New dark matter map reveals cosmic mystery(bbc.com)

87 pointsby tooltower5 tahun yang lalu108 comments

New dark matter map reveals cosmic mystery

https://www.bbc.com/news/science-environment-57244708

113 comments

fifticon5 tahun yang lalu

I am aware I have almost no knowledge about dark matter in physics, but whenever it's mentioned, I can't help but think that our modern 'dark matter', is the 'ether of the 21st century'. That is, an interrim scientific theory that will eventually be completely replaced by revisions of our physics.

da_chicken5 tahun yang lalu

I mean, we need to have a convenient term for a phenomenon even if we don't understand it. What's the alternative? "The unnamed phenomenon were some astronomic bodies behave as though a vast quantity of matter were affecting them but that matter can't be otherwise detected or identified"?

In medicine, they often call it a "syndrome". A collection of symptoms that together form a known pattern that is often closely associated with a disease or disorder but has no understood cause or origin (although some syndromes continue to be called that after they are better understood).

How do you think we arrived at terms like "planets"? We noticed that some stars in the sky moved around while the rest didn't, and we called them "wandering stars". We still call them "atoms" even though that word means "indivisible".

Weather is full of such names: rain, thunder, lightning, wind, morning glory, dew, tornado, rainbow, etc. All these words for material phenomenon existed long before we knew how they worked.

So what's wrong with "dark matter" as a name?

colordrops5 tahun yang lalu

I didn't read a negative judgment of the term in the grandparent comment, just an observation.

da_chicken5 tahun yang lalu

Yes, and I'm challenging that observation. "Making an observation" doesn't mean you're not subject to criticism or questioning.

burnished5 tahun yang lalu

You haven't meaningfully challenged anything they said, though. Your thing was an odd rant about the need to name things (not a hot take), and theirs seemed more focused, well

>>That is, an interrim scientific theory that will eventually be completely replaced by revisions of our physics.

[deleted]5 tahun yang lalu

colordrops5 tahun yang lalu

But you were reading criticism of the naming in the original comment, which wasn't there.

fifticon5 tahun yang lalu

No worry, I did not intend any criticism of 'dark matter'. I understand it as a placeholder concept for 'something we know amazingly little about, except that it appears to be in conflict with the naive version of our observations'. It reminds me of the very complex ways humans calculated the planetary motions, when they believed the sun revolved around the earth - once the proper relation was understood, a comparatively simpler model sufficed. As I understand it, we have plenty of guesses at what dark matter could be, including quantum particles that emit no energy we (currently) can observe. I believe the original theory of ether also sufficed in its time, until we had enough observations that contradicted that model. I'm not saying dark matter is wrong in any way, but it appears to me, it's a place-holder concept for until knowing what is _really_ going on, like 'planet X'.

ASalazarMX5 tahun yang lalu

> The unnamed phenomenon were some astronomic bodies behave as though a vast quantity of matter were affecting them but that matter can't be otherwise detected or identified"

How about "missing matter"? To non-astronomers, dark matter sounds like it has already been found and it doesn't emit light.

tcskeptic5 tahun yang lalu

It's not missing though, is it? It has the characteristic of having effects of mass but not being otherwise observable.

ASalazarMX5 tahun yang lalu

Other comments have suggested better names, based on the fact that we don't even know for sure that it's matter, just that gravity at large scale doesn't behave like it should.

wumpus5 tahun yang lalu

If you say "missing matter", the thing most physicists think of first is probably missing anti-matter, and then missing baryons. Not dark matter.

It's a bit late to suggest new nomenclature for something that's been around for many decades.

dnautics5 tahun yang lalu

> So what's wrong with "dark matter" as a name?

it encodes a normative, biased prejudgement about what we think explains the syndrome.

tracedddd5 tahun yang lalu

What would you prefer the concept be called?

dnautics5 tahun yang lalu

Gravitational anomalies, gravitational discrepancy, unexplained gravity effect? Hell, dark gravity, if you want.

