki/Dark_matter#Baryonic_matter
They're detectable and already taken into account.
This graphic can explain very clearly, although it’s a little old. The result is that constraints today are even tighter though, so for these purposes it’s still useful.
https://4.bp.blogspot.com/-UN8jPwzj2qI/V9hRVCG1HBI/AAAAAAAAC...
https://4.bp.blogspot.com/-UN8jPwzj2qI/V9hRVCG1HBI/AAAAAAAAC...
My "theory" is just a hunch.
It looks like Occam's razor. To explain an unexpected result, you label the problem so it looks like it must be a new entity.
It looks like Occam's razor. To explain an unexpected result, you label the problem so it looks like it must be a new entity.
"The galaxies are not rotating the way our understanding of gravity says they should....lets invent dark matter"
"Oh no, the existence of dark matter means the universe should be collapsing in on itself, lets invent dark energy to balance out the dark matter"
I always figured that the KISS principle meant maybe we just didn't understand gravity very well, but what the hell do I know. Always found it fascinating though.
"Oh no, the existence of dark matter means the universe should be collapsing in on itself, lets invent dark energy to balance out the dark matter"
I always figured that the KISS principle meant maybe we just didn't understand gravity very well, but what the hell do I know. Always found it fascinating though.
> "Oh no, the existence of dark matter means the universe should be collapsing in on itself, lets invent dark energy to balance out the dark matter"
Even if you remove dark matter from the picture, there's still the acceleration of expansion (increase of the Hubble term over time), for which there is pretty solid data, namely the discrepancy of brightness and redshift of distant supernovae.
https://en.wikipedia.org/wiki/Accelerating_expansion_of_the_...
By far the "cosmological constant" sort of outweighs all the other terms in the Universe's stress energy tensor.
Even if you remove dark matter from the picture, there's still the acceleration of expansion (increase of the Hubble term over time), for which there is pretty solid data, namely the discrepancy of brightness and redshift of distant supernovae.
https://en.wikipedia.org/wiki/Accelerating_expansion_of_the_...
By far the "cosmological constant" sort of outweighs all the other terms in the Universe's stress energy tensor.
So the item I've always wondered about this is that we don't actually know that the universe is expanding, we just infer it from redshift. But there are two components in frequency, meaning two ways to make redshift. Either change in position or change in time.
We see redshift and infer that a pulsar or other know quantity is shifted and assume it has moved because space has expanded. Why don't we ever assume that the length of a second has shrunk in the billions of years that have gone by? Space and time change in relativity, why is only change in space considered in the history of the universe? What's wrong with time?
We see redshift and infer that a pulsar or other know quantity is shifted and assume it has moved because space has expanded. Why don't we ever assume that the length of a second has shrunk in the billions of years that have gone by? Space and time change in relativity, why is only change in space considered in the history of the universe? What's wrong with time?
> we just infer it from redshift.
Not just redshift. We're using two observables: Redshift and brightness of a very specific kind of supernova which always releases close to the same energy, i.e. amount of light. Using the 1/d² law we can relate observed brightness to distance. Redshift OTOH tells us the velocity.
Then we plot "redshift vs. sqrt(1/brightness)" and if the expansion of the universe were constant, i.e. velocity/distance (i.e. the Hubble term being constant) we'd see a linear relation between distance and velocity.
However what we actually observe is that the velocity is accelerating over distance, which means something is putting "something" extra into it. So far we can describe it only as a nonvanishing extra term in the field equations. A term which originally Einstein came up with to stabilize a static universe, but you can use it as well to "push" or "pull" on the whole of spacetime to accelerate/decelerate expansion/contraction.
For all intents and purposes this term behaves like an additional energy term in the equations. We are in the _dark_ about, what's actually causing the effects we see. And when we adjust our models to accomodate for that, an extra _energy_ term shows up. Hence "dark energy". However no astrophysicist worth his salt thinks of it as something concrete.
In the cosmology lectures I attended during my studies our professor used to pick some random name, who'd not be a student in the class at the time and instead use that, instead of "Dark Energy" to refer to that term (the "dark name" used when I was there was "Geroge" BTW), just to hammer the fact, that we don't have an effin clue what's going on, only that it's something very observable that's śhows up prominently in all the data collected for the past 20 years and you can't ignore it and have to give it some name.
Not just redshift. We're using two observables: Redshift and brightness of a very specific kind of supernova which always releases close to the same energy, i.e. amount of light. Using the 1/d² law we can relate observed brightness to distance. Redshift OTOH tells us the velocity.
Then we plot "redshift vs. sqrt(1/brightness)" and if the expansion of the universe were constant, i.e. velocity/distance (i.e. the Hubble term being constant) we'd see a linear relation between distance and velocity.
However what we actually observe is that the velocity is accelerating over distance, which means something is putting "something" extra into it. So far we can describe it only as a nonvanishing extra term in the field equations. A term which originally Einstein came up with to stabilize a static universe, but you can use it as well to "push" or "pull" on the whole of spacetime to accelerate/decelerate expansion/contraction.
For all intents and purposes this term behaves like an additional energy term in the equations. We are in the _dark_ about, what's actually causing the effects we see. And when we adjust our models to accomodate for that, an extra _energy_ term shows up. Hence "dark energy". However no astrophysicist worth his salt thinks of it as something concrete.
In the cosmology lectures I attended during my studies our professor used to pick some random name, who'd not be a student in the class at the time and instead use that, instead of "Dark Energy" to refer to that term (the "dark name" used when I was there was "Geroge" BTW), just to hammer the fact, that we don't have an effin clue what's going on, only that it's something very observable that's śhows up prominently in all the data collected for the past 20 years and you can't ignore it and have to give it some name.
What would the difference be between time slowing down and the universe expanding?
> why is only change in space considered in the history of the universe
Gauge fixing.
There is no particular reason to slice up spacetime into spacelike hypersurfaces where each point in the hypersurface has the same time coordinate, other than convenience. Likewise, there is no particular reason to chose one set of time coordinates over another, other than convenience.
You can certainly work in 4 dimensional "block universe" when considering the radar distances between points on the worldlines that the centres-of-momentum of galaxies (or clusters) travel, and choose arbitrary coordinates. You're then using tensors for everything, and considering spacetime 4-volumes.
Slicing or threading into 3+1 resp 1+3 makes the calculations easier, but can introduce fictitious forces and other features that depend on one's choices when doing that sort of gauge fixing.
The standard cosmological frame of reference is a 3+1 slicing wherein the time coordinate is recoverable by an idealized observer locally measuring the spectrum of the CMB photons arriving at a point on the observer's worldline; such an observer will see matter in the bulk as homogeneous and isotropic, and will be far from any of the matter, and will see the CMB spectrum as equal to the emission spectrum of an ideal blackbody with a particular temperature. Add in adiabatic expansion, and the universe gives a quasi-universal time coordinate, which clamps many of the pseudo-forces 3+1 slicing can produce.
Since we are near lots of matter (the Earth, the solar system, our galaxy and the local cluster), we see big anisotropies and thus noise, including a gravitational and peculiar-motion redshift anisotropy in the CMB. As we get a better handle on our local environment, we can better strip out these factors that distinguish us real physical observers from an idealized non-physical "Eulerian" observer of a FLRW universe.
But since an alien astronomer in a far away galaxy could reasonably take the same approach, we could exchange messages like: "at our CMB temperature x K we observed a supernova in your galaxy", which the other side could relate to their measurement of the CMB temperature at time of supernova and at the time of receipt of message, and recover the equilvalent of a radar distance.
This also plays neatly with the Robertson-Walker (RW) spacetime that is fundamental to the cosmological frame: RW spacetimes readily admit a slicing which makes them higher-dimensional equivalents of a set of infinitesmially thin plates stacked one on top of the other, where the metric includes a function which determines the radius of each plate as one goes from the bottommost plate to the topmost. It also makes it easy to set down comoving coordinates such that matter stays at rest at the same spatial coordinates in every spacelike hypersurface: we expand the metric instead of actually moving the galaxies, which is good because there is no distortion of the shape of galaxy clusters implying a real (i.e., gauge-independent) acceleration in any particular direction, and because it makes it easy to take an Eulerian fluid mechanics approach to galaxy clusters in the large.
Additionally, this sort of slicing plays nicely with things like Hamiltonian mechanics and the time-dependent Schrödinger equation, which we are likely to care about when thinking about the microscopic details of events like a supernova inferrable from coarse-grained observables (like the emissions spectrum associated with supernova nucleosynthesis).
However, if something motivated you to do so, you could choose some other expanding-universe metric that is suitable for treating distant galaxies as at a lower gravitational potential, for instance. This would mean fixing a different gauge, and introducing some fictitious (gauge-dependent) forces to deal with some observables of high-redshift objects, and those would tend to complicate hypothetical communications with alien astronomers in distant galaxies. The "real" physics would -- just like the "real" physics in the cosmological frame -- be described correctly using generally covariant tensors. But you could do many calculations using simpler fields, and at least in principle that might highlight features that are obscured in the standard cosmological slicing.
Gauge fixing.
There is no particular reason to slice up spacetime into spacelike hypersurfaces where each point in the hypersurface has the same time coordinate, other than convenience. Likewise, there is no particular reason to chose one set of time coordinates over another, other than convenience.
You can certainly work in 4 dimensional "block universe" when considering the radar distances between points on the worldlines that the centres-of-momentum of galaxies (or clusters) travel, and choose arbitrary coordinates. You're then using tensors for everything, and considering spacetime 4-volumes.
Slicing or threading into 3+1 resp 1+3 makes the calculations easier, but can introduce fictitious forces and other features that depend on one's choices when doing that sort of gauge fixing.
The standard cosmological frame of reference is a 3+1 slicing wherein the time coordinate is recoverable by an idealized observer locally measuring the spectrum of the CMB photons arriving at a point on the observer's worldline; such an observer will see matter in the bulk as homogeneous and isotropic, and will be far from any of the matter, and will see the CMB spectrum as equal to the emission spectrum of an ideal blackbody with a particular temperature. Add in adiabatic expansion, and the universe gives a quasi-universal time coordinate, which clamps many of the pseudo-forces 3+1 slicing can produce.
Since we are near lots of matter (the Earth, the solar system, our galaxy and the local cluster), we see big anisotropies and thus noise, including a gravitational and peculiar-motion redshift anisotropy in the CMB. As we get a better handle on our local environment, we can better strip out these factors that distinguish us real physical observers from an idealized non-physical "Eulerian" observer of a FLRW universe.
But since an alien astronomer in a far away galaxy could reasonably take the same approach, we could exchange messages like: "at our CMB temperature x K we observed a supernova in your galaxy", which the other side could relate to their measurement of the CMB temperature at time of supernova and at the time of receipt of message, and recover the equilvalent of a radar distance.
This also plays neatly with the Robertson-Walker (RW) spacetime that is fundamental to the cosmological frame: RW spacetimes readily admit a slicing which makes them higher-dimensional equivalents of a set of infinitesmially thin plates stacked one on top of the other, where the metric includes a function which determines the radius of each plate as one goes from the bottommost plate to the topmost. It also makes it easy to set down comoving coordinates such that matter stays at rest at the same spatial coordinates in every spacelike hypersurface: we expand the metric instead of actually moving the galaxies, which is good because there is no distortion of the shape of galaxy clusters implying a real (i.e., gauge-independent) acceleration in any particular direction, and because it makes it easy to take an Eulerian fluid mechanics approach to galaxy clusters in the large.
Additionally, this sort of slicing plays nicely with things like Hamiltonian mechanics and the time-dependent Schrödinger equation, which we are likely to care about when thinking about the microscopic details of events like a supernova inferrable from coarse-grained observables (like the emissions spectrum associated with supernova nucleosynthesis).
However, if something motivated you to do so, you could choose some other expanding-universe metric that is suitable for treating distant galaxies as at a lower gravitational potential, for instance. This would mean fixing a different gauge, and introducing some fictitious (gauge-dependent) forces to deal with some observables of high-redshift objects, and those would tend to complicate hypothetical communications with alien astronomers in distant galaxies. The "real" physics would -- just like the "real" physics in the cosmological frame -- be described correctly using generally covariant tensors. But you could do many calculations using simpler fields, and at least in principle that might highlight features that are obscured in the standard cosmological slicing.
> the KISS principle
Known in science as Occaam's razor.
> maybe we just didn't understand gravity very well
We definitely don't understand it when it comes to quantum scales. And maybe we don't understand it properly at macro scales.
But know that the universe is in no way obligated to be simple to the layman.
E.g. Speed of light seems to be constant, so we're going to make up all this bizarre stuff about time dilation and space faring twins.
Known in science as Occaam's razor.
> maybe we just didn't understand gravity very well
We definitely don't understand it when it comes to quantum scales. And maybe we don't understand it properly at macro scales.
But know that the universe is in no way obligated to be simple to the layman.
E.g. Speed of light seems to be constant, so we're going to make up all this bizarre stuff about time dilation and space faring twins.
What is interesting to me is how recent Occaam's razor is (800 years) and why it was introduced (to understand the Trinity in western Christianity).
It's been a successful heuristic, but for understanding the things that can be understood simply - we shouldn't believe that nature "wants" to be simple though...
It's been a successful heuristic, but for understanding the things that can be understood simply - we shouldn't believe that nature "wants" to be simple though...
Occam's razor in a lot of ways is a proxy for Bayes' theorem. The theorem "punishes" more complex models by shrinking the prior term and increasing the marginal term, while the likelihood is pegged at P = 1.0 at most.
You can come up with a model that perfectly explains the outcome ( P(e|H) = 1 ) but you have to offset the insane improbability of pegging all those free parameters at the value they are at ( P(H) ).
You can come up with a model that perfectly explains the outcome ( P(e|H) = 1 ) but you have to offset the insane improbability of pegging all those free parameters at the value they are at ( P(H) ).
I wish there was some way to hide all comments on the internet about dark matter / dark energy from anyone whose understanding of the subject is below the level of even having read the wikipedia entries just once.
That's... somewhat of an over-simplification
>"This galaxy discovery, though, appears to be strong evidence against MOND or other modified-gravity theories since the dark matter is missing in this case."
What makes you say this?
What makes you say this?
The article sketches the explanation — if dark matter doesn’t actually exist and is simply an observational side effect, all galaxies should demonstrate that effect. That this galaxy doesn’t strengthens the case for dark matter being separable from regular (visible) matter.
See my post here: https://news.ycombinator.com/item?id=16702599
From what I understand of MOND, the less attraction to visible mass (as you would expect from a "ultra diffuse" galaxy like this), the smaller the effect should be. So MOND predicts the effect should be relatively small for this galaxy.
Further, in the paper that the OP references they do a calculation and get odds of something around 1/20 that a galaxy like this will exist if MOND is correct.
The interesting part is that for some reason they interpret that as ruling out MOND, while meanwhile saying "no one has any idea how this thing can exist if the dark matter theory is right". I would think "no idea how it can exist" should correspond to odds less than 1/20.
From what I understand of MOND, the less attraction to visible mass (as you would expect from a "ultra diffuse" galaxy like this), the smaller the effect should be. So MOND predicts the effect should be relatively small for this galaxy.
Further, in the paper that the OP references they do a calculation and get odds of something around 1/20 that a galaxy like this will exist if MOND is correct.
The interesting part is that for some reason they interpret that as ruling out MOND, while meanwhile saying "no one has any idea how this thing can exist if the dark matter theory is right". I would think "no idea how it can exist" should correspond to odds less than 1/20.
This galaxy is down in the 1 in 100_000 parts, not 1 in 20 fwiw. I am not going to debate MOND, I am just explaining the reasoning.
I drew the blue vertical line at 20 km/s:
https://image.ibb.co/m38AH7/galaxy.png
Are you claiming this is cutting off 1e-5 of the area of that Gaussian curve they fit? It looks very close to 1/20 to me. I don't care about .05, just to get an order of magnitude and compare it to the "this is an unbelievable result" quotes.
Are you claiming this is cutting off 1e-5 of the area of that Gaussian curve they fit? It looks very close to 1/20 to me. I don't care about .05, just to get an order of magnitude and compare it to the "this is an unbelievable result" quotes.
Ok, I see I read the figure 3b wrong (only glanced at it before). I extracted the data from their extended figure 5a:
I believe I replicated their results since it had percentile of p = .6336 for sigma_test = 0.1 and p = .1607 for sigma_test = 8.6, which matches to their sigma_int = 3.2 (+5.5, -3.2) using 16th - 84th percentiles result.
