New studies confirm existence of galaxies with almost no dark matter(news.yale.edu)
news.yale.edu
New studies confirm existence of galaxies with almost no dark matter
https://news.yale.edu/2019/03/29/new-studies-confirm-existence-galaxies-almost-no-dark-matter
84 comments
Am I wrong to conclude that this result itself is an argument for the existence of dark matter?
Because the theories that argue that physical laws behave differently at larger distances would have to work similarly for all galaxies. Physicists please correct me if I am wrong in making this conclusion!
Because the theories that argue that physical laws behave differently at larger distances would have to work similarly for all galaxies. Physicists please correct me if I am wrong in making this conclusion!
Both galaxies without dark matter are UDG's.
I'm not a physicist, but this strengthens my personal hunch that dark matter doesn't exist. My heretic theory is that it is just a product of some sort of physical law acting as a "ceiling" in local gravitational force which gets conserved by spreading over large distances.
I'm not a physicist, but this strengthens my personal hunch that dark matter doesn't exist. My heretic theory is that it is just a product of some sort of physical law acting as a "ceiling" in local gravitational force which gets conserved by spreading over large distances.
You might be interested in the work of Erik Verlinde and so called "emergent gravity". While there are still significant problems that the theory needs to explain, it is certainly an interesting perspective to consider.
https://www.forbes.com/sites/startswithabang/2017/02/28/is-d...
http://www.astronomy.com/news/2018/05/the-case-against-dark-...
https://www.forbes.com/sites/startswithabang/2017/02/28/is-d...
http://www.astronomy.com/news/2018/05/the-case-against-dark-...
I think that is a reasonable conclusion to make (assuming these findings are correct). The article says the same:
>Ironically, the lack of dark matter in these UDGs strengthens the case for dark matter, the researchers say. It proves that dark matter is a substance that is not coupled with normal matter, since they can be found separately.
>Ironically, the lack of dark matter in these UDGs strengthens the case for dark matter, the researchers say. It proves that dark matter is a substance that is not coupled with normal matter, since they can be found separately.
Also:
https://medium.com/starts-with-a-bang/mysterious-galaxy-meas...
“Ironically, the discovery of a galaxy without dark matter — amidst a sea of galaxies that require huge amounts of it — helps prove the validity of the dark matter-rich picture of the Universe. Only if normal matter can be separated out from the dark matter and left to form its own structures would such a galaxy be possible.”
https://medium.com/starts-with-a-bang/mysterious-galaxy-meas...
“Ironically, the discovery of a galaxy without dark matter — amidst a sea of galaxies that require huge amounts of it — helps prove the validity of the dark matter-rich picture of the Universe. Only if normal matter can be separated out from the dark matter and left to form its own structures would such a galaxy be possible.”
This reminds me of the discovery of oxygen, which was more about discovering the lack of oxygen in a vacuum and in combustion products.
I've been very reluctant to go 100% with the Dark Matter theories, preferring to think that Kepler's Law was somehow the root problem. Note, I had no real basis for these thoughts!
These findings do indicate that the Dark Matter theory is increasingly reliable, in that some galaxies are shown to have different densities (& thereby there's nothing wrong with Kepler's Law, and further proof that my "going with the gut" when compared with empirical science is just silly!)
These findings help reinforce that Kepler's Law are working well and seem to indicate that that dark matter is not necessarily tied with matter, but is hanging around in different densities in different galaxies.
> Ironically, the lack of dark matter in these UDGs strengthens the case for dark matter, the researchers say. It proves that dark matter is a substance that is not coupled with normal matter, since they can be found separately.
To me, it's a bit like how our planet is not uniformly the "same stuff", but we have deposits of Gold and Coal and Granite spread around the place in an uneven manner.
These findings do indicate that the Dark Matter theory is increasingly reliable, in that some galaxies are shown to have different densities (& thereby there's nothing wrong with Kepler's Law, and further proof that my "going with the gut" when compared with empirical science is just silly!)
These findings help reinforce that Kepler's Law are working well and seem to indicate that that dark matter is not necessarily tied with matter, but is hanging around in different densities in different galaxies.
> Ironically, the lack of dark matter in these UDGs strengthens the case for dark matter, the researchers say. It proves that dark matter is a substance that is not coupled with normal matter, since they can be found separately.
To me, it's a bit like how our planet is not uniformly the "same stuff", but we have deposits of Gold and Coal and Granite spread around the place in an uneven manner.
Why specifically Kepler laws and not the other dozen ways in which our current understanding of physics could be incomplete?
Actually Dark Matter is just a way of saying there's something (not necessarily a "substance") we still don't know: the old metaphor of light for knowledge.