Until the point where the observations are multimodal. Then I'd be more comfortable assigning provisionally normative names. Till then it should probably explicitly reference the fact that all observations are gravitational in nature.

slowmovintarget5 tahun yang lalu

Dark matter works just fine.

It needs to be "matter" in the cosmological sense [1] because of the ratio of protons and neutrons to photons produced during Big Bang nucleosythesis. We know the ratio of the number of protons to number of photons. We know the number of photons in the universe (most of them are in the CMB), and we therefore know the number of protons. That means we know the amount of "normal" matter in the universe, and it is far less than the total amount of matter in the universe (that must have survived from baryogenesis). This requires there to be "dark matter" in the universe.

The other effects like galaxy formation and stellar motion in galaxies are just things that might be caused by dark matter (though that's come into doubt).

Sean Carroll discusses it here: https://youtu.be/tZQadPmTd84?t=4642

[1] "Matter" in the cosmological sense is "stuff" that moves slower than the speed of light, as opposed to "energy" which moves at the speed of light.

karmakaze5 tahun yang lalu

Exactly, the fact that it's a phenomenon gets lost--all we have to do is find the missing mass (X-Y problem) that's causing the gravitational effects.

superduperycomb5 tahun yang lalu

Those are all example of observable phenomena. Dark matter is an inference made by inconsistencies with theory. I think they’re vastly different because it overstates the certainty. Maybe I’m splitting hairs

MR4D5 tahun yang lalu

Ether.

X-rays.

Black hole.

Dark matter.

I think it’s going to stick as a name even when the details have been largely worked out, especially considering the other examples I listed.

caymanjim5 tahun yang lalu

I think most academics studying dark matter are pretty upfront about not knowing what it is. The theories with the most fans tend to not be very specific about the source, but agree that it's most likely an "unknown something" with mass that causes gravity. There are some theories like MOND that instead postulate that there's no missing mass, but that our understanding of gravity needs to be altered at galactic scales.

In any case, it's a placeholder, not a specific theory. There are many candidate theories, but so far they're either untestable or the tests have had negative outcomes.

UnpossibleJim5 tahun yang lalu

Just to put it in context, I'll share one of my favorite (even though I think it's one of the more outlandish and less accepted):

In the Multiverse/Multidimensional universe theory, all of these dimensions aren't necessarily in different universes. Some are of different dimensions, laid over top of ours, but we can't see them because we can't perceive the dimensions they exist in. The only dimension that bleeds into ours, that we can track so far, is gravity. This is what we call "Dark matter". It is actually gravity leaking over from an "Nth" dimension laid over our own that we can't perceive.

Again, as to the veracity I can't say. And I wish I remembered to author, but I don't =/ maybe I'll look it up later and make an annotation.

867-53095 tahun yang lalu

sounds like Brane Cosmology https://en.wikipedia.org/wiki/Brane_cosmology

mr_mitm5 tahun yang lalu

Except the very first experiment that was supposed to confirm the ether showed that there is no ether, while every astrophysical observation we made on any scale [1] confirmed that there is dark matter.

[1] https://www.twitter.com/seanmcarroll/status/1371141258236764...

dnautics5 tahun yang lalu

We don't have any confirmation that it exists, we have more observations that Newton's law is wrong. Moreover, these observations collectively are unimodal; to date we only have observations of more gravitational discrepancies, not corroborating phenomena which correlate to the existing discrepancies.

pdonis5 tahun yang lalu

> we have more observations that Newton's law is wrong

No, we don't. We have observations that show that the motion of stars in galaxies is not what we would predict if we used our current laws of gravity with a distribution of matter that matches just what is visible to us.

There are two possible hypotheses to explain these observations:

(1) There is matter present that is not visible to us;

(2) Our current laws of gravity are wrong and need to be modified.