Where did you get 1/100,000?
dV sigma R_major
-4.16 16.41 -3.98
10.55 3.41 -1.97
15.03 7.25 -1.85
-2.24 5.12 -0.74
1.17 5.75 0.29
-14.39 5.97 1.16
-40.39 11.3 1.52
2.24 6.61 1.58
-3.52 13.43 4.49
-1.39 10.23 6.77
# Used for biweight midvariance calculation (r.bw)
require(asbio)
# Utils:
## Simulation function:
### "For a given value of sigma_test we generated 1000 samples of 10 velocities,
### distributed according to a Gaussian of width sigma_test. The ten velocities
### in each sample were then perturbed with errors, drawn from Gaussians with
### widths equal to the empirically-determined uncertainties in the measured
### dispersions.
run_sim = function(nSim, n, sigma_test, err){
replicate(nSim, rnorm(n, 0, sigma_test) + rnorm(n, 0, err))
}
## Clean up the biweight midvariance output
r.bw2 = function(x){
as.numeric(sqrt(r.bw(x)))
}
# Scraped data roughly matches what is reported in paper
## MAD (~4.7)
s_obs_nmad = mad(dat$dV)
## biweight (~8.4)
s_obs_bi = r.bw2(dat$dV)
## RMS (~14.3)
s_obs_rms = sqrt(mean(dat$dV^2))
# Run simulation to test sigma values in .1 km/s intervals
## "Using the biweight estimator, “measured” dispersions sigma_obs,test were calculated
## for all samples. If the value 8.4 is within the 16th – 84th percentile of the
## distribution of sigma_obs,test then sigma_test is within the + /-1 sigma uncertainty
## on sigma_intr. This method gives sigma_intr = 3.2 (+5.5, -3.2) kms^-1."
sigmas = seq(.1, 30, by = .1)
nSim = 1e4
out = as.data.frame(matrix(nrow = length(sigmas), ncol = 2))
colnames(out) = c("sigma_test", "s_obs_bi_percentile")
for(i in 1:length(sigmas)){
# Calculate biweights for each sim result
sim_res = run_sim(nSim, 10, sigmas[i], dat$sigma)
sigma_obs_test = apply(sim_res, 2, function(x) r.bw2(x))
# Output sigma_test and the percentile of 8.4
out[i, ] = c(sigmas[i], ecdf(sigma_obs_test)(8.4))
# Print progress
print(out[i, ])
}
# Check which sigma_test values are within the accepted percentile range
out$check = out$s_obs_bi_percentile > .16 & out$s_obs_bi_percentile < .84
# Plot the results
png("results.png", width = 960, height = 960)
plot(out[, 1], out[, 2], xlab = "sigma_test", ylab = "Percentile(8.4)", panel.first = grid())
dev.off()
I believe I replicated their results since it had percentile of p = .6336 for sigma_test = 0.1 and p = .1607 for sigma_test = 8.6, which matches to their sigma_int = 3.2 (+5.5, -3.2) using 16th - 84th percentiles result.
Where did you get 1/100,000?
I do not think this is an argument in favor or against the existence of dark matter. I think it is a very surprising result either way. Only thing we know is that this galaxy is a very special fluke.
It will surely be difficult to explain by non-dark-matter theories, but I do not think dark matter advocates are going to have any easier to explain why this galaxy (and, as far as we know, only this galaxy) doesn't have any dark matter.
It will be exciting to see what explanations both camps come up with.
It will surely be difficult to explain by non-dark-matter theories, but I do not think dark matter advocates are going to have any easier to explain why this galaxy (and, as far as we know, only this galaxy) doesn't have any dark matter.
It will be exciting to see what explanations both camps come up with.
I'm no physicist or astronomer but as far as I can tell I don't think what you said makes complete sense. While it's true that it remains to be explained why a certain galaxy doesn't have the same amount of DM as the others at a glance it doesn't seem particularly implausible that such dramatic variation could theoretically happen depending on what DM truly is and how it ends up in galaxies in the first place.
On the other hand you'd expect a modification to the theories of physics themselves like MOND would apply to all objects in the universe in the same way, if you find a galaxy where the model breaks down then the model is not good enough. That would be like finding a galaxy far away where gravity itself works differently, that'd be wild.
So as far as I can tell it does seem to discredit something like MOND more than DM because you expect physical laws to be universal while it's plausible that DM could not be distributed equally everywhere.
On the other hand you'd expect a modification to the theories of physics themselves like MOND would apply to all objects in the universe in the same way, if you find a galaxy where the model breaks down then the model is not good enough. That would be like finding a galaxy far away where gravity itself works differently, that'd be wild.
So as far as I can tell it does seem to discredit something like MOND more than DM because you expect physical laws to be universal while it's plausible that DM could not be distributed equally everywhere.
"Strongest candidate" is a bit strong. Certainly WIMPs are convenient theoretically, but so are e.g. axions.
I used "strongest candidate" not as my opinion (I don't know) but that is how physicists see the various possibilities. Certainly WIMP research has received the largest amount of funding in experimental physics as compared to other candidates.
(I am a physicist who used to work on a WIMP detector). WIMP funding has certainly gotten the largest share of funding. Part of the reason is probably that we know how to build detectors good enough to reach a swath of parameter space and the relative popularity of supersymmetry. I've heard some people suggest that part of the reason for funding such expensive WIMP experiments is that we know exactly where the limits of the detection technique lie and they are reachable given enough money.
I follow the BSM-SG model, which also has no dark matter to explain galaxy rotations.
Disclaimer: I choose my models after reduction of assumptions and stricter logic is better (Solomonoff Induction). Secondly, I understood when studying physics, that there seems something wrong, but could not put my finger on it. So, naturally it made the most sense for me, to first check all the assumptions currently used and drop those that could easily be seen differently. In fact, the BSM-SG model assumes so less, that I actually accepted it after a year of heavy studying and thinking - the first model ever for me.
It however has some "dark matter" called filaments which are the residue of the galactic crystallization egg so to say. Read some paper last year that found them, but of course, they thought it's dark matter. It is actually black black - absolutely no photon emissions even when heated to millions of degrees.
In the BSM-SG model galaxies crystallize, it does not have a big bang theory. The galactic redshift has a very different explanation.
The filaments are only located around the super massive black hole in every well formed galaxy (not globular clusters). They are the hardest and densest hard matter in the universe. Everything we know is super soft compared to this stuff. Our hole star system is maybe a cubic centimeter of this stuff.
If you take all the fragments of the fillaments and put them together they build a perfect hollow sphere that fits perfectly around the black hole. Another really wired property: if you take a cube of it and cut it in half, the mass of both parts will be more then the mass of the original cube. Sounds paradoxical at first, but once you understand how Newtonian mass is derived and that it is basically an surface property it makes sense.
If you separate ordinary matter, you don't increase to surface of atomic structures towards the vacuum. You get relativistic changes due the microcurviture, but the surface stays the same. This stuff you can cut and increase the surface until the particle is smaller then the CL node distance ~10^-20 m at which point it will loos its Newtonian mass and only the intrinsic mass will manifest. Same effect as some Neutrinos have.
But year, I'm a nut job who does not believe in what most think is true, but to be honest, I don't care. It works so wonderfully and if you can interpret papers from this models perspective, I have not found a single paper that even shakes on it's fundamentals. Some days ago a paper was posted here in which they found "magic angles" in graphene which was doted. I predicted 2 years ago that there will be some angle influence between the graphene layers.
To predict something more, you know, because I'm nuts: most of the grapheme we use is quite unordered. There are 2 possibilities in which the carbon atom can be positioned. To get a really good conductor the atoms need to be properly aligned together with the alignment of the 2 layers.
The standard perspective on the atomic nucleus is totally oversimplified and this is something quite some scientists found trough different ways. The BSM-SG model was the first model where the periodic table stared to make sense for me, together with all the isotopes.
Disclaimer: I choose my models after reduction of assumptions and stricter logic is better (Solomonoff Induction). Secondly, I understood when studying physics, that there seems something wrong, but could not put my finger on it. So, naturally it made the most sense for me, to first check all the assumptions currently used and drop those that could easily be seen differently. In fact, the BSM-SG model assumes so less, that I actually accepted it after a year of heavy studying and thinking - the first model ever for me.
It however has some "dark matter" called filaments which are the residue of the galactic crystallization egg so to say. Read some paper last year that found them, but of course, they thought it's dark matter. It is actually black black - absolutely no photon emissions even when heated to millions of degrees.
In the BSM-SG model galaxies crystallize, it does not have a big bang theory. The galactic redshift has a very different explanation.
The filaments are only located around the super massive black hole in every well formed galaxy (not globular clusters). They are the hardest and densest hard matter in the universe. Everything we know is super soft compared to this stuff. Our hole star system is maybe a cubic centimeter of this stuff.
If you take all the fragments of the fillaments and put them together they build a perfect hollow sphere that fits perfectly around the black hole. Another really wired property: if you take a cube of it and cut it in half, the mass of both parts will be more then the mass of the original cube. Sounds paradoxical at first, but once you understand how Newtonian mass is derived and that it is basically an surface property it makes sense.
If you separate ordinary matter, you don't increase to surface of atomic structures towards the vacuum. You get relativistic changes due the microcurviture, but the surface stays the same. This stuff you can cut and increase the surface until the particle is smaller then the CL node distance ~10^-20 m at which point it will loos its Newtonian mass and only the intrinsic mass will manifest. Same effect as some Neutrinos have.
But year, I'm a nut job who does not believe in what most think is true, but to be honest, I don't care. It works so wonderfully and if you can interpret papers from this models perspective, I have not found a single paper that even shakes on it's fundamentals. Some days ago a paper was posted here in which they found "magic angles" in graphene which was doted. I predicted 2 years ago that there will be some angle influence between the graphene layers.
To predict something more, you know, because I'm nuts: most of the grapheme we use is quite unordered. There are 2 possibilities in which the carbon atom can be positioned. To get a really good conductor the atoms need to be properly aligned together with the alignment of the 2 layers.
The standard perspective on the atomic nucleus is totally oversimplified and this is something quite some scientists found trough different ways. The BSM-SG model was the first model where the periodic table stared to make sense for me, together with all the isotopes.
Where can I read about this that is written by a physicist, but isn't written by Stoyan Sarge (the original author).
The reason for the second requirement is I don't know enough physics to call bullshit on bullshit. So I'd prefer to read a third party who does know enough discussing the results than the original author.
My google foo isn't finding anything.
The reason for the second requirement is I don't know enough physics to call bullshit on bullshit. So I'd prefer to read a third party who does know enough discussing the results than the original author.
My google foo isn't finding anything.
I really would like that too :(
The problem I see is that it just takes to much time for most people to really dig into this to a point where write something as deep as the original theory is possible. As always, working with fringe sciences will bring you no luck in the scientific community.
Don't get me wrong, I would also take one page that seriously falsifies the theory so I can move on. Would have been nice but isn't, that's OK. I will be sad, but letting theories go is part of development.
I have talked to countless people in person and usually I can quickly answer their questions and it works much better then mediums like this one. Unfortunately, there seem not much coming out of this except getting some fans. I have spent surely months writing emails to physicists, asking them about their opinion. Not a single good answer, some show interest but that's it.
As I also follow more critical views on climate change then IPCC, from my perspective an open mind and theories like this are the last hope for human survival. And don't come me with renewable energies like solar - we increased CO2 output by 2% in 2017 and this is without melting permafrost and other feedback loops we triggered. Most people just don't get it that we are in free fall now, the blue ocean event is right in front of our doorsteps.
To be honest, my perspective is quite dark, especially consider our track record of f*ing things up :(
The problem I see is that it just takes to much time for most people to really dig into this to a point where write something as deep as the original theory is possible. As always, working with fringe sciences will bring you no luck in the scientific community.
Don't get me wrong, I would also take one page that seriously falsifies the theory so I can move on. Would have been nice but isn't, that's OK. I will be sad, but letting theories go is part of development.
I have talked to countless people in person and usually I can quickly answer their questions and it works much better then mediums like this one. Unfortunately, there seem not much coming out of this except getting some fans. I have spent surely months writing emails to physicists, asking them about their opinion. Not a single good answer, some show interest but that's it.
As I also follow more critical views on climate change then IPCC, from my perspective an open mind and theories like this are the last hope for human survival. And don't come me with renewable energies like solar - we increased CO2 output by 2% in 2017 and this is without melting permafrost and other feedback loops we triggered. Most people just don't get it that we are in free fall now, the blue ocean event is right in front of our doorsteps.
To be honest, my perspective is quite dark, especially consider our track record of f*ing things up :(
Your post made me sad and I had to close the computer for a minute.
My hope is in a different place than yours. I don’t think we have a physics problem. We have all the technology we need to live in (roughly) closed carbon systems. It’s a mix of very old tech and some new electronics. There are thousands of people refining the details every day on YouTube. Search “permaculture vlog”.
All we are lacking is 1) a viral teaching tool. And 2) digital tools for distributed governance of shared small resources like some acres of land or a tractor.
But thousands are working feverishly on these issues right now also.
I fear a runaway carbon event as you do. But even in a return to the Cretaceous (likely), permaculture still works. People do it in the desert.
My hope is in a different place than yours. I don’t think we have a physics problem. We have all the technology we need to live in (roughly) closed carbon systems. It’s a mix of very old tech and some new electronics. There are thousands of people refining the details every day on YouTube. Search “permaculture vlog”.
All we are lacking is 1) a viral teaching tool. And 2) digital tools for distributed governance of shared small resources like some acres of land or a tractor.
But thousands are working feverishly on these issues right now also.
I fear a runaway carbon event as you do. But even in a return to the Cretaceous (likely), permaculture still works. People do it in the desert.
Interesting religion you belong to there...
For your big bang theory you have to believe in at least 20 free choose able constants that are perfectly balanced. What about an argument for an exchange ?
What’s the name for the fallacy whereby some casually informed third party is challenged by a rando to explain a complicated concept in detail, whereupon failing to do so the rando’s whacko theory becomes truth?
That's not what I said.
I said that you need at least 20 something parameters to believe in the Big Bang Theory. You can't prove theories, you can only falsify. With every "prove" you just exclude all theories out of the infinite that exist, that do not predict a certain fact.
Date of introduction and parameters you can find here:
https://www.intechopen.com/books/aspects-of-today-s-cosmolog...
I said that you need at least 20 something parameters to believe in the Big Bang Theory. You can't prove theories, you can only falsify. With every "prove" you just exclude all theories out of the infinite that exist, that do not predict a certain fact.
Date of introduction and parameters you can find here:
https://www.intechopen.com/books/aspects-of-today-s-cosmolog...
How many parameters does your theory have?
You don't need parameters when you have galactic crystallization eggs.
This needs to be my new email signature...
s/galactic crystallization egg/God
A German professor I used to take lessons from forwarded me some code sent by some random person he'd never met before.
The email he received said "unification and proof of dark matter and quantum theories through c++ simulation" in German.
To my surprise, the code compiled in clang without warnings, but it basically did nothing. Some functions did some additions and found ratios, results of which were simply ignored.
I felt kinda sad reading the code. Maybe he imagined he was at the verge of a discovery. But these guys are most probably schizophrenic in nature, going back and forth in reality never grasping that achievement they think they are so close to.
The email he received said "unification and proof of dark matter and quantum theories through c++ simulation" in German.
To my surprise, the code compiled in clang without warnings, but it basically did nothing. Some functions did some additions and found ratios, results of which were simply ignored.
I felt kinda sad reading the code. Maybe he imagined he was at the verge of a discovery. But these guys are most probably schizophrenic in nature, going back and forth in reality never grasping that achievement they think they are so close to.
Well, you know when you think everyone else is wrong and you're right - that's the definition of crazy. I'm not insulting you, as text doesn't convey that I say that with a smile, but you should consider that it's far more likely that you are actually the one who's wrong.
Going or not going with the crowd is a poor definition of craziness. Instead, consider the candidate thinking logically (e.g. do they believe P&&!P is true) or critically. Do they have reasonable arguments or do they go in circles, etc...
You're taking me too literally. If I say you're crazy with a smile, I don't mean your mentally ill, I mean your ideas are out of the ordinary.
One thing I can say for certain: I could have never come up with this model. I'm merely a student of it and so far, it was not contradicted like most other models I encountered so far. Models that are not falsifiable, like string theory go directly to the trash bin for me.
Secondly, I know for absolute certainty that most models we believe in, are wrong if you take the combined perceptive into account. There are nutjobs out there how actually believe there is a difference between physics, chemistry and astrophysics. Every model that has paradoxes must have errors.
So, please, falsify it for me, I take logical or mathematical arguments as well as publicly accepted measurements interpreted under this model. Everything else is just bullshit. Just because many people believe something does not make it right, in fact history has shown the opposite (if it concerns natural sciences).
https://www.amazon.com/Basic-Structures-Matter-Supergravitat...
This is the only more or less complete description of the model, if you think it's to expensive, and still want to falsify it (please do), I can give you a pdf. Take into account that you will need a year or so to really grasp it, it's a thick book with quite dense content.
Secondly, I know for absolute certainty that most models we believe in, are wrong if you take the combined perceptive into account. There are nutjobs out there how actually believe there is a difference between physics, chemistry and astrophysics. Every model that has paradoxes must have errors.
So, please, falsify it for me, I take logical or mathematical arguments as well as publicly accepted measurements interpreted under this model. Everything else is just bullshit. Just because many people believe something does not make it right, in fact history has shown the opposite (if it concerns natural sciences).
https://www.amazon.com/Basic-Structures-Matter-Supergravitat...