Actually Dark Matter is just a way of saying there's something (not necessarily a "substance") we still don't know: the old metaphor of light for knowledge.
I think we can increasingly say that it it almost certainly matter if some kind. I don’t think it’s a state of pure ignorance.
Why is it that no one in astrophsyics seems to understand that electricity has mass?
Darkness is not an absolute concept either. As an example, your comment is somewhat dark to me ("it it" and "if some kind") but not pitch black :)
> Am I wrong to conclude that this result itself is an argument for the existence of dark matter?
Only if competing arguments that account for the rotational speed discrepancy in galaxies fail to explain "galaxies without dark matter", which... I think they do? (I am not 100% sure on this).
Of course occam's razor is only a heuristic, and not a law, but we would prefer a theory that is simpler, and if we have to add an extra level of indirection explaining why some galaxies lack dark matter, then that is less desirable in a theory. Repeating myself, science doesn't necessarily do what we desire.
Only if competing arguments that account for the rotational speed discrepancy in galaxies fail to explain "galaxies without dark matter", which... I think they do? (I am not 100% sure on this).
Of course occam's razor is only a heuristic, and not a law, but we would prefer a theory that is simpler, and if we have to add an extra level of indirection explaining why some galaxies lack dark matter, then that is less desirable in a theory. Repeating myself, science doesn't necessarily do what we desire.
It is known that certain types of galaxies can have less dark matter than others. I believe that jury is still out. I'm a little concerned by how many people "believe" there is dark matter. This adds a bias to the proposed solutions and the funding for each. It leads me to be a little skeptical when I hear that dark matter is being found and whatnot through the speed of stars. I'd love for research to be more balanced since the phenomena can be explained through more hypothesise than just increased mass.
> I'd love for research to be more balanced since the phenomena can be explained through more hypothesise than just increased mass.
Such as?
Such as?
MOND (Modified Newtonian Dynamics) is one, though it has quite a few problems, as you can read here https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics
Another interesting one is Superfluid Dark Matter (first proposed, to my knowledge, here: https://arxiv.org/abs/1507.01019).
Those are just a few, there are probably more theories that I haven't heard about. Whatever the case, Sabine Hossenfelder recently posted three videos about Dark Matter/MOND/Superfluid Dark Matter that I wholeheartedly recommend:
Dark Matter: https://www.youtube.com/watch?v=FN2d2cmi_Gk
MOND: https://www.youtube.com/watch?v=2VNcDoLNJk8
Superfluid Dark Matter (note that it's the theory she supports): https://www.youtube.com/watch?v=468cyBZ_cq4
Another interesting one is Superfluid Dark Matter (first proposed, to my knowledge, here: https://arxiv.org/abs/1507.01019).
Those are just a few, there are probably more theories that I haven't heard about. Whatever the case, Sabine Hossenfelder recently posted three videos about Dark Matter/MOND/Superfluid Dark Matter that I wholeheartedly recommend:
Dark Matter: https://www.youtube.com/watch?v=FN2d2cmi_Gk
MOND: https://www.youtube.com/watch?v=2VNcDoLNJk8
Superfluid Dark Matter (note that it's the theory she supports): https://www.youtube.com/watch?v=468cyBZ_cq4
> Dark Matter
> Superfluid Dark Matter
As far as I see, from three theories you listed, two are the dark matter theories. The only "alternative" from your list is MOND and as far as I know it has more problems than it can solve:
https://medium.com/starts-with-a-bang/theres-a-debate-raging...
"The tests on the largest scales give us the best tests for dark matter. And these are the ones that dark matter not only universally passes, but that MOND has failed spectacularly for, on every account, for the past 35 years. Among cosmologists*, there is no debate, because there is no alternative to dark matter that reproduces the observed successes."
https://arxiv.org/abs/1112.1320
"The biggest challenge facing MOND today is the shape of the matter power spectrum. The shape depicted in Fig. 1 is related to the acoustic oscillations observed in the CMB. If the Universe is dominated by dark matter, these matter oscillations, dubbed Baryon Acoustic Oscillations (BAO), are highly suppressed as the baryons fall into the potential wells created by dark matter, leaving only percent level traces of the primordial oscillations. In a no-dark matter model, on the other hand, the oscillations should be just as apparent in matter as they are in the radiation. Indeed, Fig. 1 illustrates that – even if a generalization such as TeVeS fixes the amplitude problem – the shape of the predicted spectrum is in violent disagreement with the observed shape."
> Superfluid Dark Matter
As far as I see, from three theories you listed, two are the dark matter theories. The only "alternative" from your list is MOND and as far as I know it has more problems than it can solve:
https://medium.com/starts-with-a-bang/theres-a-debate-raging...