"Dark matter" is basically a name for #1; "MOND" and similar models are in category #2.

bopbeepboop5 tahun yang lalu

hitpointdrew5 tahun yang lalu

I also have almost no knowledge about dark matter in physics, and maybe I am a complete moron. But what do you think is more likely, there is this matter out there that no one has been able to find in decades of searching, or the mainstream assumptions/math on gravity are wrong/incomplete? I am going with the later.

pdonis5 tahun yang lalu

> what do you think is more likely, there is this matter out there that no one has been able to find in decades of searching, or the mainstream assumptions/math on gravity are wrong/incomplete?

You are mis-stating the real issue here.

The distribution of matter is already a free parameter in our current theory of gravity, General Relativity. GR in no way requires that all matter be visible to humans.

The "no one has been able to find" thing is not a problem for GR, it's a problem for the Standard Model of particle physics, which does not contain any particles that dark matter could be made out of. But most physicists already believe that there are more kinds of particles than the SM describes, we just haven't figured out how to make them in accelerators so we can study them in a controlled experimental fashion, and we haven't figured out any "smoking gun" signature that will tell us if such particles exist out there in the universe. "Dark matter" is really just a name for those kinds of particles that are already believed to exist anyway, we just can't detect them with the technology we currently have. But the technology we currently have is very limited.

So the real question you should be asking is, what do you think is more likely, that there are particles that aren't included in the Standard Model of particle physics--which is already believed to be the case anyway--or that General Relativity is wrong in its description of the dynamics of galaxies, even though this is a domain in which GR has already passed stringent experimental tests with predictions accurate to many decimal places?

I'm going with the former.

mr_mitm5 tahun yang lalu

It's interesting that this sentiment almost exclusively comes from people with almost no knowledge about dark matter.

What do you think is more likely, there is a theory that makes all observations look exactly like there is invisible matter without actually being invisible matter, or that there is invisible matter? I am going with the latter.

I also do not understand why laypeople are of the opinion that all massive particles must interact electromagnetically. If you are open to the idea that there is a massive particle that doesn't, you got dark matter.

jug5 tahun yang lalu

MOND theories etc. like this has fallen kind of out of favor because it’s just so hard to adapt our theory of gravity to resolve this.

It’s surprising how often this topic pops up. I think it’s based on a misjudgment that simply missing something in gravity follows Occams Razor better but it actually seems to be the opposite.

I think the killer is that dark matter seems to not just hold galaxies together (given their “too high” rotational speeds), but also clump together, and also sometimes be missing in galaxies and other times overwhelm galaxies.

It’s super hard to adapt the well proven theory of gravity to especially localized phenomena.

generalizations5 tahun yang lalu

I mean, the name is literally 'the stuff we can't see'. Dark matter is very much just a question mark looking for an explanation.

iainmerrick5 tahun yang lalu

The word “stuff” there is exactly the point. It’s a lot more specific than “just a question mark” -- the hypothesis is that it’s some exotic form of matter.

If something like MOND gains popularity and there’s no longer a need to postulate invisible matter, it would be fair to say that dark matter is incorrect and outdated, just like ether or phlogiston.

dnautics5 tahun yang lalu

Not really, MOND and the like falls into the category of "dark matter theory", and that's the problem with the name.

pdonis5 tahun yang lalu

> MOND and the like falls into the category of "dark matter theory"

No, it doesn't. See my response to your other post upthread about the two hypotheses. "Dark matter" is one hypothesis; "MOND" and friends are the other.

raattgift5 tahun yang lalu

No, it is unfair to call MOND (and at least much of "the like") as a dark matter theory, because it manifestly generates a solution (or solutions) in the absence of any matter at all.

MOND is characterized by an interpolating function between its fundamental constant a_0 and acceleration, such that in the "deep-MOND" regime, F = m \frac{a^2}{a_0} rather than Newton's F = ma.