This is the only more or less complete description of the model, if you think it's to expensive, and still want to falsify it (please do), I can give you a pdf. Take into account that you will need a year or so to really grasp it, it's a thick book with quite dense content.
It's one thing to discount a particular theory, is another all together to discount a huge number of them. Without understanding anything of your model versus the scientific consensus, I can easily say your odds aren't looking good. That's just statistics.
Still, that's exactly the kind of thinking that pushes science forward when it's well reasoned and takes into account all of the observations. Where your model falls on that spectrum I cannot say.
Still, that's exactly the kind of thinking that pushes science forward when it's well reasoned and takes into account all of the observations. Where your model falls on that spectrum I cannot say.
Where can I find out more?
I started with is papers at reasearch gate:
https://www.researchgate.net/profile/Stoyan_Sargoytchev
It triggered my curiosity, but I did not understand the model from it. I understood that the perspective is quite different, but I got many things wrong (my fault). Then I started to read the main book I linked in the other comment.
One advise: Start at the beginning and read linear. After some weeks/months it will start to click and things start to make much more sense then. After that you can jump in the book and it will not be confused. If you just jump into the middle because you are looking for an easy answer for something of your interest, it will just be confusing. One of the reviewer on amazon seem to have done that, that's the 1 star opinion without any quality argument.
There are two possibilities to explain the physical world: complicated or complex (sorry easy answers will not work). This model takes the complex way, nothing is really complicated in this model, but the level of detail is enormous compared to the default perspective. We normally thing the neutrino is small, in this model it's huge.
https://www.researchgate.net/profile/Stoyan_Sargoytchev
It triggered my curiosity, but I did not understand the model from it. I understood that the perspective is quite different, but I got many things wrong (my fault). Then I started to read the main book I linked in the other comment.
One advise: Start at the beginning and read linear. After some weeks/months it will start to click and things start to make much more sense then. After that you can jump in the book and it will not be confused. If you just jump into the middle because you are looking for an easy answer for something of your interest, it will just be confusing. One of the reviewer on amazon seem to have done that, that's the 1 star opinion without any quality argument.
There are two possibilities to explain the physical world: complicated or complex (sorry easy answers will not work). This model takes the complex way, nothing is really complicated in this model, but the level of detail is enormous compared to the default perspective. We normally thing the neutrino is small, in this model it's huge.
> We normally thing the neutrino is small, in this model it's huge.
We have measurements of the size of neutrino (well, for the definition of size that makes sense for such objects)
https://en.wikipedia.org/wiki/Neutrino#Size
We have measurements of the size of neutrino (well, for the definition of size that makes sense for such objects)
https://en.wikipedia.org/wiki/Neutrino#Size
Thanks. It amuses me that somehow that the jury is out on the relationship between general relativity and quantum mechanics – up to the point where the basic primitives of reality are considered unknown, and viable candidates include miniature knots or networks of foam – and yet people are quick to assume the ontological nature of dark matter, a primitive ostensibly responsible for an effect on gravitation.
I dont know much about it, but that may not necessarily be the case. Lets assume dark matter does not exist and the people talking about it believe it does. It could be that the people describing it are only describing it through the lens of what they already believe.
As in, I believe there are invisible blobs that keep cars close together. I observe some cars far apart and i say "hey look at all that missing blobs"
As in, I believe there are invisible blobs that keep cars close together. I observe some cars far apart and i say "hey look at all that missing blobs"
That's missing the point entirely, which is that under theories like MOND, you would not expect to see any galaxies that behave like galaxies with no dark matter, as it posits that dark matter does not exist.
This "galaxy" is so sparse, what if the key to dark matter is some unknown property of the central black hole. Maybe an unknown space-time interaction we have yet to conceive?
Well, then you encounter problems with things like the triangulum Galaxy. Which has dark matter, but no supermassive black hole.
According to my [limited] reading of MOND, it revolves around an unknown function μ(x). That means it could be adjusted to account for this. Which is a handwavium thesis, but that's what I like about it: how long have we been doing science for? Are we so arrogant that we can claim to know nearly everything? What if we simply started from the beginning? That is the essence that MOND captures.
The fact that galaxies form and light is being bent is not evidence: it is the problem. We don't actually have any classical scientific evidence of the stuff. Dark matter is handwavium. It may be real, but it not being is just as plausible. When the Earth was flat and the universe revolved around it, we solved problems with our theories by adding escape clauses to them: epicycles. An effect was explained with an effect, and dark matter fits that's profile perfectly.
MOND could be incorrect, but it is valuable as an alternative line of thought. There needs to be more like it. Science needs to get into the routine of failure once again, we are making no progress without it.
The fact that galaxies form and light is being bent is not evidence: it is the problem. We don't actually have any classical scientific evidence of the stuff. Dark matter is handwavium. It may be real, but it not being is just as plausible. When the Earth was flat and the universe revolved around it, we solved problems with our theories by adding escape clauses to them: epicycles. An effect was explained with an effect, and dark matter fits that's profile perfectly.
MOND could be incorrect, but it is valuable as an alternative line of thought. There needs to be more like it. Science needs to get into the routine of failure once again, we are making no progress without it.
Cosmologists would agree with you. They are looking to define the hand waving they have to do in order to explain the universe we can see. Whatever that explanation is, it must be “shaped” like dark matter, though. This is different than epicycles, which were a specific “solution” to the problem of Keplerian motion. Dark matter is not an explanation, it is an observation in search of an explanation.
Here’s a not-so-bold prediction: sometime this century some team will detect something new that explains some but not all of the effects we currently attribute to dark matter. We will call it the explanation to the problem and the remainder will be given a new catchy name and people on the HN of the future will complain that this thing is simple hand waving.
Here’s a not-so-bold prediction: sometime this century some team will detect something new that explains some but not all of the effects we currently attribute to dark matter. We will call it the explanation to the problem and the remainder will be given a new catchy name and people on the HN of the future will complain that this thing is simple hand waving.
Copernicus also used epicycles - since he assumed orbits to be circular instead of elliptical (as Kepler deduced).
Thoughtful comment, thanks.
> people on the HN of the future will complain that this thing is simple hand waving.
So long as we don't fall into a local minimum in the theory space forever, I'd be happy. Progress is progress, no matter how slow.
> people on the HN of the future will complain that this thing is simple hand waving.
So long as we don't fall into a local minimum in the theory space forever, I'd be happy. Progress is progress, no matter how slow.
Does not appear to have one:
https://en.wikipedia.org/wiki/Triangulum_Galaxy
Nice shot!
https://en.wikipedia.org/wiki/Triangulum_Galaxy
Nice shot!
In fact that article states that the upper limit for the size of a central black hole in the Triangulum galaxy is estimated at 3000 solar masses, so it simply wouldn't be "supermassive" - but there is not a claim that no black hole exists there.
noetic_techy:
> what if the key to dark matter is some unknown property of the central black hole [...] ?
cowboysauce:
> Well, then you encounter problems with things like the triangulum Galaxy. Which has dark matter, but no supermassive black hole.
The initial comment was about a "central" black hole (which may indeed exist), not a "supermassive" black hole.
noetic_techy:
> what if the key to dark matter is some unknown property of the central black hole [...] ?
cowboysauce:
> Well, then you encounter problems with things like the triangulum Galaxy. Which has dark matter, but no supermassive black hole.
The initial comment was about a "central" black hole (which may indeed exist), not a "supermassive" black hole.
I still don't think it's good logic. There is no mechanism explaining why dark matter exists. So not seeing it in this case doesn't shake that theory because it gives no explanation in the first place. MOND does give some explanation so this becomes problematic. But what about a possible new 'dark force' being a possible explanation? Maybe this 'dark force' only radiates under certain circumstances.
I seem to have to qualify my statements with this but I am not attacking the idea of dark matter (nor am I defending MOND) I am just questioning the logic of this one point.
----Edit------ I think it could be confusing what i mean by saying there is no mechanism explaining why dark matter exists. I just mean specifically why. It is a theory that postulates the existence of something but does not give a mechanism to how it gets there. This is not my field of research though so I could be terribly wrong about that.
I seem to have to qualify my statements with this but I am not attacking the idea of dark matter (nor am I defending MOND) I am just questioning the logic of this one point.
----Edit------ I think it could be confusing what i mean by saying there is no mechanism explaining why dark matter exists. I just mean specifically why. It is a theory that postulates the existence of something but does not give a mechanism to how it gets there. This is not my field of research though so I could be terribly wrong about that.
There are indeed many reasonable dark matter candidates: sterile neutrinos, lightest supersymmetric partner, axions... These are particles which are theoretically motivated, but by their nature would be difficult to detect because they don't interact electromagnetically. If we exclude the existence of such particles it is more difficult to construct a coherent theory.
Dark matter is exciting in part because it intersects particle physics with cosmology, and has the potential to solve problems in both (e.g. rotation curves and neutrino masses). It's appealing in part because it's parsimonious.
Dark matter is exciting in part because it intersects particle physics with cosmology, and has the potential to solve problems in both (e.g. rotation curves and neutrino masses). It's appealing in part because it's parsimonious.
> "you would not expect to see any galaxies that behave like galaxies with no dark matter"
you miss my point entirely, your explanation is around an assumption of dark matter. You are not saying this galaxy behaves in such a way that MOND cannot explain, you are saying the galaxy behaves in such a way that based on believing in dark matter, this galaxy moves as if it doesnt have it thus MOND is wrong.
you miss my point entirely, your explanation is around an assumption of dark matter. You are not saying this galaxy behaves in such a way that MOND cannot explain, you are saying the galaxy behaves in such a way that based on believing in dark matter, this galaxy moves as if it doesnt have it thus MOND is wrong.
> "You are not saying this galaxy behaves in such a way that MOND cannot explain"
No, that's exactly what's being stated. MOND doesn't predict this galaxy. This galaxy rotates slower than others of similar directly observable mass. In other words, it confirms the current model of gravity, which by definition is evidence against any modified model of gravity.
No, that's exactly what's being stated. MOND doesn't predict this galaxy. This galaxy rotates slower than others of similar directly observable mass. In other words, it confirms the current model of gravity, which by definition is evidence against any modified model of gravity.
Was any of that said above? No.
GP actually said exactly that, just in a slightly different way:
> "This galaxy discovery, though, appears to be strong evidence against MOND or other modified-gravity theories since the dark matter is missing in this case."
Since MOND assumes that dark matter does not exist (or at least does not interact with gravity), then under MOND this galaxy should not exist. MOND stipulates that all galaxies of a given size should rotate at the same speed, and that speed is greater than that predicted by newtonian mechanics. This galaxy defies MOND predictions, and appears to obey Newton's. Does that make sense or am I missing something else?
> "This galaxy discovery, though, appears to be strong evidence against MOND or other modified-gravity theories since the dark matter is missing in this case."
Since MOND assumes that dark matter does not exist (or at least does not interact with gravity), then under MOND this galaxy should not exist. MOND stipulates that all galaxies of a given size should rotate at the same speed, and that speed is greater than that predicted by newtonian mechanics. This galaxy defies MOND predictions, and appears to obey Newton's. Does that make sense or am I missing something else?
This finding certainly weakens the case for MOND. The authors write that in the conclusion of their paper, as quoted in this article: https://arstechnica.com/science/2018/03/galaxy-seems-to-lack...
MOND can not explain a galaxy without dark matter as it is a proposal for the modification of the laws of gravity at large distances, and can explain the already-observed galaxies that behave as if they have dark matter (or, as MOND posits, the laws of gravity are different than what we have observed on earth.)
MOND can not explain a galaxy without dark matter as it is a proposal for the modification of the laws of gravity at large distances, and can explain the already-observed galaxies that behave as if they have dark matter (or, as MOND posits, the laws of gravity are different than what we have observed on earth.)
@rmcpherson I am not defending MOND or attacking dark matter. I am stating based on what they have said, it doesnt make sense unless they provide some basis on why its problematic for MOND
Consider galaxy 1, from visible matter, we might expect rotation curve A, but we see curve B.
Explanation 1: MOND -- modified gravity at that scale, just works differently than we guessed.
Explanation 2: Dark Matter
Now this is galaxy 2, and from visible matter, we expect rotation curve B, and we get rotation curve A!
So no, if it's just visible matter interacting w/ a modified version of gravity, it should predict consistent rotation curves. But we see two types. Thus it is directly problematic for MOND.
Explanation 1: MOND -- modified gravity at that scale, just works differently than we guessed.
Explanation 2: Dark Matter
Now this is galaxy 2, and from visible matter, we expect rotation curve B, and we get rotation curve A!
So no, if it's just visible matter interacting w/ a modified version of gravity, it should predict consistent rotation curves. But we see two types. Thus it is directly problematic for MOND.
In this case, the theory also predicts that airplanes will need extra thrust when they fly over roads, to pull away from the blobs. The new discovery is a road where that doesn't happen, but the cars still stick together as expected.
Phil Plait's take on it: http://www.syfy.com/syfywire/what-is-this-galaxy-doing-witho...
Man do I feel dumb when these things hit the front page. I managed to make it through a couple "layman's guides to the impenetrable world of theoretical physics," including all variations of "A brief history as time" as well as a noob's introduction to string theory, title of which escapes me.
Still don't fucking get it lol. Between this, AI, and even just the drumset, I wish there was more time in the world to learn all the things I wanna learn :(
Still don't fucking get it lol. Between this, AI, and even just the drumset, I wish there was more time in the world to learn all the things I wanna learn :(
You're not dumb; sometimes you just need something to click, and you never know what that something might be.
Try this: when we look at a galaxy, we can see how fast it's rotating. We can do this using spectrometers that can see where light from stars is red-shifted (which indicates those stars are away from us) or blue-shifted (moving toward us).
The faster a galaxy is rotating, the more massive it must be, because without that gravity the stars would just be flung out into space like toddlers on a malfunctioning carousel.
We can also make a pretty good guess at the amount of "normal" matter in a galaxy (i.e., not dark matter) by measuring how bright a galaxy is, since nearly all light comes from stars, which are, as far as we know, composed entirely of normal matter.
So, putting those two things together, we encounter a mysterious thing: having observed thousands of galaxies, nearly all are rotating much faster than is expected given their estimated mass. One possible explanation for this is that there's another kind of matter in there that doesn't emit any light, which we call dark matter. To be more precise, not only can this dark matter not emit radiation, it cannot interact with radiation at all -- it can't absorb or reflect it. Otherwise we'd see it, just as we see clouds of dust in galaxies (which is accounted for in our mass estimates).
Other proposed explanations involve changing the way gravity works. These are very interesting and compelling ideas, however this newly discovered galaxy would appear to weaken the case for modified gravity theories, because under those theories we would expect this galaxy to be rotating as fast as all other galaxies, since gravity is a function of mass. But that's not what we see; this new galaxy is rotating much slower. It is, in fact, rotating at the rate we would predict if it didn't contain any mysterious invisible "dark" matter. Which is why we think it doesn't!
Try this: when we look at a galaxy, we can see how fast it's rotating. We can do this using spectrometers that can see where light from stars is red-shifted (which indicates those stars are away from us) or blue-shifted (moving toward us).
The faster a galaxy is rotating, the more massive it must be, because without that gravity the stars would just be flung out into space like toddlers on a malfunctioning carousel.
We can also make a pretty good guess at the amount of "normal" matter in a galaxy (i.e., not dark matter) by measuring how bright a galaxy is, since nearly all light comes from stars, which are, as far as we know, composed entirely of normal matter.
So, putting those two things together, we encounter a mysterious thing: having observed thousands of galaxies, nearly all are rotating much faster than is expected given their estimated mass. One possible explanation for this is that there's another kind of matter in there that doesn't emit any light, which we call dark matter. To be more precise, not only can this dark matter not emit radiation, it cannot interact with radiation at all -- it can't absorb or reflect it. Otherwise we'd see it, just as we see clouds of dust in galaxies (which is accounted for in our mass estimates).
Other proposed explanations involve changing the way gravity works. These are very interesting and compelling ideas, however this newly discovered galaxy would appear to weaken the case for modified gravity theories, because under those theories we would expect this galaxy to be rotating as fast as all other galaxies, since gravity is a function of mass. But that's not what we see; this new galaxy is rotating much slower. It is, in fact, rotating at the rate we would predict if it didn't contain any mysterious invisible "dark" matter. Which is why we think it doesn't!
I wish that concise, jargon-free explanations like this one were more common in discussions of theoretical physics and cosmology; thank you for posting.
With you on that but I think it takes a special kind of skill to bridge that gap. This is why I like Hacker News and Reddit, there's always someone who has the time and skills to find a good balance between precise and concise.
That makes a lot of sense! Thank you.
So if whatever a bunch of stars were orbiting weren't massive enough (all the other stars in the galaxy), they wouldn't really even be orbiting, huh? That's what necessitates dark matter - there doesn't appear to be enough matter to justify the amount of mass it would take for all the stars in a given galaxy to actually be in a galaxy style orbit?