"The tests on the largest scales give us the best tests for dark matter. And these are the ones that dark matter not only universally passes, but that MOND has failed spectacularly for, on every account, for the past 35 years. Among cosmologists*, there is no debate, because there is no alternative to dark matter that reproduces the observed successes."
https://arxiv.org/abs/1112.1320
"The biggest challenge facing MOND today is the shape of the matter power spectrum. The shape depicted in Fig. 1 is related to the acoustic oscillations observed in the CMB. If the Universe is dominated by dark matter, these matter oscillations, dubbed Baryon Acoustic Oscillations (BAO), are highly suppressed as the baryons fall into the potential wells created by dark matter, leaving only percent level traces of the primordial oscillations. In a no-dark matter model, on the other hand, the oscillations should be just as apparent in matter as they are in the radiation. Indeed, Fig. 1 illustrates that – even if a generalization such as TeVeS fixes the amplitude problem – the shape of the predicted spectrum is in violent disagreement with the observed shape."
I'm just a layman, so I may have misunderstood Sabine's explanation, but to my knowledge Dark Matter, as a theory, is fundamentally different from Superfluid Dark Matter. In the latter, there's a new kind of force explaining why MOND, in certain cases, works. She notes at the end of the third video I listed above that Superfluid Dark Matter should be considered a combination of both Dark Matter and MOND. I would recommend taking a look at her video, it's short (about 6 mins) and I found it quite clear. Anyway, this is far from a solved problem, and no theory is sufficiently developed to be the last word on the subject.
Dark matter is a supersolid that fills 'empty' space, strongly interacts with ordinary matter and is displaced by ordinary matter. What is referred to geometrically as curved spacetime physically exists in nature as the state of displacement of the supersolid dark matter. The state of displacement of the supersolid dark matter is gravity.
The supersolid dark matter displaced by a galaxy pushes back, causing the stars in the outer arms of the galaxy to orbit the galactic center at the rate in which they do.
Displaced supersolid dark matter is curved spacetime.
In the Bullet Cluster collision the dark matter has not separated from the ordinary matter. The collision is analogous to two boats that collide, the boats slow down and their bow waves continue to propagate. The water has not separated from the boats, the bow waves have. In the Bullet Cluster collision the galaxy's associated dark matter displacement waves have separated from the colliding galaxies, causing the light to lense as it passes through the waves.
The supersolid dark matter displaced by a galaxy pushes back, causing the stars in the outer arms of the galaxy to orbit the galactic center at the rate in which they do.
Displaced supersolid dark matter is curved spacetime.
In the Bullet Cluster collision the dark matter has not separated from the ordinary matter. The collision is analogous to two boats that collide, the boats slow down and their bow waves continue to propagate. The water has not separated from the boats, the bow waves have. In the Bullet Cluster collision the galaxy's associated dark matter displacement waves have separated from the colliding galaxies, causing the light to lense as it passes through the waves.
> to my knowledge Dark Matter, as a theory, is fundamentally different from Superfluid Dark Matter.
To to be able to fit with the CMB measurements, at that scale this theory behaves exactly like a normal dark matter, so there is still 5 times more dark matter than a baryonic matter.
So it's just "more complicated" at the galaxy level than a normal dark matter, to fit with these measurements "better" too: it involves an "additional force" there. That also means, considering everything together, it's nothing that should make happy those that consider the normal dark matter "unintuitive."
To to be able to fit with the CMB measurements, at that scale this theory behaves exactly like a normal dark matter, so there is still 5 times more dark matter than a baryonic matter.
So it's just "more complicated" at the galaxy level than a normal dark matter, to fit with these measurements "better" too: it involves an "additional force" there. That also means, considering everything together, it's nothing that should make happy those that consider the normal dark matter "unintuitive."
> I'd love for research to be more balanced since the phenomena can be explained through more hypothesise than just increased mass.
I'd love for climate science research to be more balanced...
Seriously, the "balance" tends to fall towards the theories that have the strongest indications of being on the right track, and that's what's happened with dark matter.
In the video on the only non-dark-matter theory that you linked, MOND, it is pointed out that MOND is known to be wrong, and is considered to at best be an approximation to an unknown theory of modified gravity. MOND itself also has no real theoretical justification, i.e. no explanation for how or why the gravitational equations should be modified.
I'd love for climate science research to be more balanced...
Seriously, the "balance" tends to fall towards the theories that have the strongest indications of being on the right track, and that's what's happened with dark matter.