Both MOND and Newton work in everywhere-vacuum, and in everywhere-vacuum perturbed by a point mass. Neither the vacuum case nor the point mass case qualifies as a "dark matter theory" any more than the vacuum Schwarzschild solution of General Relativity does. These are model universes where one can calculate a Gauss law for gravity.

MOND would behave differently for deep deep extragalactic space with small icy body moving around a large gassy body than Newton would, or than we would get from e.g. a roughly Einstein-de Sitter solution.

Milgrom has pursued several relativistic corrections to the simple MOND formula, and they generally allow for solutions where the stress-energy tensor is everywhere 0, and everywhere 0 except for one point. Milgrom originated MOND. He has collected numerous thoughts and many links at http://www.scholarpedia.org/article/The_MOND_paradigm_of_mod...

When applied to large systems like galaxies and especially galaxy clusters, Milgrom's corrections may be insufficient, and other corrections (catalogued well in https://arxiv.org/abs/1112.3960 section 7). Several of these are very hard to make work with the stress-energy tensor everywhere zero, and those (certainly not all of them) one might ungraciously call "just another dark matter theory". On the other hand, several of these theories admit plausible gravitational radiation in the total absence of matter, just as does General Relativity. (Gravitational radiation is not a feature of Newton or the simple original Milgrom formula.) McGaugh runs tritonstation and is a well known sceptic of particle dark matter.

Relativistic dark matter theories deliberately introduce nonzeros into the stress-energy tensor. You need a distribution of dark matter and trace out how that generates curvature, or you don't have a (relativistic) dark matter theory. You can also of course set out a distribution of cold dark matter and see how it behaves under F = ma or F = m \frac{a^2}{a_0}. Neither MOND nor Newton requires dark matter; likewise neither MOND nor Newton is incompatible with dark matter. Likewise, General Relativity doesn't require dark matter. But one gets more physically realistic solutions at a wide range of scales when one adds dark matter to a defined large-scale distribution of matter and radiation. MOND or the various attempts at generally covariant Milgrom gravitation would also behave differently when one adds dark matter to a large-scale distribution of matter and radiation. That is unlikely to produce more physically realistic solutions to a wide range of scales, however.

Indeed, Milgrom et al's most concrete argument is that their formula for gravitation is worse if one adds in a distribution of dark matter, and that their formula is better than e.g. a Lemaître-Tolman-Bondi solution equipped with a distribution of cold dark matter. That's as much a non-dark-matter theory as I think it is possible to have!

dnautics5 tahun yang lalu

Forgive me, but I was under the impression the phrase "dark matter theory" can confusingly refer to something that generally explains galactic rotation curves whether or not it's a theory that involves dark matter. What would you call the general category of theories which explain galactic rotation curve deviations?

raattgift5 tahun yang lalu

Maybe just solutions for non-Keplerian redshifts, see http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/darmat.html second image and bracketing paragraphs.

As in computing, naming things is hard.

The redshifting things are mostly molecular gas clouds which we would expect would orbit in a Keplerian fashion given the overall distribution of luminous matter and occluding/extinguishing dust visible in galaxies. Astrophysicists love hydrogen gas clouds because they ionize easily from background emitters (stars, quasars) producing very sharp spectral lines, and you can pick up their changes by looking on different sides of an edge-on spiral or are at nearly perpendicular angles to the rotational axis of spheroidals. For giant ellipticals, which have practically zero axial rotation, an instrument that gets a sweeping view of them will see a more chaotic red-and-blue shifting of the Lyman-alpha hydrogen line, rather than an obvious gradient between advancing and retreating edges. (Face-on spirals often also have gas clouds on excursions back and forth in front of the disk plane, where "forth" means "towards us" means "relatively blueshifted" and that's hard to explain without a halo rather than a ring of dark matter).

So really it's a question explaining the non-Keplerian https://en.wikipedia.org/wiki/Peculiar_velocity of structures from ionized hydrogen to molecular clouds to supernovae with highly predictable light curves.