So if whatever a bunch of stars were orbiting weren't massive enough (all the other stars in the galaxy), they wouldn't really even be orbiting, huh? That's what necessitates dark matter - there doesn't appear to be enough matter to justify the amount of mass it would take for all the stars in a given galaxy to actually be in a galaxy style orbit?
That's exactly right, and there's really nothing more to it than that, conceptually anyway.
Now, one possible alternative to dark matter is that gravity itself doesn't behave the way we think it does. There is a group of theories that we generally call MOND (MOdified Newtonian Mechanics), which propose that gravity is stronger than we think at very low levels of acceleration. When is acceleration very low? Well, it's low when an object is very far away from any other sources of gravity, and this actually describes most stars in galaxies. If a star isn't too near the center of a galaxy, the gravitational force on it is extremely small -- much smaller, for example, than the pull of the sun on the earth. Most stars are traveling in nearly a straight line, bending only ever so slightly on an enormous circular path. That bending is caused by a teeny tiny acceleration -- a gravitational pull -- toward the center of the galaxy's mass. Which is always more or less right in the middle.
MOND theories propose that as objects get farther away from each other and experience less gravity, the gravity still remains stronger than is predicted by classical newtonian mechanics. Thus, the gravity experienced by most stars in a galaxy is greater than we think, and therefore they must rotate faster for the galaxy to maintain a stable shape.
These are attractive theories IMO, and they explain some things that dark matter has difficulty explaining. However, the galaxy described in the article is a serious problem for MOND, because MOND says that it should be rotating as fast as all other galaxies of its size. But it isn't. It is rotating at the rate predicted by the classic newtonian model of gravity, which would naturally seem to rule out any other modified model of gravity.
Now, one possible alternative to dark matter is that gravity itself doesn't behave the way we think it does. There is a group of theories that we generally call MOND (MOdified Newtonian Mechanics), which propose that gravity is stronger than we think at very low levels of acceleration. When is acceleration very low? Well, it's low when an object is very far away from any other sources of gravity, and this actually describes most stars in galaxies. If a star isn't too near the center of a galaxy, the gravitational force on it is extremely small -- much smaller, for example, than the pull of the sun on the earth. Most stars are traveling in nearly a straight line, bending only ever so slightly on an enormous circular path. That bending is caused by a teeny tiny acceleration -- a gravitational pull -- toward the center of the galaxy's mass. Which is always more or less right in the middle.
MOND theories propose that as objects get farther away from each other and experience less gravity, the gravity still remains stronger than is predicted by classical newtonian mechanics. Thus, the gravity experienced by most stars in a galaxy is greater than we think, and therefore they must rotate faster for the galaxy to maintain a stable shape.
These are attractive theories IMO, and they explain some things that dark matter has difficulty explaining. However, the galaxy described in the article is a serious problem for MOND, because MOND says that it should be rotating as fast as all other galaxies of its size. But it isn't. It is rotating at the rate predicted by the classic newtonian model of gravity, which would naturally seem to rule out any other modified model of gravity.
This is a good explanation, but there's a bit more to be said about MOND. MOND only argues that dark matter is unnecessary, it does not argue against the existence of undetected matter.
If you look at a galaxy, you can measure the amount of visible mass using star luminosity, and you can calculate the total mass of the galaxy under standard cosmology using orbits, radial velocities and so forth. Comparing the visible mass to the calculated mass reveals a huge discrepancy. Some of the undetected matter can be normal, baryonic matter, but there is to much missing matter for it all to be normal matter. So we postulate the existence of an unknown type of matter that interacts gravitationally but not by radiation (light).
Using MOND, when you calculate the the mass of a galaxy and compare it to the visible mass, you still get a discrepancy! This discrepancy is much smaller however. Under standard cosmology it's a factor of 10-100 times, under MOND, the mass discrepancy is usually in the neighborhood of 2. In other words, under MOND, there's still matter in the universe that we can't detect by its light, but the missing mass is small enough that it can be explained by normal matter that's to dim to detect. This is why things like the bullet cluster are actually expected under MOND.
I don't know the particulars of this exact galaxy, but it's possible that the radial velocities that astronomers measured for its stars have a high enough margin of error such that it still fits within MOND's predictions. Something I've noticed is that MOND is still fringe enough that you can't necessarily trust non-MOND physicists when they talk about it. One thing is for sure however, this galaxy is as puzzling for Dark Matter as it is for MOND.
If you look at a galaxy, you can measure the amount of visible mass using star luminosity, and you can calculate the total mass of the galaxy under standard cosmology using orbits, radial velocities and so forth. Comparing the visible mass to the calculated mass reveals a huge discrepancy. Some of the undetected matter can be normal, baryonic matter, but there is to much missing matter for it all to be normal matter. So we postulate the existence of an unknown type of matter that interacts gravitationally but not by radiation (light).
Using MOND, when you calculate the the mass of a galaxy and compare it to the visible mass, you still get a discrepancy! This discrepancy is much smaller however. Under standard cosmology it's a factor of 10-100 times, under MOND, the mass discrepancy is usually in the neighborhood of 2. In other words, under MOND, there's still matter in the universe that we can't detect by its light, but the missing mass is small enough that it can be explained by normal matter that's to dim to detect. This is why things like the bullet cluster are actually expected under MOND.
I don't know the particulars of this exact galaxy, but it's possible that the radial velocities that astronomers measured for its stars have a high enough margin of error such that it still fits within MOND's predictions. Something I've noticed is that MOND is still fringe enough that you can't necessarily trust non-MOND physicists when they talk about it. One thing is for sure however, this galaxy is as puzzling for Dark Matter as it is for MOND.
This is great, thank you.
Things like this in theoretical physics, cosmology, quantum physics, etc. aren't really supposed to "click," are they? My impression is that you just learn the math and the predictions made by the mathematical models, then learn about the observations we've made that fit that model's predictions, and that's really it. "Click" implies to me that you can achieve some sort of "intuitive" grasp of these concepts, perhaps in terms of more familiar everyday phenomena, which from what I've heard isn't the case even for the experts in these fields.
(Theoretical physicist, working in a different area)
"Intuitive" means two different things in this context. One is that, over time, theoreticians certainly gain a lot of intuition about how the relevant math works. That's no substitute for doing the work, but it does cut down on the number of mistakes you make, you often have a decent sense of what's definitely not going to work, etc.
The other meaning is what people usually imply - that some combination of analogies with everyday phenomena finally "clicks" and you "understand" the physics all of a sudden. That's just nonsense. Past the natural limits of our everyday intuitions (which are very narrow indeed, consider how something as trivial as terminal velocity is often unintuitive to students in basic mechanics) physics is applied math, everything else is a crude approximation at best.
"Intuitive" means two different things in this context. One is that, over time, theoreticians certainly gain a lot of intuition about how the relevant math works. That's no substitute for doing the work, but it does cut down on the number of mistakes you make, you often have a decent sense of what's definitely not going to work, etc.
The other meaning is what people usually imply - that some combination of analogies with everyday phenomena finally "clicks" and you "understand" the physics all of a sudden. That's just nonsense. Past the natural limits of our everyday intuitions (which are very narrow indeed, consider how something as trivial as terminal velocity is often unintuitive to students in basic mechanics) physics is applied math, everything else is a crude approximation at best.
Thanks for the insight. I was just going off of what I’ve heard, and it sounds like you mostly agree.
I will say that what can become intuitive to us (in your second sense) can certainly change over time, at least after generations. Netwton’s first law must not have been intuitive, mostly because of the ubiquity of friction in everyday life, but I think it is fairly intuitive now for people with a solid basic science education. Maybe more advanced concepts will get more intuitive over the generations, as science concepts get taught at younger ages.
I will say that what can become intuitive to us (in your second sense) can certainly change over time, at least after generations. Netwton’s first law must not have been intuitive, mostly because of the ubiquity of friction in everyday life, but I think it is fairly intuitive now for people with a solid basic science education. Maybe more advanced concepts will get more intuitive over the generations, as science concepts get taught at younger ages.
[deleted]
Yes, they are.
They click about as much as a well designed but strange piece of code suddenly start to make sense.
They click about as much as a well designed but strange piece of code suddenly start to make sense.
Just try to dedicate some of your time with the basics. It really just takes you putting some time out of your day to understand.
Also feeling dumb is part of the process .
Also feeling dumb is part of the process .
Make sure to watch the video (in 4K) and see just how far away this galaxy is:
http://www.spacetelescope.org/videos/heic1806a/
Mind blowing far.
http://www.spacetelescope.org/videos/heic1806a/
Mind blowing far.
Makes me wonder how they found this one. Did they just point it at one random place and happened to find this galaxy there, or did they have to look around for a long time.
If we take that telescope array and point it at a new random place will we just find another one of these galaxies in short order?
If we take that telescope array and point it at a new random place will we just find another one of these galaxies in short order?
(I'm no astrophysicist, so forgive me if my comment is stupid or naive)
From the Wikipedia article [0] on NGC 1052:
Or do we know what happens when galaxies collide well enough to conclude with reasonable certainty that this is not the case?
[0]: https://en.wikipedia.org/wiki/NGC_1052
From the Wikipedia article [0] on NGC 1052:
NGC 1052 shows also two small jets emerging from its
nucleus as well as a very extended disc of neutral
hydrogen, far larger than the galaxy itself,
all these features suggesting a gas-rich galaxy
collided and merged with it 1 billion years ago
producing all the above features.
Or do we know what happens when galaxies collide well enough to conclude with reasonable certainty that this is not the case?
[0]: https://en.wikipedia.org/wiki/NGC_1052
The point is that the galaxy shows that dark matter exists /independently/ (i.e. as its own particles) of normal matter. This is huge.
Up until now, there was a branch of physics that tried to explain dark matter as a phenomenon that is caused by normal matter. This discovery shows that such an approach is unlikely to work.
For all of this, it does not matter /why/ there is no (not a lot) dark matter in that galaxy. Although that is probably the next question physicists are after!
Up until now, there was a branch of physics that tried to explain dark matter as a phenomenon that is caused by normal matter. This discovery shows that such an approach is unlikely to work.
For all of this, it does not matter /why/ there is no (not a lot) dark matter in that galaxy. Although that is probably the next question physicists are after!
Minor clarification in case someone else similarly misreads your post at first glance:
Dark matter is not normal matter, and does not exist as particles of normal matter. It's completely distinct, and we don't even know what it is made of. This finding gives good evidence that it's actually real, rather than either simply not existing ("gravity behaves strangely at long distances" hypotheses) or just being normal matter that isn't easily detectable, thus contributing to a galaxy's gravity but not what we can see of it.
This new discovery is of a galaxy whose mass does correspond to what we can see, meaning that we aren't missing obvious explanations for other galaxies, which rules out a lot of hypotheses that might otherwise explain the mass that we can't see for other galaxies.
I know you know this, but it's possible to misinterpret your comment.
Dark matter is not normal matter, and does not exist as particles of normal matter. It's completely distinct, and we don't even know what it is made of. This finding gives good evidence that it's actually real, rather than either simply not existing ("gravity behaves strangely at long distances" hypotheses) or just being normal matter that isn't easily detectable, thus contributing to a galaxy's gravity but not what we can see of it.
This new discovery is of a galaxy whose mass does correspond to what we can see, meaning that we aren't missing obvious explanations for other galaxies, which rules out a lot of hypotheses that might otherwise explain the mass that we can't see for other galaxies.
I know you know this, but it's possible to misinterpret your comment.
Dark matter is (probably) pretty damn normal, while we don't know precisely what particles make up the majority of it there's half a dozen particles predicted by the Standard Model (how much more normal than standard can you get?) that are dark.
Normal aka ordinary aka baryonic matter. Dark matter is decidedly not normal. The Standard Model encompasses everything that could theoretically exist, while normal matter (the kind of stuff you and I are made of) is a small subset of that.
That is true of some forms of dark matter (e.g. WIMPs). It is not true of others (e.g. MACHOs) which ARE classical matter, just not visible to telescopes.
This finding rules out most if not all of the MACHO candidates though. There's no way you could strip them away cleanly like this.
This galaxy (NGC 1052-DF2) is very sparse. The size of the Milky Way, but only 0.5% as many stars. So I think there is still some wiggle room for the modified gravity theories here.
> The point is that the galaxy shows that dark matter exists /independently/ (i.e. as its own particles) of normal matter. This is huge.
Didn't the bullet cluster already show that?
Didn't the bullet cluster already show that?
This feels like a leap in logic to me. It seems that it is possible that a 'dark force' could be not operating as strongly in this galaxy as easily as the possibility that dark matter is not being present. Without a mechanism to explain why either one exists, I don't see how this observation could rule one or the other out.
Not sure if this analogy works?
Imagine one group postulated a new type of matter to explain the effects of EM radiation. Another group theorized there was a new force. Then there was observed some charge particles moving at constant velocity and the conclusion was the new matter theory had to be correct since we observed regular matter without the effects of EM radiation.
Not sure if this analogy works?
Imagine one group postulated a new type of matter to explain the effects of EM radiation. Another group theorized there was a new force. Then there was observed some charge particles moving at constant velocity and the conclusion was the new matter theory had to be correct since we observed regular matter without the effects of EM radiation.
Ah, gotcha! That is huge indeed. I appreciate the clarification.
My father has a wonderful old book published in 1896. It's a textbook for University students on physics. What makes it interesting is that the electron was discovered in 1897.
I can't say I've studied the book in detail, but what I did find was something that reminds me a lot of where we are today with dark matter. We understand the problem, and where something ought to fit into it and there's lots of good theories being tried out- but we just don't know what the answer is yet.
I suspect we're writing this generation's version of that book right now, and we call the subject "dark matter".
I can't say I've studied the book in detail, but what I did find was something that reminds me a lot of where we are today with dark matter. We understand the problem, and where something ought to fit into it and there's lots of good theories being tried out- but we just don't know what the answer is yet.
I suspect we're writing this generation's version of that book right now, and we call the subject "dark matter".
I wonder if there are also galaxies that are just dark matter. (Of course, it'd be hard to observe them. Maybe it would gravitationally affect galaxies near it.)
for some definition of "galaxy" - sort of https://news.nationalgeographic.com/news/2012/03/120306-dark...
I think Dragonfly 44 is pretty close to all-dark-matter.
Could it be just because that galaxy has no central black hole? And if so that rises more questions:
Does each galaxy have black hole associated with it? What if black hole is not a "point mass source" (a.k.a. singularity) but rather something with different curvature of space/time that we treat as a dark matter effect?
Does each galaxy have black hole associated with it? What if black hole is not a "point mass source" (a.k.a. singularity) but rather something with different curvature of space/time that we treat as a dark matter effect?
And Dragonfly 44 is only dark matter ...
http://www.wired.co.uk/article/dark-matter-galaxy-dragonfly-...
If there are many galaxy collisions like the Bullet Cluster, which separate dark from visible, then it's not surprising that many such pure galaxies exist, and their creation cannot be unknown (except for the purposes of melodramatic popsci quotes).
http://www.wired.co.uk/article/dark-matter-galaxy-dragonfly-...
If there are many galaxy collisions like the Bullet Cluster, which separate dark from visible, then it's not surprising that many such pure galaxies exist, and their creation cannot be unknown (except for the purposes of melodramatic popsci quotes).
> Merritt remarks: "There is no theory that predicts these types of galaxies [without dark matter] — how you actually go about forming one of these things is completely unknown."
Is there a theory that predicts galaxies with dark matter, or states how they come to be that way? I thought the whole dark matter concept was "we don't know anything about it".
Is there a theory that predicts galaxies with dark matter, or states how they come to be that way? I thought the whole dark matter concept was "we don't know anything about it".
The "dark" in "dark matter" means invisible, not unknowable. Every theory of cosmology predicts dark matter, because if it didn't, it would have been thrown out. That's how the scientific method works.
Any theory of what dark matter is (and there are many) would need to explain how galaxies came to be the way they are. Indeed, explaining anomalies in galaxy behavior is the main reason the idea of “dark matter” came about.
> Is there a theory that predicts galaxies with dark matter, or states how they come to be that way?
Yes, the concordance cosmology (because it concords with all the available evidence), also called the standard cosmology, has an evidence-matching theory of structure formation. The European Space Agency has a good overview at http://www.esa.int/Our_Activities/Space_Science/Planck/Histo...
The evidence for the concordance cosmology comes from terrestrial laboratory physics, tests of general relativity and the standard model at various other places in our solar system, and astrophysical observation. It continues to deliver good predictions (notably about small-scale fluctuations in the cosmic microwave background) but strictly speaking it is an effective theory (it is not complete for the extremely early universe, and it is coarse-grained at length scales larger than galaxies) and it is frequently updated as new information arises, so it's not so much falsifiable (barring falsification of General Relativity, statistical mechanics, thermodynamics, or quantum mechanics, which underpin the cosmology and give it self-consistency) as requiring completion. [ one starting point: https://en.wikipedia.org/wiki/Lambda-CDM_model ]
"CDM" stands for cold dark matter, and its name represents the key features for the (cosmological scale) fluid implied by the behaviour of visible matter: it is [a] moving slowly compared to the speed of light (cold, rather than hot) [b] oblivious to electromagnetism, having and feeling no charge and not decaying to visible matter, and [c] some form of matter rather than an adaptation of General Relativity that amounts to an alternative theory of gravitation.