In the video on the only non-dark-matter theory that you linked, MOND, it is pointed out that MOND is known to be wrong, and is considered to at best be an approximation to an unknown theory of modified gravity. MOND itself also has no real theoretical justification, i.e. no explanation for how or why the gravitational equations should be modified.
This leads to an interesting question the article didn't mention...
If a galaxy can exist without any dark matter and others with a balance of dark and normal matter, can one exist made entirely of dark matter?
If a galaxy can exist without any dark matter and others with a balance of dark and normal matter, can one exist made entirely of dark matter?
Yup, that seems to be it, similar mass to our own with 1% off the light emitted.
How does dark matter interact with another dark matter? I know that dark matter can only interact with normal matter by gravitational force..
It surely wouldn't be a galaxy then? I think a galaxy has to have stars in order to be a galaxy.
I am not a physicist, just someone with an armchair interest but...
If dark matter is theorized to explain more gravity than there would be without it, that implies dark matter has a gravitational effect, which means it too might coalesce into very dense things. Perhaps dark matter also forms star-like gravitational bodies, but whatever makes it "dark" hides those too.
It seems weird because I'm almost suggesting... Cloaked stars. Which on the face seems terribly unscientific, given what we know about EM radiation of things. But i think dark matter implies things that don't radiate in the EM spectrum.
Part of me wants to speculate that dark matter is a gravitational force echoing from 4th dimensional matter. Like if a cube affected a plane over which it hovered. But that's just me being an ignorant speculator.
If dark matter is theorized to explain more gravity than there would be without it, that implies dark matter has a gravitational effect, which means it too might coalesce into very dense things. Perhaps dark matter also forms star-like gravitational bodies, but whatever makes it "dark" hides those too.
It seems weird because I'm almost suggesting... Cloaked stars. Which on the face seems terribly unscientific, given what we know about EM radiation of things. But i think dark matter implies things that don't radiate in the EM spectrum.
Part of me wants to speculate that dark matter is a gravitational force echoing from 4th dimensional matter. Like if a cube affected a plane over which it hovered. But that's just me being an ignorant speculator.
The thing about dark matter is that because it doesn't seem to interact in any way other than gravitationally, there is no way for it to lose energy and clump together. So, collections of dark matter tend to stay rather diffuse. The way you could notice such a collection without any visible matter is through gravitational lensing.
Why is loosing energy required for particles to clump together?
Think about two dark matter clouds crashing into each other.
Each cloud just continues with the same speed in the direction it had. They dont interact other than gtavitationally.
After passing each other they start to pull on each other gravitationally and slow each other down. Eventually stop and accelerate towards each other again.
And so they can oscillate forever back and forth, if left to themselves without other clumps. But at no point would they "clump"... they are only close to one another in space when their speeds are most opposite one another. Being clumped means being im the same location and having the same speed.
Each cloud just continues with the same speed in the direction it had. They dont interact other than gtavitationally.
After passing each other they start to pull on each other gravitationally and slow each other down. Eventually stop and accelerate towards each other again.
And so they can oscillate forever back and forth, if left to themselves without other clumps. But at no point would they "clump"... they are only close to one another in space when their speeds are most opposite one another. Being clumped means being im the same location and having the same speed.
I expect they would lose energy and clump together due GR effects- the system will emit gravitational waves.
https://en.wikipedia.org/wiki/Gravitational_wave#Binaries
https://en.wikipedia.org/wiki/Gravitational_wave#Binaries
If they don't lose energy by interacting through some force other than gravity when they get close together, then they will whip right past each other with nothing to slow them down.
If you could play God and place blobs of dark matter in the right places and moving at the right speeds, it seems plausible that such a dark matter Galaxy could physically exist and be stable.
But I don't think we know of a mechanism for such a structure to form naturally. Normal particles collide and transform some of their gravitational energy into heat. If dark matter only interacts gravitationally, blobs of it would pass right through each other without losing energy. I think(?) you need these sorts of collisional energy losses to get matter to coalesce into a galaxy.
But I don't think we know of a mechanism for such a structure to form naturally. Normal particles collide and transform some of their gravitational energy into heat. If dark matter only interacts gravitationally, blobs of it would pass right through each other without losing energy. I think(?) you need these sorts of collisional energy losses to get matter to coalesce into a galaxy.
You can get the equivalent of evaporative cooling in an unstable multi body system. You occasionally kick out an object which takes more energy away than the average body that remains.
The formation of a dark matter galaxy in such a fashion is plausible, but would take a long time.
The formation of a dark matter galaxy in such a fashion is plausible, but would take a long time.
That require that the bodies exchange energy to get near the lower average, and transfer some energy to a few hot particles. Otherwise, you won't get any cooling.
That could be done entirely through gravitational mechanisms. There's something like that which happens to globular clusters.