While basic MOND can explain edge-on spirals and axis-perpendicular spheroidals and lenticulars, it does a bad job with face-on spirals, ellipticals, and lots of other unusual shapes in the galaxy zoo. It also doesn't do well with entire clusters, where there are enormous gas clouds between the member galaxies, and often a bright quasar (or at least central galaxy) really lighting them up. Corrections to basic MOND patch some of these up, mostly starting by adding extra terms to the initial basic Milgrom formula which modified F = ma.

A ball-like distribution of dark matter around these very large structures (galaxies, clusters) on the other hand is a decent explanation, with the major drawback that the only sufficiently transparent stuff we've produced in labs is simply too light to stay constrained within these structures (electron neutrinos and their antis would fly away to infinity, for example, rather than stick around in a hierarchy of halo/shell/ball structures; free neutrons aren't stable). We also don't have a really good explanation about why these proposed dark matter structures don't have a strong density gradient leading to grossly non-Keplerian orbits of stars in the cores of galaxies (including our own). That's why you get wildly speculative ideas like phase transitions, "dark chemistry", dark-matter/dark-matter annihilation, and so on. There are similar questions about why the outer reaches of dark matter halos don't blow off into infinity rather than lingering around. ( See the excellent III A and III E of Adams & Laughlin https://arxiv.org/abs/astro-ph/9701131 or the Binney & Tremaine textbook https://press.princeton.edu/books/paperback/9780691130279/ga... )

Good explanations should cover the entire lifetime of a galaxy including the things that lead up to it and its ultimate decay. This also means it should be good for the oldest galaxies that we can see. It also means that as we get better at simulating initial value problems we can take a large number of observed quantities of a highly-redshifted galaxy and evolve that initial snapshot step by step such that after trillions of steps we end up with something that looks remarkably like a galaxy with low redshift. We're not really there yet. Work harder, supercomputer builders! :D :D

dnautics5 tahun yang lalu

The problem, IIRC, is that we don't have an explanation of why those distributions of DM are balls around galaxies and not balls in the intergalactic medium. It seems like the distribution of lambda-CDM could be anything. So much so, that people are postulating that the core of the milky way galaxy could be an ultradense ball of lambda-CDM.

https://phys.org/news/2019-12-dark-center-milky.html

raattgift5 tahun yang lalu

> we don't have an explanation of why those distributions of DM are balls around galaxies and not balls in the intergalactic medium

We do. Structure formation is part of the standard cosmology. https://www.esa.int/Science_Exploration/Space_Science/Planck... https://kipac.stanford.edu/research/cosmic-structure etc etc.

Matter fell into place wherever it could, forming overdense areas. More matter tended to fall into overdense areas from underdense areas, and this allowed the underdense areas to expand more easily while the overdense areas were constrained to expand more slowly or even to collapse. Dark matter, not being able to collide and radiate away the collision-energy collapses much more slowly than heavy matter.

Dark matter connects fluctuations in the cosmic microwave background with large scale structures -- the cosmic web -- extremely well, as long as it moves nonrelativistically (that's why it's "cold" compared to electron neutrinos which move ultrarelativistically) and collapses very slowly.

> ultradense ball of lambda-CDM

Mu?

Lambda (capital greek letter) is the term that encodes the expansion of space and the consequent separation of clusters of galaxies from one another. It's irrelevant within clusters of galaxies, let alone individual galaxies. The solar system is not expanding at all, and neither is Manhattan.

Lambda is also a constant. It doesn't form balls, there are no overdensities or underdensities, it has the same tiny value at every point in spacetime.