[a] and [b] also imply that CDM is essentially collisionless, meaning that it (at least mostly) interacts only gravitationally with other matter and itself, which means that it cannot ditch angular momentum that would let it fall into a structure like a galaxy or star (note that normal visible matter -- mostly protons by mass -- can collide, releasing photons and other daughter products, and that lots of collisions make matter hotter and brighter generally, and also more likely to densify into e.g. stars, which are ultimately mostly very hot very frequently colliding very visible matter).
Cold dark matter mostly behaves like heavy neutrinos. Known neutrinos are "hot" because they are so low mass that they are hard to keep from accelerating to speeds comparable to the speed of light, and thus can't keep their (individually low, but collectively less low) mass localized around a galaxy cluster. Neutrinos also interact only via the weak force, so while there are trillions passing through you as you read this, probably none at all will be "felt" by any of the regular matter in your body. But they are matter, and they have been observed in laboratory experiments since 1959, and solar neutrinos have been spotted in detectors in more recent decades.
There are also almost certainly cold regular neutrinos that have redshifted as a relic field, like the cosmic microwave background (a relic field of neutrinos). They will be much much harder to detect because the recoil of atomic nuclei encountering a cold regular neutrino will be much smaller than one encountering a neutrino moving relativistically.
We may at some point be able to doppler-cool neutrino emitting substances, producing cold regular neutrinos in terrestrial laboratories.
So the missing piece is preventing already-cold neutrinos from heating up (extra mass can do that, so can a lower interaction cross-section, e.g. gravitation-only interaction rather than weak force interaction; both can combine). Consequently, the existence of a CDM particle is not much of a stretch, particle physicists are doing what they can to try to find it (and block out interaction cross-section vs mass areas that have been probed without finding a CDM particle), and observational astrophysicists are also looking for evidence from cosmic rays and their sources (e.g. supernovae). There is still a lot of room for a CDM particle to be hiding, and there is a lot of not-so-low-hanging fruit that will have to be probed. :-(
There are quite a few proposed extensions of the standard model (of particle physics) that have suggested places to look first, since a variety of sterile heavy neutrinos could fix some problems specific to the standard model (i.e., not involving gravitation at all). The LHC has killed off a few such particle-physics proposals, but certainly nowhere near all; it could still end up finding a particle (or more than one) that would be a good candidate for at least some of the observationally-inferred dark matter.
Yes, the concordance cosmology (because it concords with all the available evidence), also called the standard cosmology, has an evidence-matching theory of structure formation. The European Space Agency has a good overview at http://www.esa.int/Our_Activities/Space_Science/Planck/Histo...
The evidence for the concordance cosmology comes from terrestrial laboratory physics, tests of general relativity and the standard model at various other places in our solar system, and astrophysical observation. It continues to deliver good predictions (notably about small-scale fluctuations in the cosmic microwave background) but strictly speaking it is an effective theory (it is not complete for the extremely early universe, and it is coarse-grained at length scales larger than galaxies) and it is frequently updated as new information arises, so it's not so much falsifiable (barring falsification of General Relativity, statistical mechanics, thermodynamics, or quantum mechanics, which underpin the cosmology and give it self-consistency) as requiring completion. [ one starting point: https://en.wikipedia.org/wiki/Lambda-CDM_model ]
"CDM" stands for cold dark matter, and its name represents the key features for the (cosmological scale) fluid implied by the behaviour of visible matter: it is [a] moving slowly compared to the speed of light (cold, rather than hot) [b] oblivious to electromagnetism, having and feeling no charge and not decaying to visible matter, and [c] some form of matter rather than an adaptation of General Relativity that amounts to an alternative theory of gravitation.
[a] and [b] also imply that CDM is essentially collisionless, meaning that it (at least mostly) interacts only gravitationally with other matter and itself, which means that it cannot ditch angular momentum that would let it fall into a structure like a galaxy or star (note that normal visible matter -- mostly protons by mass -- can collide, releasing photons and other daughter products, and that lots of collisions make matter hotter and brighter generally, and also more likely to densify into e.g. stars, which are ultimately mostly very hot very frequently colliding very visible matter).
Cold dark matter mostly behaves like heavy neutrinos. Known neutrinos are "hot" because they are so low mass that they are hard to keep from accelerating to speeds comparable to the speed of light, and thus can't keep their (individually low, but collectively less low) mass localized around a galaxy cluster. Neutrinos also interact only via the weak force, so while there are trillions passing through you as you read this, probably none at all will be "felt" by any of the regular matter in your body. But they are matter, and they have been observed in laboratory experiments since 1959, and solar neutrinos have been spotted in detectors in more recent decades.
There are also almost certainly cold regular neutrinos that have redshifted as a relic field, like the cosmic microwave background (a relic field of neutrinos). They will be much much harder to detect because the recoil of atomic nuclei encountering a cold regular neutrino will be much smaller than one encountering a neutrino moving relativistically.
We may at some point be able to doppler-cool neutrino emitting substances, producing cold regular neutrinos in terrestrial laboratories.
So the missing piece is preventing already-cold neutrinos from heating up (extra mass can do that, so can a lower interaction cross-section, e.g. gravitation-only interaction rather than weak force interaction; both can combine). Consequently, the existence of a CDM particle is not much of a stretch, particle physicists are doing what they can to try to find it (and block out interaction cross-section vs mass areas that have been probed without finding a CDM particle), and observational astrophysicists are also looking for evidence from cosmic rays and their sources (e.g. supernovae). There is still a lot of room for a CDM particle to be hiding, and there is a lot of not-so-low-hanging fruit that will have to be probed. :-(
There are quite a few proposed extensions of the standard model (of particle physics) that have suggested places to look first, since a variety of sterile heavy neutrinos could fix some problems specific to the standard model (i.e., not involving gravitation at all). The LHC has killed off a few such particle-physics proposals, but certainly nowhere near all; it could still end up finding a particle (or more than one) that would be a good candidate for at least some of the observationally-inferred dark matter.
https://en.wikipedia.org/wiki/Lambda-CDM_model has this to say about the role of dark matter in the theory, which I find worrying:
> Dark matter is postulated in order to account for gravitational effects observed in very large-scale structures (the "flat" rotation curves of galaxies; the gravitational lensing of light by galaxy clusters; and enhanced clustering of galaxies) that cannot be accounted for by the quantity of observed matter.
(My emphasis.)
This sounds very much like someone wanted a theory that would predict the observed rotation of galaxies, and created one that assumed the existence of dark matter as a way of doing that. (This was also basically my understanding of where the theory of dark matter came from.)
I'm asking about a theory that predicts dark matter, from some other set of assumptions.
Have I understood this right?
> Dark matter is postulated in order to account for gravitational effects observed in very large-scale structures (the "flat" rotation curves of galaxies; the gravitational lensing of light by galaxy clusters; and enhanced clustering of galaxies) that cannot be accounted for by the quantity of observed matter.
(My emphasis.)
This sounds very much like someone wanted a theory that would predict the observed rotation of galaxies, and created one that assumed the existence of dark matter as a way of doing that. (This was also basically my understanding of where the theory of dark matter came from.)
I'm asking about a theory that predicts dark matter, from some other set of assumptions.
Have I understood this right?
I think you're basically right, but I'd encourage you to feel comfortable with it as a normal part of the discovery process.
I would phrase it as "There's something happening that we see only indirectly—by its diffuse gravitational influence. Let's give it a name."
That's happened many times before in science. For example, the measurable perturbation of Mercury's orbit due to special relativity—it was measured before it was explained, and there were multiple competing explanations.
Or, long before that, the causes of disease. What we believe now (invisibly small germs) was laughed at.
I would phrase it as "There's something happening that we see only indirectly—by its diffuse gravitational influence. Let's give it a name."
That's happened many times before in science. For example, the measurable perturbation of Mercury's orbit due to special relativity—it was measured before it was explained, and there were multiple competing explanations.
Or, long before that, the causes of disease. What we believe now (invisibly small germs) was laughed at.
I agree that this is a normal part of the discovery process.
But I don't agree that it constitutes a theory of dark matter, any more than the observation of the precession of Mercury constitutes a theory of general relativity. I was contrasting the claim from the article, "there is no theory that predicts galaxies without dark matter" with the implied claim that there is a theory that predicts galaxies with dark matter.
If the extent of our theory is "we conclude that the galaxies we've observed are permeated by a substance which is massless[1] but nevertheless generates gravitational fields, because their gravity is all messed up", that is not a theory that predicts the nonexistence of galaxies that lack this substance and display normal gravity, nor is it a theory that predicts how a galaxy would acquire any of this weird substance. If it doesn't do those things, what's surprising about the quote I highlighted?
If the dark matter concept is purely observational -- "I'm going to call the weird gravity of this galaxy 'dark matter'" -- it cannot possibly conflict with any other observations. As far as this theory is concerned, it does predict the existence of galaxies without weird gravity, because we observe those, and that is how we've defined "predictions" as generated by this theory. If that sounds stupid to you -- and it does to me -- either we shouldn't be talking about "predictions" or there's something better qualified to be called a "theory". Is there?
[1] I'm using "massless" to refer to the property called "collisionless" in the wikipedia article. Physicists may define mass as the property by which gravitational fields are generated, but I think the ability to collide with other masses is more fundamental to the basic concept.
But I don't agree that it constitutes a theory of dark matter, any more than the observation of the precession of Mercury constitutes a theory of general relativity. I was contrasting the claim from the article, "there is no theory that predicts galaxies without dark matter" with the implied claim that there is a theory that predicts galaxies with dark matter.
If the extent of our theory is "we conclude that the galaxies we've observed are permeated by a substance which is massless[1] but nevertheless generates gravitational fields, because their gravity is all messed up", that is not a theory that predicts the nonexistence of galaxies that lack this substance and display normal gravity, nor is it a theory that predicts how a galaxy would acquire any of this weird substance. If it doesn't do those things, what's surprising about the quote I highlighted?
If the dark matter concept is purely observational -- "I'm going to call the weird gravity of this galaxy 'dark matter'" -- it cannot possibly conflict with any other observations. As far as this theory is concerned, it does predict the existence of galaxies without weird gravity, because we observe those, and that is how we've defined "predictions" as generated by this theory. If that sounds stupid to you -- and it does to me -- either we shouldn't be talking about "predictions" or there's something better qualified to be called a "theory". Is there?
[1] I'm using "massless" to refer to the property called "collisionless" in the wikipedia article. Physicists may define mass as the property by which gravitational fields are generated, but I think the ability to collide with other masses is more fundamental to the basic concept.
Haha, I'm no physicist, so take anything I say with a grain of salt. Also, I don't think I can address everything you wrote, so apologies for picking and choosing. I'll say what I can:
> the ability to collide ... is more fundamental
That's exactly the funny thing—when physicists say "dark matter" they're saying matter that attracts but doesn't collide. They can only detect it indirectly. I think they normally want to agree with you that matter interacts with light and other matter. "Dark matter" would not be their first choice of explanations.
> the observation of the precession of Mercury constitutes a theory of general relativity
I'm going to get pedantic here and separate observation from theory, because I think it helps talk about it. Mercury was observed to precess oddly, and nobody knew why. For quite a while, the theory was that an unseen planet was doing it. So I'd phrase it only slightly differently:
> the observation of the precession of Mercury constitutes evidence for a theory of general relativity
I draw the distinction because it's central to the debate about dark matter. Weird gravity isn't a theory at all—it's an observation. And it's one physicists pretty much agree on—galaxies are definitely acting weird, with respect to their visible mass.
One possible theory to explain it is dark matter. Another is MOND. Any of them could be true—it's kind of probabilistic. For some physicists this new observation is shifting the probabilities in favor of dark matter, but it's obviously still flawed because we can't really explain dark because we have no positive confirmation of its existence outside of gravity. We'd love to find a WIMP, for example, but we haven't.
> the ability to collide ... is more fundamental
That's exactly the funny thing—when physicists say "dark matter" they're saying matter that attracts but doesn't collide. They can only detect it indirectly. I think they normally want to agree with you that matter interacts with light and other matter. "Dark matter" would not be their first choice of explanations.
> the observation of the precession of Mercury constitutes a theory of general relativity
I'm going to get pedantic here and separate observation from theory, because I think it helps talk about it. Mercury was observed to precess oddly, and nobody knew why. For quite a while, the theory was that an unseen planet was doing it. So I'd phrase it only slightly differently:
> the observation of the precession of Mercury constitutes evidence for a theory of general relativity
I draw the distinction because it's central to the debate about dark matter. Weird gravity isn't a theory at all—it's an observation. And it's one physicists pretty much agree on—galaxies are definitely acting weird, with respect to their visible mass.
One possible theory to explain it is dark matter. Another is MOND. Any of them could be true—it's kind of probabilistic. For some physicists this new observation is shifting the probabilities in favor of dark matter, but it's obviously still flawed because we can't really explain dark because we have no positive confirmation of its existence outside of gravity. We'd love to find a WIMP, for example, but we haven't.
Er, I don't really understand your objection. In \Lambda-CDM General Relativity is also postulated, as are general covariance and gauge theory. Each of these in turn rest on other postulates. All of which are testable within some limit, many of which have already been confirmed to exquisite precision. If compelling evidence arises that is incompatible with any of these postulates, \Lambda-CDM would cease to be a concordance cosmology.
> someone ... created [a theory] that assumed ... dark matter
Yes, that's what theorists do. Then their theories must confront evidence from direct experiment and astrophysical observation. Most theories fail very hard and very quickly; there are an awful lot of different lines of evidence from modern experimental physics and astronomy, and all of them have to be met in a concordance cosmology.
Indeed, the article at the top highlights that a "hm, that's odd, I can't explain it if the observations hold up" will disfavour a family of serious alternatives to \Lambda-CDM. \Lambda-CDM is just fine with galaxies having substantial underdensities and overdensities of dark matter, since the coupling between CDM and luminous matter is very weak. More galaxies like the one in the article, and indeed their opposites where there is a lensing event with surprisingly little luminous matter, would be more evidence supporting CDM.
On the other hand, a cosmology which does not postulate General Relativity and instead proposes a modified theory of gravitation wherein gravitational interactions are sourced exclusively by luminous matter (and the six non-luminous matter particles already in the Standard Model) cannot be a concordance cosmology in the face of a zoo of galaxies like the one in the article.
> a theory that predicts dark matter from some other set of assumptions
Cold Dark Matter was driven by a set of large scale observations starting in the 1960s. Other lines of evidence, starting in the 1990s (BOOMERaNG experiment) also began to demand it at wholly different scales. What was not demanded was any particular microscropic description -- CDM was an "in the large" matter field with some particular characteristics in the Friedmann-Lemaître-Robertson-Walker model, and could in principle be a large mix of types of non-luminous matter including a large fraction of merely hard-to-see isolated Jupiter-esque objects made of ordinary Standard Model particles.
Entirely separately, the application of gauge theory to solve problems in the Standard Model -- wholly unconnected to the gravitational sector -- suggested the addition of extra particles, several of which could in large number fulfil the large-scale requirements of Cold Dark Matter. "Oh, neat, our proposal for a sterile neutrino or a lowest-mass superpartner, or an axion can behave like Cold Dark Matter, let's look for astrophysical evidence of our proposed particle, as well as evidence from laboratory experiments".
One can consider the reverse: neutrinos were proposed before much was known about galaxies (and even directly detected years before galaxy rotation curves were studied by Rubin et al.). Lots and lots and lots of them are produced in common astrophysical processes like stellar nucleosynthesis, and thus would have to enter into the total energy density of a cosmological model (and they do, as Hot Dark Matter, as a component of the \Lambda-CDM parameter \Omega_rad). Lots and lots and lots of them are also expected in cosmological nucleosynthesis (baryogenesis), and so also have to enter into the total energy density (and they do, as relic neutrinos of the Cosmic Neutrino Background, parameterized as \Omega_\nu). As we discover more about the microscopic details of the total energy-density, more components of the total \Omega are likely to be added. We are in serious trouble if we have to remove an existing component, however, or if we cannot resolve conflicting evidence for the total energy density \Omega_tot.
A modified gravity theory that produces a cosmology without \Omega_c h^2 ~ 0.12 (the parameter and value representing dark matter density) will tend to struggle with the lines of evidence other than galaxy rotation curves that support the current value.
> someone ... created [a theory] that assumed ... dark matter
Yes, that's what theorists do. Then their theories must confront evidence from direct experiment and astrophysical observation. Most theories fail very hard and very quickly; there are an awful lot of different lines of evidence from modern experimental physics and astronomy, and all of them have to be met in a concordance cosmology.