You might be interested in Lynden-Bell’s papers
Thanks. I'm just going by a tidbit from DeepSkyVideos on YouTube. They specifically mention a similarity to evaporative cooling.
I just found the video! ("Gravothermal Catastrophe")
https://www.youtube.com/watch?v=LYnoV2n5ptg
I just found the video! ("Gravothermal Catastrophe")
https://www.youtube.com/watch?v=LYnoV2n5ptg
Are you aware of the concept of Dynamical Friction? There is nothing speculative about it. It slows down objects above the average speed by ordinary Newton-style gravitational interactions.
They are called "Dark Galaxies", there are a few candidates, but IIUC they are not confirmed. https://en.wikipedia.org/wiki/Dark_galaxy
[deleted]
I think the main theory for how the dark matter free galaxies form is that they somehow lost their dark matter component. Perhaps in a collision with another galaxy.
So in that scenario it's quite possible that the dark matter part of those galaxies is drifting around on it's own somewhere. I don't know that we'll have any way of detecting them soon though...
So in that scenario it's quite possible that the dark matter part of those galaxies is drifting around on it's own somewhere. I don't know that we'll have any way of detecting them soon though...
That would fit nicely with dark matter being perfectly ordinary stuff like chunks of rock that happens to be difficult to observe from afar.
That's a great common sense theory that's sadly been thoroughly disproven.
As another commenter has noted, they wouldn't then be galaxies. And the question of whether there are (or how common and at what mass) galaxyless dark matter overdensities (think of it as a clump, in the literature they are called "halos") is still open.
One of the problems with the current best model of the universe (Lambda CDM - a universe that is primarily make up of some cosmological constant driving expansion and cold dark matter whose gravity drives structure formation) is that it predicts that there should be far more of these dark matter overdensities than we see galaxies. This is the "Too big to fail" problem - https://arxiv.org/abs/1103.0007 is a pretty readable description. Basically the model predicts lots of dark matter clumps orbiting the milky way and we don't see enough satellite galaxies to populate them.
So there are now two options: 1) Galaxy formation becomes stochastic at lower dark matter halo masses - and so there are lots of these galaxyless overdensities of dark matter. 2) There is something wrong with our model and there might not be these overdensities. 3) The milky way is an outlier.
I think the short summary is that, at very low masses (say ~1e6 solar masses) we wouldn't be surprised if not all dark matter halos form galaxies. At high masses (1e12 solar masses, roughly that of the milky way halo) we would be very surprised if there weren't galaxies. But in between we are not sure.
Edit: To be clear, these structures would still likely contain baryons (normal matter), they just haven't formed stars (or would have formed only a very few stars).
One of the problems with the current best model of the universe (Lambda CDM - a universe that is primarily make up of some cosmological constant driving expansion and cold dark matter whose gravity drives structure formation) is that it predicts that there should be far more of these dark matter overdensities than we see galaxies. This is the "Too big to fail" problem - https://arxiv.org/abs/1103.0007 is a pretty readable description. Basically the model predicts lots of dark matter clumps orbiting the milky way and we don't see enough satellite galaxies to populate them.
So there are now two options: 1) Galaxy formation becomes stochastic at lower dark matter halo masses - and so there are lots of these galaxyless overdensities of dark matter. 2) There is something wrong with our model and there might not be these overdensities. 3) The milky way is an outlier.
I think the short summary is that, at very low masses (say ~1e6 solar masses) we wouldn't be surprised if not all dark matter halos form galaxies. At high masses (1e12 solar masses, roughly that of the milky way halo) we would be very surprised if there weren't galaxies. But in between we are not sure.
Edit: To be clear, these structures would still likely contain baryons (normal matter), they just haven't formed stars (or would have formed only a very few stars).
Can a physics minded HNer confirm something for me (or set me straight)?
One piece of evidence for dark matter is observed when galaxies collide. Two galaxies collide, but by observation of gravitational lensing on the light from stars "behind" the colliding galaxies show that much of the matter from each continues moving. This matter (dark matter) is seemingly unobstructed by the collision (because of dark matter's lack of electromagnetic interaction).
I heard this explained to me once, wondering if it is true.
One piece of evidence for dark matter is observed when galaxies collide. Two galaxies collide, but by observation of gravitational lensing on the light from stars "behind" the colliding galaxies show that much of the matter from each continues moving. This matter (dark matter) is seemingly unobstructed by the collision (because of dark matter's lack of electromagnetic interaction).
I heard this explained to me once, wondering if it is true.