CDM is cold dark matter, and your phys.org link discusses evidence for a possible explanation about why there isn't a high density "ball" of dark matter in the galactic centre compared to around our solar system: maybe dark matter can interact electromagnetically under conditions found in galactic cores. The electromagnetic interaction in this study would produce a gamma ray and the vanishing of a dark matter particle. This could be some complicated way dark matter can self-annihilate in the presence of very hot standard model gas like that around black holes or produced by supernovae, for instance. Whatever the specifics, this conversion of dark matter to gammas would keep the central dark matter relatively sparse.

(Other mechanisms for avoiding high dark matter densities in galaxy cores are available. These do have a good collective name: galactic outflow, of which dark matter heating and dark matter decay are subcategories that are at the root of your phys.org link. Generally dark matter turns into hot matter, or drags hot matter along with it for a ride. Outflow must enrich the metallicity of stars far from galactic cores, and quench core star formation, and these generate observables for individual proposed mechanisms, and astronomers are already happily gathering up data that probe those observables. Good proposals for outflow mechanisms also work on the scale of structures like the Coma Cluster).

generalizations5 tahun yang lalu

That would be fair. However, from what I've read MOND is relatively unlikely. If its proponents can address the biggest difficulties (e.g. not every galaxy appears to have "dark matter") and make it somewhat more feasible, then I'd agree the name is insufficiently broad.

Hemospectrum5 tahun yang lalu

You've got plenty of scientists as company. There are cute names like WIMP and MOND for different types of hypotheses about what the heck dark matter actually is, or how to explain what's going on if it doesn't exist at all. Each of these is a huge headache for astrophysicists in one respect or another.

[deleted]5 tahun yang lalu

keithnz5 tahun yang lalu

Sabine is a great science (physics) communicator to the "masses", she recently did an update on dark matter. https://www.youtube.com/watch?v=4_qJptwikRc

Hypx_5 tahun yang lalu

Summary: Both dark matter and modified gravity are true. Could be caused by wave-particle duality.

interestica5 tahun yang lalu

> 'ether of the 21st century'

Or 'caloric' or 'phlogiston'

https://en.wikipedia.org/wiki/Superseded_theories_in_science

tigerlily5 tahun yang lalu

Phlogiston, now there's a word I haven't seen in a while. A lot of big names on that list of superseded theories by the way :)

kiba5 tahun yang lalu

Dark matter is not a theory, it's a name for an unexplained phenomena.

passivate5 tahun yang lalu

But in modern science we differentiate between facts/hypotheses/laws/theories. What you're referring to is a hypothesis - and those should be replaced with a more concrete explanation - once we have it. I don't quite understand your point about connecting it to ether though. I can't think of any established theory in modern science that was completely replaced - but I'm not a physicist either.

salty_biscuits5 tahun yang lalu

As a counterpoint, Neptune and Pluto were "dark matter" prior to confirmation with observations. They were theorized to exist due to their perturbations to observed motion of known planets.

dnautics5 tahun yang lalu

Well by that token so was the discrepancy in the precession of mercury (which was not, as it turns out,matter).

pdonis5 tahun yang lalu

> so was the discrepancy in the precession of mercury

No, it wasn't, because, as you note, that observation was accounted for by adopting new laws of gravity, not by adding more matter to the distribution of matter we used in our model. Those are two different things.

nimish5 tahun yang lalu

It was thought for decades the anomalous precession of Mercury could be due to matter, including an entirely new planet, Vulcan.

pdonis5 tahun yang lalu

Yes, that was one hypothesis, but it didn't turn out to be the correct one. The correct hypothesis was that Newtonian gravity was wrong and had to be replaced by General Relativity. That hypothesis is not a "dark matter" hypothesis. But the poster I was responding to, dnautics, is claiming (not just here but elsewhere in this thread) that it is.

dnautics5 tahun yang lalu

That's exactly the point. It was a dark matter hypothesis until they found out it wasn't actually matter. Physicists couldn't go back in time and tell themselves, hey, this isn't actually matter.