Indeed, the article at the top highlights that a "hm, that's odd, I can't explain it if the observations hold up" will disfavour a family of serious alternatives to \Lambda-CDM. \Lambda-CDM is just fine with galaxies having substantial underdensities and overdensities of dark matter, since the coupling between CDM and luminous matter is very weak. More galaxies like the one in the article, and indeed their opposites where there is a lensing event with surprisingly little luminous matter, would be more evidence supporting CDM.
On the other hand, a cosmology which does not postulate General Relativity and instead proposes a modified theory of gravitation wherein gravitational interactions are sourced exclusively by luminous matter (and the six non-luminous matter particles already in the Standard Model) cannot be a concordance cosmology in the face of a zoo of galaxies like the one in the article.
> a theory that predicts dark matter from some other set of assumptions
Cold Dark Matter was driven by a set of large scale observations starting in the 1960s. Other lines of evidence, starting in the 1990s (BOOMERaNG experiment) also began to demand it at wholly different scales. What was not demanded was any particular microscropic description -- CDM was an "in the large" matter field with some particular characteristics in the Friedmann-Lemaître-Robertson-Walker model, and could in principle be a large mix of types of non-luminous matter including a large fraction of merely hard-to-see isolated Jupiter-esque objects made of ordinary Standard Model particles.
Entirely separately, the application of gauge theory to solve problems in the Standard Model -- wholly unconnected to the gravitational sector -- suggested the addition of extra particles, several of which could in large number fulfil the large-scale requirements of Cold Dark Matter. "Oh, neat, our proposal for a sterile neutrino or a lowest-mass superpartner, or an axion can behave like Cold Dark Matter, let's look for astrophysical evidence of our proposed particle, as well as evidence from laboratory experiments".
One can consider the reverse: neutrinos were proposed before much was known about galaxies (and even directly detected years before galaxy rotation curves were studied by Rubin et al.). Lots and lots and lots of them are produced in common astrophysical processes like stellar nucleosynthesis, and thus would have to enter into the total energy density of a cosmological model (and they do, as Hot Dark Matter, as a component of the \Lambda-CDM parameter \Omega_rad). Lots and lots and lots of them are also expected in cosmological nucleosynthesis (baryogenesis), and so also have to enter into the total energy density (and they do, as relic neutrinos of the Cosmic Neutrino Background, parameterized as \Omega_\nu). As we discover more about the microscopic details of the total energy-density, more components of the total \Omega are likely to be added. We are in serious trouble if we have to remove an existing component, however, or if we cannot resolve conflicting evidence for the total energy density \Omega_tot.
A modified gravity theory that produces a cosmology without \Omega_c h^2 ~ 0.12 (the parameter and value representing dark matter density) will tend to struggle with the lines of evidence other than galaxy rotation curves that support the current value.
I'm no cosmologist, but hasn't the effort to build detectors for these "heavy neutrino's" failed thus far? How much low hanging fruit is actually left?
https://en.wikipedia.org/wiki/Large_Underground_Xenon_experi...
https://en.wikipedia.org/wiki/Large_Underground_Xenon_experi...
It took twenty-five years to go from the solidification of the theory of a neutrino to account for missing momentum in beta decays to direct detection of the neutrino, and along the way atomic reactors and nuclear bombs were engaged in the effort.
The problem is that detection relies upon the details of the recoil of atomic nuclei, and the recoil is much smaller for cold neutrinos than relativistic neutrinos, so we have detected pretty much exactly zero Cosmic Neutrino Background neutrinos directly, although there is plenty of indirect evidence from astrophysical observation. The interaction cross section of a "heavy neutrino"-like particle might be smaller than that of a neutrino of the same temperature (it might not feel the weak force at all, so caeteris paribus it might be many orders of magnitude less detectable just because the common interaction is purely gravitational), so not only is such a Cold Dark Matter candidate harder to detect (because cold ~ low recoil) but the interaction cross-section might be even smaller than that between the neutrino and a big atomic nucleus.
> how much low hanging fruit is actually left
There's no real way to answer that rigorously, as it really depends on what one means by "low hanging". Above neutrino floor? Below detector volumes in low numbers of cubic metres? Somewhere within the reach of existing technology at budgets less than the LHC? Somewhere within the reach of technology likely available in twenty years at budgets (adjusted for inflation and economic conditions) less than Super-Kamiokande?
The problem is that detection relies upon the details of the recoil of atomic nuclei, and the recoil is much smaller for cold neutrinos than relativistic neutrinos, so we have detected pretty much exactly zero Cosmic Neutrino Background neutrinos directly, although there is plenty of indirect evidence from astrophysical observation. The interaction cross section of a "heavy neutrino"-like particle might be smaller than that of a neutrino of the same temperature (it might not feel the weak force at all, so caeteris paribus it might be many orders of magnitude less detectable just because the common interaction is purely gravitational), so not only is such a Cold Dark Matter candidate harder to detect (because cold ~ low recoil) but the interaction cross-section might be even smaller than that between the neutrino and a big atomic nucleus.
> how much low hanging fruit is actually left
There's no real way to answer that rigorously, as it really depends on what one means by "low hanging". Above neutrino floor? Below detector volumes in low numbers of cubic metres? Somewhere within the reach of existing technology at budgets less than the LHC? Somewhere within the reach of technology likely available in twenty years at budgets (adjusted for inflation and economic conditions) less than Super-Kamiokande?
Interesting commentary. Are you a cosmologist?
> although there is plenty of indirect evidence from astrophysical observation
I'd be curious to see this if you have any links.
> it really depends on what one means by "low hanging". Above neutrino floor?
After reviewing this and neutrino masses, I see your point. Its hard to say.
> although there is plenty of indirect evidence from astrophysical observation
I'd be curious to see this if you have any links.
> it really depends on what one means by "low hanging". Above neutrino floor?
After reviewing this and neutrino masses, I see your point. Its hard to say.
Ah, sorry, wires crossed in my head. I think you were asking for information about cosmic neutrino background detectability versus astrophysical/indirect evidence, right?
On the latter: https://arxiv.org/abs/1412.5948
On direct detection: KATRIN collaboration: https://arxiv.org/abs/1602.03347
KATRIN is pretty cool. Pretty pictures of some of the apparatus: http://www.dw.com/en/the-katrin-tritium-neutrino-experiment-...
But KATRIN is to detect warm neutrinos from a known source. As noted in the collab's paper (preprint link above), detecting ambient cold neutrinos would require a different, and larger, apparatus.
We may need something much bigger even than that for direct neutrino-like cold dark matter detection. :-( So that gives you a sense of physical scale of what "low hanging fruit" might entail.
ETA: also wikipedia is reasonable on this point
https://en.wikipedia.org/wiki/Cosmic_neutrino_background?old...
On the latter: https://arxiv.org/abs/1412.5948
On direct detection: KATRIN collaboration: https://arxiv.org/abs/1602.03347
KATRIN is pretty cool. Pretty pictures of some of the apparatus: http://www.dw.com/en/the-katrin-tritium-neutrino-experiment-...
But KATRIN is to detect warm neutrinos from a known source. As noted in the collab's paper (preprint link above), detecting ambient cold neutrinos would require a different, and larger, apparatus.
We may need something much bigger even than that for direct neutrino-like cold dark matter detection. :-( So that gives you a sense of physical scale of what "low hanging fruit" might entail.
ETA: also wikipedia is reasonable on this point
https://en.wikipedia.org/wiki/Cosmic_neutrino_background?old...
> if you have any links
D'Amico et al.'s course notes, section 2: https://arxiv.org/abs/0907.1912
And from TASI (Theoretical Advanced Study Institute in Elementary Particle Physics) 2016, section III: https://arxiv.org/abs/1710.05137
D'Amico et al.'s course notes, section 2: https://arxiv.org/abs/0907.1912
And from TASI (Theoretical Advanced Study Institute in Elementary Particle Physics) 2016, section III: https://arxiv.org/abs/1710.05137
Here is the article referenced. The link on spacetelescope isn't working. http://imgsrc.hubblesite.org/hvi/uploads/science_paper/file_...
Main message:
“Although counterintuitive, the existence of a galaxy without dark matter negates theories that try to explain the Universe without dark matter being a part of it [3]: The discovery of NGC 1052-DF2 demonstrates that dark matter is somehow separable from galaxies. This is only expected if dark matter is bound to ordinary matter through nothing but gravity.”
“Although counterintuitive, the existence of a galaxy without dark matter negates theories that try to explain the Universe without dark matter being a part of it [3]: The discovery of NGC 1052-DF2 demonstrates that dark matter is somehow separable from galaxies. This is only expected if dark matter is bound to ordinary matter through nothing but gravity.”
That does seem fairly convincing, though it also highlights the lack of precise theories about dark matter: what is it, and how did it get there? Hopefully these are answerable questions some day.
The theory is as precise as we can observe it, the match is quite good compared to the alternatives:
https://www.duo.uio.no/bitstream/handle/10852/52374/FinalRep...
https://www.duo.uio.no/bitstream/handle/10852/52374/FinalRep...
How do we know that gravity is constant in proportion to mass and not a function of time? General relativity tells us mass slows down time, but we assume slowing down action doesn't affect gravity. If gravitons, or other quanta of gravity, exists wouldn't general relativity infer that massive objects have less gravity in proportion to mass than less massive objects?
We know from general relativity that massive galactic cores have less time in proportion to their mass compared with galactic arms, which have more time (action) relative to their mass.
Why wouldn't general relativity's slow down in time also mean less gravity in proportion to mass?
We know from general relativity that massive galactic cores have less time in proportion to their mass compared with galactic arms, which have more time (action) relative to their mass.
Why wouldn't general relativity's slow down in time also mean less gravity in proportion to mass?
Well, if the strength of gravity was linear in redshift, astronomers would have detected it decades ago.
Apologies if this is naive, I was looking for a free book for a flight a week ago and I downloaded 'The Einstein theory of relativity; a concise statement' by H A Lorentz, 1920, available here https://archive.org/details/einsteintheoryr00einsgoog as well.
On page 22, Lorentz states a 3rd experimental test for the theory, "If his (Einstein) theory is correct as it stands, there ought, in a gravitational field, to be a displacement of the lines of the spectrum towards the red." He adds, "No such effect has been discovered. ... there is no way of accounting for this failure if Einstein's theory in its present form is assumed."
He goes on to say that some modification would be necessary. What eventually happened to this 3rd test?
On page 22, Lorentz states a 3rd experimental test for the theory, "If his (Einstein) theory is correct as it stands, there ought, in a gravitational field, to be a displacement of the lines of the spectrum towards the red." He adds, "No such effect has been discovered. ... there is no way of accounting for this failure if Einstein's theory in its present form is assumed."
He goes on to say that some modification would be necessary. What eventually happened to this 3rd test?
Here's the wikipedia page for the test you're talking about:
https://en.wikipedia.org/wiki/Gravitational_redshift
>The first accurate measurement of the gravitational redshift of a white dwarf was done by Popper in 1954, measuring a 21 km/sec gravitational redshift of 40 Eridani B.
The effect has been seen in experiments. (See the section on experimental verification.)
>The first accurate measurement of the gravitational redshift of a white dwarf was done by Popper in 1954, measuring a 21 km/sec gravitational redshift of 40 Eridani B.
The effect has been seen in experiments. (See the section on experimental verification.)
[deleted]
I wouldn't expect gravity to be linear, but exponential just like time in general relativity.
Does the galaxy spin correctly according to our current understanding of gravity?
Looks like. So this would be the first galaxy that actually seems to rotate following the laws of gravity as we know them.
The data on the 10 globular clusters the team tracked showed them moving much more slowly than would be expected. That led to an estimated mass that was extremely low for a galaxy—on the order of 10^8 solar masses. Using the amount of light emitted by the galaxy produced an estimate of the total mass of stars in the galaxy that was also in the neighborhood of 10^8. Normally, we infer that there's dark matter around because the galaxy appears to have a lot more matter than the amount provided by the stars we can see. But in this case, there's a minimal difference between the two.
https://arstechnica.com/science/2018/03/galaxy-seems-to-lack...
Edit: 10^8, not 108.
The data on the 10 globular clusters the team tracked showed them moving much more slowly than would be expected. That led to an estimated mass that was extremely low for a galaxy—on the order of 10^8 solar masses. Using the amount of light emitted by the galaxy produced an estimate of the total mass of stars in the galaxy that was also in the neighborhood of 10^8. Normally, we infer that there's dark matter around because the galaxy appears to have a lot more matter than the amount provided by the stars we can see. But in this case, there's a minimal difference between the two.
https://arstechnica.com/science/2018/03/galaxy-seems-to-lack...
Edit: 10^8, not 108.
First of all, this is super cool but I'm confused about these sentences:
>The discovery of NGC 1052-DF2 demonstrates that dark matter is somehow separable from galaxies. This is only expected if dark matter is bound to ordinary matter through nothing but gravity.
Why is this the only explanation? Was this a prediction of dark matter theorists that they will find galaxies without any dark matter and that proves it only interacts via gravity?
Lately, I've pondered that the observed behavior dark matter explains could be the result of a new force that only has an appreciable effect in regimes of extremely high mass/energy. Or possibly gravity is more complex than predicted by GR. Just like newtonian gravity needed to be extended by GR possibly GR still needs further extension to account for unexpected behavior in these extreme mass/energy regimes? If all the forces are ultimately just one force than these two ideas I've been thinking about are essentially the same.
I would assume there has been some work exploring this avenue of thought? Has it been fruitless?
>The discovery of NGC 1052-DF2 demonstrates that dark matter is somehow separable from galaxies. This is only expected if dark matter is bound to ordinary matter through nothing but gravity.
Why is this the only explanation? Was this a prediction of dark matter theorists that they will find galaxies without any dark matter and that proves it only interacts via gravity?
Lately, I've pondered that the observed behavior dark matter explains could be the result of a new force that only has an appreciable effect in regimes of extremely high mass/energy. Or possibly gravity is more complex than predicted by GR. Just like newtonian gravity needed to be extended by GR possibly GR still needs further extension to account for unexpected behavior in these extreme mass/energy regimes? If all the forces are ultimately just one force than these two ideas I've been thinking about are essentially the same.
I would assume there has been some work exploring this avenue of thought? Has it been fruitless?
The idea of modifying general relativity is called as MOND and has been thoroughly investigated by physicists. There are still proponents, but I think the models are not matching the observational data. Also, the fact that Galaxies with similar (visible) masses have different levels of rotational momentum seems to hint that dark matter is a thing. I don't understand why people think having something that interacts with fewer forces than normal matter is more complicated than Jerry rigging a change to the elegant general relativity.
MOND is only one theory in a whole class of modified gravity theories:
https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics
https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics
Dark matter is easily the most popular theory for this observed phenomena, so clearly most people do NOT think having "something that interacts with fewer forces than normal matter is more complicated than Jerry rigging a change to the elegant general relativity."
I'm not sure if you are saying that I think one is more complicated than the other because I do not. I'm simply wondering about ideas and asking questions. Surely we should still ask questions, right?
I'm not sure if you are saying that I think one is more complicated than the other because I do not. I'm simply wondering about ideas and asking questions. Surely we should still ask questions, right?
I'm not a physicist, but my laymans understanding is that it's fairly easy to account for some cherry-picked observations with modified gravity, but to account for all of them requires something at least as contrived as dark matter. So the evidence is still pointing in that direction for now.
Keep in mind that each Galaxy must have its own curve-fit distribution of dark matter whereas alternative gravity theories fit all galaxies using a single free parameter, which makes it en face a more appealing model.
Plus to me dark matter feels like a hack to make the math work. It feels inelegant. But that doesn't make it wrong - nature doesn't care what I like or don't like.
There are physicists working on the problem, yes:
https://en.wikipedia.org/wiki/Alternatives_to_general_relati...
https://en.wikipedia.org/wiki/Alternatives_to_general_relati...
Thanks for googling for me. :)
>"Based on these data the team discovered that NGC 1052-DF2 larger than the Milky Way, but contains about 250 times fewer stars, leading it to be classified as an ultra diffuse galaxy."
Interesting, the mass discrepancy (ie, "dark matter") has been noted to be proportional to the predicted acceleration due to the visible mass (according to Newtonian mechanics): https://arxiv.org/abs/astro-ph/0403610
How does this galaxy fit into that relationship? If the galaxy is ultra diffuse it sounds like very little deviation would be predicted, but I am no expert here.
Edit:
To clarify since the relationship is somewhat tortured: According to my understanding this observation sounds surprising in light of dark matter, but 100% consistent with what MOND proponents have been saying. I'd love to hear from someone who knows more.
Interesting, the mass discrepancy (ie, "dark matter") has been noted to be proportional to the predicted acceleration due to the visible mass (according to Newtonian mechanics): https://arxiv.org/abs/astro-ph/0403610
How does this galaxy fit into that relationship? If the galaxy is ultra diffuse it sounds like very little deviation would be predicted, but I am no expert here.
Edit:
To clarify since the relationship is somewhat tortured: According to my understanding this observation sounds surprising in light of dark matter, but 100% consistent with what MOND proponents have been saying. I'd love to hear from someone who knows more.
Honestly, MOND proponents seem to be saying everything and its inverse.