> because of dark matter's lack of electromagnetic interaction
This is still an assumption, because we don't know for sure that dark matter is only weakly interacting, but this is what was observed in the Bullet Cluster collision: https://en.wikipedia.org/wiki/Bullet_Cluster
There's a good explanation here: https://astrobites.org/2016/11/04/the-bullet-cluster-a-smoki...
The lensing was found to be strongest near the visible matter (stars), and the amount of observed lensing couldn't be accounted for with the total stellar mass alone. So the assumption is that the dark matter, like the stars, wasn't slowed much by the collision. The gas, on the other hand, interacts electromagnetically and slows down in the middle.
That's not to say that the stuff that stars are made up of isn't affected by electromagnetism, but it's a short-range effect and even in a collision, interstellar separation is large (i.e. contrary to what you might think, a galaxy collision doesn't involve all the stars smashing into each other, most of them will just sail right on through). Gravity will dominate in that case.
This is still an assumption, because we don't know for sure that dark matter is only weakly interacting, but this is what was observed in the Bullet Cluster collision: https://en.wikipedia.org/wiki/Bullet_Cluster
There's a good explanation here: https://astrobites.org/2016/11/04/the-bullet-cluster-a-smoki...
The lensing was found to be strongest near the visible matter (stars), and the amount of observed lensing couldn't be accounted for with the total stellar mass alone. So the assumption is that the dark matter, like the stars, wasn't slowed much by the collision. The gas, on the other hand, interacts electromagnetically and slows down in the middle.
That's not to say that the stuff that stars are made up of isn't affected by electromagnetism, but it's a short-range effect and even in a collision, interstellar separation is large (i.e. contrary to what you might think, a galaxy collision doesn't involve all the stars smashing into each other, most of them will just sail right on through). Gravity will dominate in that case.
"Gas", here, as usual, means "plasma". Gas does not interact electromagnetically.
Use of the term "plasma" invites banishment from the community of astronomers, and loss of access to telescopes.
Use of the term "plasma" invites banishment from the community of astronomers, and loss of access to telescopes.
It's referred to as plasma in the primary literature: https://arxiv.org/abs/astro-ph/0608407
Many people use hot/ionised gas and plasma interchangeably though. For example: https://academic.oup.com/mnras/article/389/2/967/974617
Many people use hot/ionised gas and plasma interchangeably though. For example: https://academic.oup.com/mnras/article/389/2/967/974617
Fascinating, thank you for the specific example. With a memorable name like "Bullet Cluster" I won't forget it.
The most famous example: the Bullet Cluster
https://www.forbes.com/sites/startswithabang/2017/11/09/the-...
“This argument has been made for over a decade, now, with no satisfactory counterargument coming from detractors of dark matter. It isn’t the displacement of gravitation from normal matter that “proves” dark matter exists, but rather the fact that the displacement only occurs in environments where dark matter and normal matter would be separated by astrophysical processes.”
https://www.forbes.com/sites/startswithabang/2017/11/09/the-...
“This argument has been made for over a decade, now, with no satisfactory counterargument coming from detractors of dark matter. It isn’t the displacement of gravitation from normal matter that “proves” dark matter exists, but rather the fact that the displacement only occurs in environments where dark matter and normal matter would be separated by astrophysical processes.”
When galaxies "collide" stars barely ever get into contact. The interaction is completely dominated by gravity.
What I've never understood about dark matter is if it doesn't interact with the electromagnetic force, why is it found in voluminous clouds instead of collapsing into dense discs or balls? Radiation pressure doesn't affect it, it experiences no impact or friction effects, no thermal effects, so why doesn't it fall to the middle of the galaxies and stay there?
Why should it stay there? Imagine you had a sphere of particles with an initial velocity of 0 that interact only through gravity. At first, these particles would all accelerate towards the center of gravity. Once they reach the center of gravity, they would have accumulated some velocity, and so continue to travel away from the center of gravity (but still be accelerating towards it). Eventually, the acceleration towards the center of gravity would 0 out their velocity again, and they would start falling back towards it. The end result is that your original sphere oscillates indefinantly, but always returns to its original state.
As your initial configuration becomes more random, this discernible time structure begins to fade, and you end up with a voluminous cloud.
What allows regular matter to collapse into dense structures is that it interacts with itself to shed velocity.
As your initial configuration becomes more random, this discernible time structure begins to fade, and you end up with a voluminous cloud.
What allows regular matter to collapse into dense structures is that it interacts with itself to shed velocity.
So then why is its structure and location correlated with galaxies at all? Shouldn't it be relatively even throughout the universe?