By analogy suppose we find out in 35 years that some non-lambda-cdm, non-particle theory is correct? It would be revisionist to say we didn't believe in dark matter today.

pdonis5 tahun yang lalu

> It was a dark matter hypothesis until they found out it wasn't actually matter.

If you mean that nobody considered modifying the theory of gravity to explain the precession until Einstein and General Relativity, that is not correct. By the 1890s pretty much everyone had given up on finding another planet and a number of astronomers were considering modifications to the law of gravity.

In any case, you are shifting your ground from the comments you have made elsewhere in this thread. Elsewhere in this thread, you have been claiming that "dark matter hypothesis" includes the hypothesis that we should change the laws of gravity instead of changing the matter distribution in our models. Now you are admitting that those are two different things, which is the point I have been trying to make all along.

> suppose we find out in 35 years that some non-lambda-cdm, non-particle theory is correct? It would be revisionist to say we didn't believe in dark matter today.

"Believe in" is the wrong term. Dark matter is a hypothesis (just as suggesting there was another planet causing Mercury's excess orbital precession was a hypothesis), which appears to be the one most cosmologists currently think will end up being the right one. But that does not mean anyone claims that it is the only hypothesis, or that that hypothesis includes the hypothesis that we should modify the laws of gravity. The latter is understood by all cosmologists to be a separate hypothesis. Just as modifying the laws of gravity to explain Mercury's excess orbital precession was understood by all at the time to be a separate hypothesis.

dnautics5 tahun yang lalu

all hypotheses fundamentally are a belief system, it does no good to assert otherwise.

> you have been claiming that "dark matter hypothesis"

No. I have claimed that the class of dark matter theories includes those which reject the existence of "dark matter" under the (possibly incorrect) assumption that the historical term "dark matter theory" refers to any theory that explains galactic rotation curves, as coined by Zwicky and cataloged by Rubin. In general, "A theory of X" can certainly reject X, and replace it with something else.

pdonis5 tahun yang lalu

> I have claimed that the class of dark matter theories includes those which reject the existence of "dark matter"

And this claim is incorrect. "Dark matter" is a particular hypothesis for explaining the discrepancy in galaxy rotation curves (and other observations as well, such as features of the CMB that cannot be explained with a model that only includes the matter visible to us). It is not just another name for the discrepancy itself, or for any theory that claims to explain the discrepancy by any mechanism whatever.

> under the (possibly incorrect) assumption that the historical term "dark matter theory" refers to any theory that explains galactic rotation curves

And this is an incorrect assumption. That's what I've been saying all along. See above.

RedComet5 tahun yang lalu

Ether very much exists. Its just now referred to as emptiness or "the vacuum of space".

Ono-Sendai5 tahun yang lalu

Also known as fields (as in fields from quantum field theory)

1_player5 tahun yang lalu

It doesn't. Aether was supposed to be an element, substance that filled the emptiness of space, not the emptiness itself.

RedComet5 tahun yang lalu

You seem to be mistakenly conflating the modern conception of chemical elements with classical philosophical notions of elements.

megablast5 tahun yang lalu

>I am aware I have almost no knowledge about dark matter in physics

Thank you for introducing your comment like this so I can give it the true weight it deserves.

claytongulick5 tahun yang lalu

Even "Ether" has come back around a bit in certain circles. It's not called that anymore, I think it's called the "Grid" or something similar. Milo Wolff discusses some of this in his WSM theories, IIRC.

floxy5 tahun yang lalu

Off topic a little bit, but has anyone checked the work of the guy who was trying to explain galactic rotation curves using general relativity and gravitomagnetism? Which would lessen the need for dark matter.

https://link.springer.com/article/10.1140/epjc/s10052-021-08...

https://news.ycombinator.com/item?id=26442021

knzhou5 tahun yang lalu

Gravitomagnetism is a well-understood and experimentally measured effect. It is also a very small effect, of the order v^2 / c^2 where v is the speed of the sources. In the galaxy, stars move with v/c ~ 1/1000, which means the gravitomagnetic correction is one in a million. So while N-body simulations do sometimes account for general relativistic corrections like these, they're not nearly large enough to remove the requirement for dark matter.