What is not really a complaint, because they should be looking everywhere. But yes, that will invalidate some MOND theories, while keeping other ones unharmed, and will be evidence of some other set of them. And I don't think anybody should be surprised by any of that.
(But I'm no expert either, so if one wants to correct me, it will be welcome.)
What is not really a complaint, because they should be looking everywhere. But yes, that will invalidate some MOND theories, while keeping other ones unharmed, and will be evidence of some other set of them. And I don't think anybody should be surprised by any of that.
(But I'm no expert either, so if one wants to correct me, it will be welcome.)
I could be a MOND proponent only in that "dark matter" looks exactly like epicycles to me. I make no claims to understand the details of MOND, so please do not blame them for any of my errors.
That said, what do you mean by "MOND proponents seem to be saying everything and its inverse"?
That said, what do you mean by "MOND proponents seem to be saying everything and its inverse"?
There are many MOND theories, and you will find some that fit almost any observation you can imagine.
The paper considers the class of MOND theories that made a precise prediction of deltaV = 20 km/s for this galaxy. I guess I am referring to only those.
I see the paper says:
>"[MOND predicts] the expected velocity dispersion of NGC1052–DF2 is ... 20 km s−1, a factor of two higher than the 90% upper limit on the observed dispersion." http://imgsrc.hubblesite.org/hvi/uploads/science_paper/file_...
So they chose to use a 90% CI along with assuming the error is normally distributed, etc. Looking at figure 3b it seems that if they instead arbitrarily chose to use a 95% CI that 20 km/s would be within the interval.
Ie, these results appear to be moderately unlikely if MOND were correct but extremely unlikely if "dark matter" is correct. Thus, according to Bayes' rule, the probability that MOND is correct has increased.
>"[MOND predicts] the expected velocity dispersion of NGC1052–DF2 is ... 20 km s−1, a factor of two higher than the 90% upper limit on the observed dispersion." http://imgsrc.hubblesite.org/hvi/uploads/science_paper/file_...
So they chose to use a 90% CI along with assuming the error is normally distributed, etc. Looking at figure 3b it seems that if they instead arbitrarily chose to use a 95% CI that 20 km/s would be within the interval.
Ie, these results appear to be moderately unlikely if MOND were correct but extremely unlikely if "dark matter" is correct. Thus, according to Bayes' rule, the probability that MOND is correct has increased.
Not sure where the downvotes are coming from. The reasoning that this evidence is moderately unlikely given that MOND is true can be found in the parent post. The authors worked it out to ~1/10 to 1/20.
They don't put a number on how unlikely it would be given dark matter is true, but it sounds pretty extreme:
>Merritt remarks: "There is no theory that predicts these types of galaxies — how you actually go about forming one of these things is completely unknown." http://www.spacetelescope.org/news/heic1806/
>"And NGC 1052-DF2 appears to have none. Or at best, about as much dark matter as normal matter. And that just doesn't make sense." http://www.syfy.com/syfywire/what-is-this-galaxy-doing-witho...
The denominator of Bayes rule for Pr(MOND|data) is just the sum of all Pr(Theory) x Pr(data|Theory) where the main theory is "dark matter" and second is MOND (everything else is some small value epsilon), while the numerator is Pr(MOND) x Pr(data|MOND).
Where is the mistake?
They don't put a number on how unlikely it would be given dark matter is true, but it sounds pretty extreme:
>Merritt remarks: "There is no theory that predicts these types of galaxies — how you actually go about forming one of these things is completely unknown." http://www.spacetelescope.org/news/heic1806/
>"And NGC 1052-DF2 appears to have none. Or at best, about as much dark matter as normal matter. And that just doesn't make sense." http://www.syfy.com/syfywire/what-is-this-galaxy-doing-witho...
The denominator of Bayes rule for Pr(MOND|data) is just the sum of all Pr(Theory) x Pr(data|Theory) where the main theory is "dark matter" and second is MOND (everything else is some small value epsilon), while the numerator is Pr(MOND) x Pr(data|MOND).
Where is the mistake?
> missing most — if not all — of its dark matter
Is it just me or does this seem a bit like saying a landline is a wired cell phone? I thought dark matter represented what's missing, so if it's not there then nothing is missing, right?
Is it just me or does this seem a bit like saying a landline is a wired cell phone? I thought dark matter represented what's missing, so if it's not there then nothing is missing, right?
We know that dark matter makes up a large fraction of the universe's overall mass. And we observe dark matter (indirectly) in just about every galaxy we look at. This makes sense, since we believe that galaxies are formed when local concentrations of matter collapse under their own gravity; you'd expect this process to affect both types of matter the same way. When we see a galaxy with no dark matter, it suggests that something unusual is going on, in a way that affects dark matter differently.
> We know that dark matter makes up a large fraction of the universe's overall mass.
Do we?
I thought it was that we know a significant amount of mass is missing from our observations and we explain it with dark matter.
Dark matter to me is a crutch to explain things until we really know.
Is it not at all possible our understanding is just wrong and that nothing is missing?
Do we?
I thought it was that we know a significant amount of mass is missing from our observations and we explain it with dark matter.
Dark matter to me is a crutch to explain things until we really know.
Is it not at all possible our understanding is just wrong and that nothing is missing?
It's possible, but I think it's worthwhile to examine the intuitions that so many people have that leads them to dismiss the notion of dark matter, or at least the more "exotic" sounding theories like WIMPs.
We know that every particle we've been able to track down and verify the existence of responds to the gravitational force. We also know that some particles (like neutrinos) have virtually no interaction with anything other than gravitational force, to the point where the vast majority of them will pass through the entire earth undetected and having virtually no effect on anything or anyone.
The notion of particles that are even harder to detect than neutrinos doesn't seem that absurd. It's kind of anthropocentric to assume that most of the mass in the universe is going to be the same kind of mass that we are, or that we are at least capable of easily observing.
And think about what it would imply if there were no dark matter. It would almost imply that, as of the late 20th century, humanity finally had a basic comprehension of all of material existence.
The universe that humanity understood before Copernicus and Galileo is less than 1% of what we can see now. If our math is right and there really is enough dark matter and dark energy to throw off our models, then the universe we can see now is about 5% of everything that exists. That seems pretty plausible for a civilization that can barely get out of its own atmosphere.
We know that every particle we've been able to track down and verify the existence of responds to the gravitational force. We also know that some particles (like neutrinos) have virtually no interaction with anything other than gravitational force, to the point where the vast majority of them will pass through the entire earth undetected and having virtually no effect on anything or anyone.
The notion of particles that are even harder to detect than neutrinos doesn't seem that absurd. It's kind of anthropocentric to assume that most of the mass in the universe is going to be the same kind of mass that we are, or that we are at least capable of easily observing.
And think about what it would imply if there were no dark matter. It would almost imply that, as of the late 20th century, humanity finally had a basic comprehension of all of material existence.
The universe that humanity understood before Copernicus and Galileo is less than 1% of what we can see now. If our math is right and there really is enough dark matter and dark energy to throw off our models, then the universe we can see now is about 5% of everything that exists. That seems pretty plausible for a civilization that can barely get out of its own atmosphere.
Sure, it's just a lot more likely that it's stuff we can't see than our fundamental understanding of a loooot of other things is so incredibly wrong while still being very accurate in practice. I'm sure there are people putting effort into trying to reconcile things in that direction though.
I mean, the short answer is that, of course dark matter is a hack and our understanding of the universe could be wildly wrong or grossly incomplete.
The longer answer is that dark matter isn't particularly special or unique in this. We make observations, and where they don't match our expectations, we hypothesize new things to fill the gaps. Sometimes they're borne out by later observations, sometimes not.
We've hypothesized the existence of planets to explain deviations in planets' orbits. Sometimes the hypothesized planets existed, sometimes they didn't. In the early days of chemistry, we came up with a whole bunch of elements; sometimes they turned out to be compounds that were particularly difficult to break down. We came up a whole host of particles and subparticles, and the ones that we still think are real aren't necessarily the ones you would have guessed at the time they were proposed. If you pick up an old text book on quantum mechanics, you'll find they keep referring to it as "the new quantum mechanics", not just because it was being nailed down at the same time the text books were written but to distinguish it from "the old quantum mechanics" from twenty years earlier that it was replacing.
Sometimes the hacky models are the best we have to offer; most people will call the Standard Model of particle physics just a hacky description of the observed phenomenon, but all the elegant models to replace it haven't really paid off.
It's possible that dark matter will go the way of luminiferous aether, but so far all the observations astrophysicists are making really jive well with the idea of dark matter, so for now it gets to stay in the "weird things about the universe many people don't like" category instead of the "stupid ideas no one can believe that people used to believe" category. I mean, it makes me angry that a neutron isn't just an electron and proton mashed together, but them's the breaks.
The longer answer is that dark matter isn't particularly special or unique in this. We make observations, and where they don't match our expectations, we hypothesize new things to fill the gaps. Sometimes they're borne out by later observations, sometimes not.
We've hypothesized the existence of planets to explain deviations in planets' orbits. Sometimes the hypothesized planets existed, sometimes they didn't. In the early days of chemistry, we came up with a whole bunch of elements; sometimes they turned out to be compounds that were particularly difficult to break down. We came up a whole host of particles and subparticles, and the ones that we still think are real aren't necessarily the ones you would have guessed at the time they were proposed. If you pick up an old text book on quantum mechanics, you'll find they keep referring to it as "the new quantum mechanics", not just because it was being nailed down at the same time the text books were written but to distinguish it from "the old quantum mechanics" from twenty years earlier that it was replacing.
Sometimes the hacky models are the best we have to offer; most people will call the Standard Model of particle physics just a hacky description of the observed phenomenon, but all the elegant models to replace it haven't really paid off.
It's possible that dark matter will go the way of luminiferous aether, but so far all the observations astrophysicists are making really jive well with the idea of dark matter, so for now it gets to stay in the "weird things about the universe many people don't like" category instead of the "stupid ideas no one can believe that people used to believe" category. I mean, it makes me angry that a neutron isn't just an electron and proton mashed together, but them's the breaks.
Surely all theories are amendable given appropriate data. But we do have a few direct examples of “darkish matter”. Neutrinos definitely exist and pass through the sun faster than their counterparts.
Dark matter is only “missing” in the sense that an explanation of it is missing from established physical theories. It (whatever it is) certainly appears to be present, because it has a major effect on observed galaxy behavior. Otherwise there would be no need to bring up the topic!
I get that, it's not that I don't get the reasoning. It's just that it seems backwards to me (like in the phone example). But maybe my comment was an artifact of the fact that I (layman) haven't really bought into the whole idea that we really ever had any "theory" for dark matter in the first place. To me the appropriate headline would've been more like "astronomers find the first galaxy that actually behaves as theories predict".
I don’t think we do have a working theory that explains all the observed evidence. We know the theory without dark matter is wrong, because the vast majority of galaxies don’t agree with it. But we don’t have a new theory that holds together that does explain the observations, with or without dark matter.
Basically “dark matter” just means “something that makes the observations work” and the choice of words “dark matter” is because it would work to have some kind of yet-unexplained matter that doesn’t interact with the matter we know about. It looked like it might also work to change the theory of gravity, in which case that was a bad choice of placeholder name — but now this is a counterexample to that idea.
Basically “dark matter” just means “something that makes the observations work” and the choice of words “dark matter” is because it would work to have some kind of yet-unexplained matter that doesn’t interact with the matter we know about. It looked like it might also work to change the theory of gravity, in which case that was a bad choice of placeholder name — but now this is a counterexample to that idea.
I don’t know the math, but my intuition based guess is it’s related to the range of interaction possible with other particles through QMs probability functions. Ie it’s important to remember that at quantum scale matter/energy transfer through space as probability density waves. Dark matter are at a low bar of the interaction distribution. Something about the empirical values of those equations gives way to many more particles at that low end than expected, but their existence couldn’t ever be ruled out.
Dark matter isn't "missing". It's invisible. It's something that has gravity but can't be observed. This galaxy is weird because it's mass is equal to the sum of its stars. In every other galaxy, its hundreds or thousands of times more heavy than all the stars put together.
Woah!
I found this article from 2007 where Hubble found a "ring" of dark matter that seemed to ripple out from a collision : https://www.nasa.gov/mission_pages/hubble/news/dark_matter_r...
Could this galaxy be an example of what the galaxy with the "ring" might look like some day far in the future? Fascinating stuff.
I found this article from 2007 where Hubble found a "ring" of dark matter that seemed to ripple out from a collision : https://www.nasa.gov/mission_pages/hubble/news/dark_matter_r...
Could this galaxy be an example of what the galaxy with the "ring" might look like some day far in the future? Fascinating stuff.
Wouldn’t this finding mean we can rule out modified gravity theories?
if Modified gravity were real wouldn’t all galaxies have to show the effects?
if Modified gravity were real wouldn’t all galaxies have to show the effects?
What about lack of super massive black hole singularity?
Smbcs are tiny on the scale of galaxies. You can measure their mass, they have a negligible impact on the rest of the galaxy.
Smbcs exist because crap tends to fall towards the middle of galaxies and pile up. Galaxies do not exist because of SMBCs
Smbcs exist because crap tends to fall towards the middle of galaxies and pile up. Galaxies do not exist because of SMBCs
I'm not even talking about their mass gravitation, I'm saying its possible that central singularity has some sort of unknown gravitational effect that ripples outward. When you talk about dark matter, you cant limit yourself to conventional gravitation theory. Maybe the SMB's are the heavy neutrino/WIMP source just as our sun is the source of the neutrino's we detect. We still have no clue what happens at the center of these singularities.
Can you imagine being in a civilization in that galaxy, they must really think they are the center of the universe.
Sounds like a cause for the Krikkit Wars...
Interesting, though I wonder if using multiple $1k lenses is as cost effective as buying one 16" telescope (<$3k) and a sensor/camera ($1k?) that would give ~3x more light gathering capacity than the array of 24 lenses.
> local Universe
What? What's the alternative of non-local universe that Hubble could possibly see?
What? What's the alternative of non-local universe that Hubble could possibly see?
I believe the headline is using local in the plainest sense of proximity. NGC 1052-DF2, the galaxy in question here, is only 65 million light-years away. That's not terribly far away, cosmically speaking.
Dumb question here but how do we know dark matter isn't just regular ol matter that is inside the event horizon of black holes? It's something I've always wondered and this discovery reminded me of it..
I’m pretty sure that one piece of evidence for dark matter is that galaxies with dark matter have flat rotation curves. This means as you move outwards from the centre of the galaxy, it turns out counterintuitively that objects have the same orbital speed. Because of Kepler’s laws, this implies that each farther orbit is encompassing more and more mass to give the farther object the same speed as the inner ones. Dark matter is spread out through the whole galaxy extending far out past the luminous matter like stars. If this mass was all in the centre inside the event horizon like you’re saying, the rotation curve of the galaxy would not be flat. The orbital speeds would drop off like they do in our solar system.
Because we can measure the mass of blackholes and account for it.
I have a noob question. How do astronomers know that the stuff they don't see is this exotic "dark matter" rather than just regular matter that their telescopes can't see?
Microlensing excludes large unseen objects and the CMB power spectrum suggests most matter is nonbaryonic.
Hi just curious. What is nonbaryonic matter and what is the CMB power spectrum?
Also doesn't microlensing involve gravity? Thus the effects of dark matter on microlensing would be identical to regular unseen matter which also interacts with gravity in the same way? What's the difference?
Also doesn't microlensing involve gravity? Thus the effects of dark matter on microlensing would be identical to regular unseen matter which also interacts with gravity in the same way? What's the difference?
The CMB is the cosmic microwave background, essentially the earliest light in the universe from when atoms are formed. The power spectrum of the temperature fluctuations in the CMB contains information about the constituents of the universe because each constituent (radiation, baryonic matter, non-baryonic matter) behaves differently in the early universe. Here is a Scientific American article that offers some explanation: http://background.uchicago.edu/~whu/SciAm/sym1.html
Microlensing involves gravity. The difference is that normal matter that could be dark matter would be relatively things massive but compact things like black holes and brown dwarfs as opposed to a diffuse halo. The compactness would make them detectable by lensing.
Microlensing involves gravity. The difference is that normal matter that could be dark matter would be relatively things massive but compact things like black holes and brown dwarfs as opposed to a diffuse halo. The compactness would make them detectable by lensing.
What's the difference between the two things you described?
To my understanding, the two options you gave are the same: Dark matter is a hypothesized form of matter which does not interact with normal matter or light (i.e. It can't be seen).
To my understanding, the two options you gave are the same: Dark matter is a hypothesized form of matter which does not interact with normal matter or light (i.e. It can't be seen).
Right I'm not questioning the differences between the two. I'm questioning how we know that the unseen stuff out there is regular matter vs Dark matter. Both matter and dark matter can appear dark when not interacting with light or glowing.
According to my knowledge dark matter is identified through its gravitational properties. Matter and dark matter both are gravitational hence my curiosity on why we're identifying this unaccounted gravitational effect as "dark matter" rather than regular matter.