I think this is roughly the question why are there galaxies (and clusters, and stars) at all? And the answer is that the primordial soup wasn't perfectly smooth, and gravitational attraction has the tendency to make clumpy things more clumpy. Precisely how un-smooth the soup was, and how clumpy galaxies are today, is quite a large part of the information we have about the history of the universe -- the CMB is a snapshot of the soup right when it came out of the kitchen.
If dark matter was just like matter, then you'd expect it to go along for the ride, and exist now mostly in the same places (and at the same contrast) as normal matter. If it were something else, then its distribution could be much less clumpy, and I think this is a constraint on what kinds of things are plausible candidates.
If dark matter was just like matter, then you'd expect it to go along for the ride, and exist now mostly in the same places (and at the same contrast) as normal matter. If it were something else, then its distribution could be much less clumpy, and I think this is a constraint on what kinds of things are plausible candidates.
I thought in practice it appears to form halos or donut-like formations. It doesn't seem to like the gravitational centers, doesn't like the far reaches of space, but instead is a kind of Goldilocks force that hovers on the outskirts of galaxies.
I should probably qualify that I am not a physisist, but that also strikes me as expected. In theory, a dark matter particle is in orbit of the center of gravity [0]. However, an object is fastest when it is at the lowest point in its object. This would mean that no individual particle is hovering on the outskirts, but each particle spends most of its time there. As a result, at any given time, that is where the majority of the particles are.
I have no idea how elliptical their orbits are, so it is also possible that their orbits just never take them near the center, in which case my initial condition of 0 velocity would be innacurate. (Which is good news unless we want to reinvent the Ether)
[0] I practice, this is probably a nasty n-body problem as the mass is so diffuse, so "orbit" may be misleading here.
I have no idea how elliptical their orbits are, so it is also possible that their orbits just never take them near the center, in which case my initial condition of 0 velocity would be innacurate. (Which is good news unless we want to reinvent the Ether)
[0] I practice, this is probably a nasty n-body problem as the mass is so diffuse, so "orbit" may be misleading here.
I think it's like this: Imagine you have a cloud of dark matter, with a combined centre of gravity. Now imagine a "piece" of dark matter falling down to that centre of gravity. It doesn't impact, or slow down with friction, or anything like that, it just keeps going, through the centre, and out the other side, just as far as it fell before. There's no way to clump together, or stay in the centre.
But it's not electromagnetism etc. which stops the ordinary stars from falling into the middle of the galaxy, either. It's just gravity, they are slowly orbiting the centre of mass of the galaxy.
Maybe it has some sort of internal pressure. Just like our planet doesn't simply collapse into a black hole.
The fact that it doesn’t interact should make it less likely to fall in as it still has intertia (apparently).
Possibly dumb question: Do we know the dark matter proportion of our own galaxy? (in other words, how hard is it to take a selfie, at galactic scales?)
Isn't it to do with the rotational velocity? If it isn't what you'd expect from the matter that you see, you need DM for it to add up.
The existence of dark matter in galactic halos is inferred from the velocity dispersion of stars in a galaxy (sigma). It’s important to realize that this is a very rough kind of estimate because every aspect of it (as in the case of the M-sigma relation) is roughly estimated. Virial mass is inferred from average temperatures of gas by way of detexting X-rays. Central black hole mass is inferred through the M-sigma relation, and stellar masses are inferred through luminosity. Every so often a new paper comes out revising the overall estimates, but the proportion of DM to luminous matter stays pretty constant.
Now, in a sense this makes a “selfie” easier, because you just need a series of snapshots which help you make all of these inferences. On the other hand because we’re in the disk of our galaxy it is difficult to image because we have to deal with all of that hot interstellar gas and dust, and the extremely bright galactic core. I’m not sure how much, when you take into account a long period of time and many wavelengths can be observed, but I’d say we can observe the majority of our galaxy in at least one wavelength. Still, we’re not talking about “selfies” here, just really well educated guesses based on observations of our own galaxy and similar spiral galaxies.
So as far as inferring the existence of a dark matter halo in our own galaxy we’re pretty certain, and it seems to make up about 95% of the galactic mass. It’s based on something a lot more tenuous and complex than a “selfie” though. Once you’ve done your best to estimate the total luminous mass of the galaxy and it’s rough distribution though, and ruled out MACHOs with lending surveys all that’s left in the remaining narrow window is dark matter. The only other theory with any chance at all is MOND, but it has to thread crushingly narrow passages between observation and tests of GR.
Now, in a sense this makes a “selfie” easier, because you just need a series of snapshots which help you make all of these inferences. On the other hand because we’re in the disk of our galaxy it is difficult to image because we have to deal with all of that hot interstellar gas and dust, and the extremely bright galactic core. I’m not sure how much, when you take into account a long period of time and many wavelengths can be observed, but I’d say we can observe the majority of our galaxy in at least one wavelength. Still, we’re not talking about “selfies” here, just really well educated guesses based on observations of our own galaxy and similar spiral galaxies.