According to my knowledge dark matter is identified through its gravitational properties. Matter and dark matter both are gravitational hence my curiosity on why we're identifying this unaccounted gravitational effect as "dark matter" rather than regular matter.
Because you can have dark matter in a hot gas cluster which means it should heat up and be observable by its radiation spectrum. Also note dark matter isn’t black like normal matter in the absence of light. It’s literally transparent because it doesn’t interact with light.
so if dark matter effects are missing from tiny galaxies is that evidence that dark matter is not undetectable massy particles but that gravity just behaves differently at huge scales?
Honestly it's probably more evidence for dark matter; even tiny galaxies are large enough that MOND formulations should have effects.
James Webb
James Webb
James Webb
It must be heaven.
Seems like just further confirmation that we actually don't know how gravity works. Inventing some magic modifier that makes our theories (and formulas based on those theories) "work" (dark matter) was always rather absurd.
Wouldn't this observation do the opposite?
If dark matter doesn't exist and gravity works differently at large scales how do you explain this galaxy? A rare galaxy having its dark matter blown away or something seems more reasonable.
If dark matter doesn't exist and gravity works differently at large scales how do you explain this galaxy? A rare galaxy having its dark matter blown away or something seems more reasonable.
Look at how sparse this galaxy is. What if its simply an unknown property of the black hole singularity at the center of the galaxy that acts as some sort of gravity modifier? What is galaxy scale has nothing to do with it? No one really knows for sure what the hell is going on and everyone wants to cling to the mystery particle theories because its the easier explanation.
>how do you explain this galaxy?
I can't, and neither can anyone else - which is why people are struggling with invented variables like "dark matter". Its fine to speculate, its absurd to claim with certainty (as many so-called scientists do).
I can't, and neither can anyone else - which is why people are struggling with invented variables like "dark matter". Its fine to speculate, its absurd to claim with certainty (as many so-called scientists do).
It's not "magic" it's a model that fits the observations: otherwise known as science. Let's be clear here, every other model does not explain the observations, by a pretty significant margin. Moreover, the concept of weakly interacting massive particles is not outlandish, we already know of some particles that behave very similarly (neutrinos).
Dark matter also struggles to explain this galaxy. Standard cosmology has built explanations of galaxy formation using dark matter, yet none of those explanations seem to apply here. This is not yet a fatal flaw since some explanation may be found. But the same thing is true for MOND. It may take time for them as well to analyze this galaxy and incorporate its lessons into MOND.
>It's not "magic" it's a model that fits the observations: otherwise known as science.
Epicycles.
Epicycles.
[deleted]
Except dark matter isn't an observation. Its an unobserved variable that's been inserted to make models that don't match observations add up.
Dark matter is a theory that fits the observations. Prior to the 1980s nobody had ever been able to image a single atom as well, atoms are just theories that fit the observations. Proton-proton fusion has never been observed directly experimentally, and yet we confidently say that it is the primary method of energy generation inside our own Sun and many other stars, because it's the only theory that fits the observations. This is how science works.
>Dark matter is a theory that fits the observations.
You are mistaking "theory" and "hypothesis". Dark matter is a hypothesis. It is unproven, it has not been observed. Describing this as a "galaxy devoid of dark matter" rather than "a galaxy where the dark matter hypothesis does not add up" is absurd. There is a difference between speculation about potential causes for observed phenomena and observed phenomena themselves.
You are mistaking "theory" and "hypothesis". Dark matter is a hypothesis. It is unproven, it has not been observed. Describing this as a "galaxy devoid of dark matter" rather than "a galaxy where the dark matter hypothesis does not add up" is absurd. There is a difference between speculation about potential causes for observed phenomena and observed phenomena themselves.
Au contraire - in that galaxy the dark matter hypothesis really adds up. It's existence kills theories that "dark matter" is just an artefact of how gravity works.
A theory is a hypothesis that is well-substantiated by data. Dark matter is such a theory, those who believe otherwise almost always do not understand the data.
What is your definition of the word "observation"?
Well, we were right about induction currents, gravitational waves and anti-matter, so it's not as if filling in the gaps is always incorrect.
Right eventually. Before that the explanations were a lot wackier. I could easily see "dark matter" the same way I see "ether".
Don't forget neutrinos... or a million other things.
Are you a physicist or cosmologist?
Positing some mystery matter that doesn't behave like anything we've observed strikes me as more absurd than gravity working differently at galactic scales, so I guess it's all a matter of perspective.
It is more absurd. Firstly, WIMPs do not "behave unlike anything we've observed". They behave similarly to neutrinos except are much more massive and instead of almost always having so much energy that they travel at relativistic speeds they travel at much slower speeds. No new fundamental forces are required to explain WIMPs, they are simply particles we haven't discovered yet. Given that in the 20th century alone we discovered photons, protons, neutrons, anti-particles, quarks (and their whole family of composite particles in the form of baryons and mesons), neutrinos, and weak bosons while in the 21st century we've discovered the Higgs it would be ridiculous to say that new particle discoveries are beyond the realm of possibility. Moreover, we know that the standard model of particle physics is incomplete due to the existence of neutrino oscillations (among other things). So we can be fairly confident there are new particles yet to be discovered, and we have a model of dark matter which fits a hugely diverse set of data fairly well that no other model fits. To deny that dark matter is the best model for the data at present is nothing more than superstition.
I have a engineering-physics degree, and while I will admit my day job involves more down to earth physics software simulation rather than advanced cosmology and particle theory, I would categorize WIMPS more so as the model with the least amount of gotcha's when compared with alternative gravity theories. I for one lean towards alternative gravity theories.
"They behave similarly to neutrinos except are much more massive..." And yet we can build neutrino detectors and "see" neutrinos. So far similar detectors for dark matters have failed to detect anything:
https://en.wikipedia.org/wiki/Large_Underground_Xenon_experi...
Lack of detection, plus this sparse galaxy with no sign of dark matter, is starting to slowly rule out the simpler explanations by virtue of outliers that don't fit any model.
You yourself mentioned it, but I always remind detractors that the standard model IS WRONG.
The sparseness of this galaxy is what intrigues me. What if the lack singularity at the center is the key?
"They behave similarly to neutrinos except are much more massive..." And yet we can build neutrino detectors and "see" neutrinos. So far similar detectors for dark matters have failed to detect anything:
https://en.wikipedia.org/wiki/Large_Underground_Xenon_experi...
Lack of detection, plus this sparse galaxy with no sign of dark matter, is starting to slowly rule out the simpler explanations by virtue of outliers that don't fit any model.
You yourself mentioned it, but I always remind detractors that the standard model IS WRONG.
The sparseness of this galaxy is what intrigues me. What if the lack singularity at the center is the key?
> The sparseness of this galaxy is what intrigues me.
But none of the ultra-diffuse galaxies discovered so far have been found to be lacking in dark matter. So even among this unusual class of galaxy, NGC 1052-DF2 is an oddball.
https://www.nasa.gov/feature/goddard/2018/dark-matter-goes-m...
But none of the ultra-diffuse galaxies discovered so far have been found to be lacking in dark matter. So even among this unusual class of galaxy, NGC 1052-DF2 is an oddball.
https://www.nasa.gov/feature/goddard/2018/dark-matter-goes-m...
> They behave similarly to neutrinos except are much more massive and instead of almost always having so much energy that they travel at relativistic speeds they travel at much slower speeds
You've mixed this up a bit.
Because neutrinos have such a low invariant mass, they have little inertia, so are readily accelerated to speeds comparable to the speed of light at production time or when scattering off an atomic nucleus. A heavy neutrino has a higher invariant mass by definition, and thus more inertia, and thus are likely to move less than a lower-inertia regular neutrino when encountering an an atomic nucleus.
Since there is a lot of gas and dust around at the start of structure formation, you need some mechanism to keep energy-momentum localized around the matter that will become luminous matter like stars and hot gas. Adding inertia to a particle that is otherwise highly comparable to a neutrino would do the job. Normal neutrinos approaching close enough ("scattering") to an atomic nucleus would tend to get a large kick, which can be considered as a substantial Lorentz boost. Heavy neutrinos suffer a smaller kick.
There are is an additionally point worth considering here: there are almost certainly cold regular neutrinos in a relic field called the cosmic neutrino background, which is analogous to the cosmic microwave background. These neutrinos rarely interact with atomic nuclei, but when they do they are liable to get a big kick. They also are so low-energy that there would have to be an enormous amount of them if they were a major component of Cold Dark Matter; enough that there would be visible nuclear-reaction signatures in our sky as they get heated up by collision with hot gas and the like in the galaxies around which the standard cosmology expects there to be lots of Cold Dark Matter. The heating up of such huge densities of cold standard neutrinos (by weak interactions with hot baryons) would also kick many of the neutrinos out of galaxies over time, which produces a smearing out of visible matter as an observable.
Those galaxy-scale observables and the peaks in the CMB power spectrum (plotting the fluctuations in the CMB temperature spectrum at different angular scales) preclude primordial cold standard neutrinos as an important component of Cold Dark Matter operating since the formation of the earliest galaxies.
You've mixed this up a bit.
Because neutrinos have such a low invariant mass, they have little inertia, so are readily accelerated to speeds comparable to the speed of light at production time or when scattering off an atomic nucleus. A heavy neutrino has a higher invariant mass by definition, and thus more inertia, and thus are likely to move less than a lower-inertia regular neutrino when encountering an an atomic nucleus.
Since there is a lot of gas and dust around at the start of structure formation, you need some mechanism to keep energy-momentum localized around the matter that will become luminous matter like stars and hot gas. Adding inertia to a particle that is otherwise highly comparable to a neutrino would do the job. Normal neutrinos approaching close enough ("scattering") to an atomic nucleus would tend to get a large kick, which can be considered as a substantial Lorentz boost. Heavy neutrinos suffer a smaller kick.
There are is an additionally point worth considering here: there are almost certainly cold regular neutrinos in a relic field called the cosmic neutrino background, which is analogous to the cosmic microwave background. These neutrinos rarely interact with atomic nuclei, but when they do they are liable to get a big kick. They also are so low-energy that there would have to be an enormous amount of them if they were a major component of Cold Dark Matter; enough that there would be visible nuclear-reaction signatures in our sky as they get heated up by collision with hot gas and the like in the galaxies around which the standard cosmology expects there to be lots of Cold Dark Matter. The heating up of such huge densities of cold standard neutrinos (by weak interactions with hot baryons) would also kick many of the neutrinos out of galaxies over time, which produces a smearing out of visible matter as an observable.
Those galaxy-scale observables and the peaks in the CMB power spectrum (plotting the fluctuations in the CMB temperature spectrum at different angular scales) preclude primordial cold standard neutrinos as an important component of Cold Dark Matter operating since the formation of the earliest galaxies.
> You've mixed this up a bit.
You've used different words to describe exactly the same thing I said.
You've used different words to describe exactly the same thing I said.
No, the rest energy of a neutrino is always less than that of a heavy neutrino, by definition. A heavy neutrino can certainly move relativistically; a neutrino likewise can move non-relativistically. The cosmic neutrino background is a huge number of non-relativistic, i.e. thermal, neutrinos.
So, "... except are much more massive and instead of almost always having so much energy that they travel at relativistic speeds ..." is wrong on two fronts, and "... [heavy neutrinos] travel at much slower speeds" also will not be true if they feel the weak force, and if they don't it will still likely be true for heavy neutrinos that find themselves near a sufficiently dramatic event like a highly asymmetrical star-degenerate white dwarf supernova.
So, "... except are much more massive and instead of almost always having so much energy that they travel at relativistic speeds ..." is wrong on two fronts, and "... [heavy neutrinos] travel at much slower speeds" also will not be true if they feel the weak force, and if they don't it will still likely be true for heavy neutrinos that find themselves near a sufficiently dramatic event like a highly asymmetrical star-degenerate white dwarf supernova.
Did I say rest energy? You keep reading my words and finding different words than I wrote.
No, you didn't, you said "energy", which is an observer-dependent quantity. "Rest energy" is on the other hand Lorentz-invariant.
You can always find some observer which will see even a chosen massless particle as having a lot more energy than a chosen massive particle, although then you can find observers who will see the opposite. The invariant quantity is what matters.
A comoving observer with much better detector technology than we have today will see relativistic neutrinos and thermal neutrinos in the comoving frame, and (if they exist) relativistic heavy neutrinos and thermal heavy neutrinos in the comoving frame. If cold dark matter is mostly heavy neutrinos, then the energy density measured at a typical point in the comoving frame will have thermal heavy neutrinos as the largest component of these four.
However, nature doesn't single out comoving observers' measurements as more real than anyone else's, the real universe has overdensities (there are relativsitic neutrino jets produced here on Earth, for instance) and underdensities, and the motion of even planetary bodies might kick thermal neutrinos and thermal heavy neutrinos up to relativistic speeds, or conversely wrench relativistic ones down to speeds comparable to Earth's orbital speed (in e.g. a solar system barycentric frame). You would have to know at least two not-yet-known parameters for heavy neutrinos in order to say whether (still in a solar system barycentric frame) Earth would kick a CNB neutrino from thermal to relativistic speeds with greater probability than a CDM heavy neutrino from thermal to relativistic speeds.
But, justifying a paragraph I wrote a bit earlier, in a lab frame with a neutrino or heavy neutrino held at rest at the origin of a set of coordinates when we elastically collide the particle with a heavy atomic nucleus (thus fixing interaction cross section), the former is more likely to wind up moving away from the origin at relativistic speeds than the latter, thanks to its rest mass. I had underspecified the coordinate conditions.
You can always find some observer which will see even a chosen massless particle as having a lot more energy than a chosen massive particle, although then you can find observers who will see the opposite. The invariant quantity is what matters.
A comoving observer with much better detector technology than we have today will see relativistic neutrinos and thermal neutrinos in the comoving frame, and (if they exist) relativistic heavy neutrinos and thermal heavy neutrinos in the comoving frame. If cold dark matter is mostly heavy neutrinos, then the energy density measured at a typical point in the comoving frame will have thermal heavy neutrinos as the largest component of these four.
However, nature doesn't single out comoving observers' measurements as more real than anyone else's, the real universe has overdensities (there are relativsitic neutrino jets produced here on Earth, for instance) and underdensities, and the motion of even planetary bodies might kick thermal neutrinos and thermal heavy neutrinos up to relativistic speeds, or conversely wrench relativistic ones down to speeds comparable to Earth's orbital speed (in e.g. a solar system barycentric frame). You would have to know at least two not-yet-known parameters for heavy neutrinos in order to say whether (still in a solar system barycentric frame) Earth would kick a CNB neutrino from thermal to relativistic speeds with greater probability than a CDM heavy neutrino from thermal to relativistic speeds.
But, justifying a paragraph I wrote a bit earlier, in a lab frame with a neutrino or heavy neutrino held at rest at the origin of a set of coordinates when we elastically collide the particle with a heavy atomic nucleus (thus fixing interaction cross section), the former is more likely to wind up moving away from the origin at relativistic speeds than the latter, thanks to its rest mass. I had underspecified the coordinate conditions.
Now you're just being needlessly pedantic. I wasn't writing a paper, I was making a comment aimed at the public at large. There's no confusion about "energy" in that context, the observer is obvious, the reference frame is obvious.
Also, I made no mention of a "heavy neutrino", again you keep reading beyond and misinterpreting my words for no good reason other than to jump into a thread and shout "well, actually!" Which is not helpful towards educating the lay public on the matter whatsoever.
Also, I made no mention of a "heavy neutrino", again you keep reading beyond and misinterpreting my words for no good reason other than to jump into a thread and shout "well, actually!" Which is not helpful towards educating the lay public on the matter whatsoever.
I too have this feeling that with dark matter we’re at the epicycles phase...
Yep. Occam's razor is just not -that- flexible.
What is this epicycle phase you reference?
https://en.m.wikipedia.org/wiki/Deferent_and_epicycle
The canonical example of how tweaking your model to fit observed data doesn’t necessarily results in a good theory.
The canonical example of how tweaking your model to fit observed data doesn’t necessarily results in a good theory.
Crazy uniformed but fun theory !:
Quantum theory suggests that universes split when a decision is taken. This is my crude understanding of parallel universe theory.
So, while universe splits, it creates a superimposing copy of itself where decision is different. Like a git repository, where only changes are saved, instead of copying everything.
Gravity however continues to interact with this changes and so we observe the dark matter.
If their is any weight to this theory,:
1. We can say where changes occurred
2. We can predict dark matter
3. We might be able to jump to parallel universe as we know where they are! Only if we can solve how gravity exactly works!
Apparently, some of the newer Canon lenses have a newfangled anti-reflective coating with unprecedented performance. So some enterprising Canucks said, "you know, maybe we can buy a bunch of those and tape them together and make a world-class scientific instrument, eh?" And then they fucking did it.
That there is the DIY hacker spirit in broad daylight. It's so awesome I can barely stand it.
You can read about it here, which found via Phil Plait's excellent blog: http://www.dunlap.utoronto.ca/instrumentation/dragonfly/