So as far as inferring the existence of a dark matter halo in our own galaxy we’re pretty certain, and it seems to make up about 95% of the galactic mass. It’s based on something a lot more tenuous and complex than a “selfie” though. Once you’ve done your best to estimate the total luminous mass of the galaxy and it’s rough distribution though, and ruled out MACHOs with lending surveys all that’s left in the remaining narrow window is dark matter. The only other theory with any chance at all is MOND, but it has to thread crushingly narrow passages between observation and tests of GR.
>"[dark matter] seems to make up about 95% of the galactic mass...crushingly narrow passages between observation and tests of GR"
GR only matches observation because you added in 20x more stuff that is undetectable other than as a deviation from the predictions of GR.
GR only matches observation because you added in 20x more stuff that is undetectable other than as a deviation from the predictions of GR.
That’s true, but it’s also true that hypothesis is preferred because of the abject failure or other theories and hypotheses to match those observations. The window for something other than DM is exceedingly narrow and only MOND has a hope of fitting through, and currently not in a form that’s very compelling.
The point is this "window" isn't narrow at all once you allow dark matter.
Practically any theory plus 95% "dark matter" can match the data.
Practically any theory plus 95% "dark matter" can match the data.
On the tangentially related topic of dark matter + black holes, here's a YouTube video I found interesting on topic. It poses the question "do black holes contain dark matter?"
https://youtu.be/9Qis5VDOd18
The TL;DR is that dark matter can fall into black holes, but black holes don't hoover up nearly as much dark matter as regular matter.
https://youtu.be/9Qis5VDOd18
The TL;DR is that dark matter can fall into black holes, but black holes don't hoover up nearly as much dark matter as regular matter.
Wow. From my armchair it seemed to me much more likely that physics just works a different way at that scale than that there was an invisible substance magically sprinkled evenly throughout most of the universe.
Also from my armchair view as well.
Both of these galaxies are UDG's, keep in mind.
In my uneducated opinion dark matter is an outcome from cluttering too much gravitational force in too small a space.
Both of these galaxies are UDG's, keep in mind.
In my uneducated opinion dark matter is an outcome from cluttering too much gravitational force in too small a space.
Are there any alternative theories that invoke strange interactions of matter/gravity/etc with time? I guess the gravitational wave results out of LIGO would indicate that it propagates across time in a fashion similar to electromagnetic radiation, but if gravitational effects extended beyond 'now' there would seemingly be a possibly small cumulative effect.
One alternative is 'Process Physics' which is not widely accepted but has a fundamentally different explanation for dark matter: Turbulence in the in-flow of space into matter. This flow is PP's model for gravity itself, as if matter continuously 'consumes' space to continue its existence. It's an interesting, quite different and generally totally rejected point of view.
i dont understand what you are saying. What would accumulate?
At least I can say, everything propagates across time. No time, no propagation.
At least I can say, everything propagates across time. No time, no propagation.
I had a naive theory that there is a limit to black hole size/mass that once reached causes gravity to have a more linear strength instead of the inverse square. This discovery blows out that pet theory.
This would seem at first glance to be evidence against entropic gravity?
Could it be that we simply cannot observe all of the actual objects a galaxy contains, and thus we maybe do not need another theory or dark matter?
Pretty much all massive objects except black holes emit light/radiation, and black holes, afaik, are detectable. Thus there's something else that we give a placeholder name "dark matter". It's possible it's not a matter of any kind at all, but just some very smooth but irregular field that interacts with the space-time curvature (gravity).
Today it is dark matter, but tomorrow it is...
https://i.imgur.com/YcsVKGX.png
I don't believe in the dark matter as they are currently selling it. I think we have small eyes on the universe and are missing information we'll probably never get at our scale.
The missing energy could be giant elementary particles (in the standard model), and these "particles" go to bigger-than-black-holes energy, which we could never gauge in a lab, tho I'm pretty sure they can be "predicted" along an energy scale.
The missing energy could be giant elementary particles (in the standard model), and these "particles" go to bigger-than-black-holes energy, which we could never gauge in a lab, tho I'm pretty sure they can be "predicted" along an energy scale.
https://tritonstation.wordpress.com/2018/04/04/the-dwarf-gal...
And keep in mind these uncertainties are arbitrary cutoffs...
Also, others claimed the distance they use (20 Mpc) is wrong. It should be 13 Mpc and this throws off the calculations: https://arxiv.org/abs/1806.10141
I see them still using the 20 Mpc value without addressing those concerns.