The main thing the paper should do is explain why they think the correction is a million times larger than the back of the envelope estimate. But they don't. Instead, they try to solve everything analytically, never plugging in numbers or reasoning about what's big or small, leading to a forest of long combinations of special functions. That's a reliable recipe for making a mistake.

That is the simple reason the paper has been ignored by everyone in the scientific community and rejected from decent journals. Of course, this hasn't stopped hundreds of fluffy pop articles being written on it, or it getting posted every week on HN. The blind leading the blind.

floxy5 tahun yang lalu

> Instead, they try to solve everything analytically...

This seems like a common refrain it lots of things I see (not just this one paper). Can anyone give a lay man's explanation why we can't just numerically simulate general relativity? As in, plug a simulation with 100 billion stars in to a super computer and see what comes out.

ncmncm5 tahun yang lalu

It ought to suffice to simulate the motion of exactly one star, in a circular orbit, for exactly one time-step, just adding up all the effects of each of the 1e11 or so other stars, plus interstellar medium. There are two possible end states: either it follows the circle--no dark matter needed--or it swings wide.

ncmncm5 tahun yang lalu

That is not the reason the paper has been ignored. If it really were wrong, somebody would say where. But most astrophysicists are nowhere near as familiar with the maths involved as the paper's author is.

It is ignored because it is inconvenient. There is no practical consequence for continuing to be wrong, in cosmology or astrophysics. You can be wrong and publish papers, be wrong and get hired, be wrong and get tenure. Meanwhile, there is no upside in letting dark matter have no role in galactic rotation curves. Feeling smug knowing everybody else is still deluded is a solitary vice. If it's right, that will probably have to be acknowledged someday, but there is no personal benefit to getting ahead of the curve, only irritation.

Cosmology has found myriad uses for dark matter besides patching up galactic rotation. Accepting reality means you need to explain why all the dark matter you have been using for these other things doesn't clump up into galaxies; or find some other way to explain what you have been using dark matter for. Dark matter is just too convenient: like the Schmoo, it can be almost anything you like, as much as you need, wherever you need it. Your use doesn't even need to be consistent with (almost) anybody else's.

When it finally becomes necessary to accept reality, no one will be embarrassed, because everyone will have lots of company, and it will never be mentioned again, at least anywhere polite.

knzhou5 tahun yang lalu

This is why I left this site. Endless smug engineers explaining condescendingly to physicists why they’re stupid sheep, without knowing the first thing about anything. Intellectual curiosity, my ass. Do you have a reply to my concrete criticism or not?

ncmncm5 tahun yang lalu

You don't have a concrete criticism. You just have a complaint. Either the math in the paper is right, or it's wrong. If it's wrong, say where. If you can't find anything wrong, say that.

the84725 tahun yang lalu

Thanks, I assume the same cricitcism also applies to other applying-GR-corrections papers? E.g. I saw one about gravitational self-interaction leading to concentrating gravity inside galaxies and starving the outside or something like that.

knzhou5 tahun yang lalu

The same criticism applies for any other crappy paper HN likes. Whenever I check this site, half the time the front page has something even worse. Just assume everything you see here is wrong.

nimish5 tahun yang lalu

The math is correct. Linearized GR and Newtonian gravity are qualitatively and quantitatively different and at "relativistic" scales we should probably see a difference!

I don't have the cosmology background to evaluate more than that unfortunately; it's just that the "Standard Model" of cosmology uses a toy solution of GR (FLRW + newton).

That said, it'd be wild if any major case of dark matter is just an artifact of incorrect approximations.

[deleted]5 tahun yang lalu

raattgift5 tahun yang lalu

"Has anyone checked the work?"

tl;dr: no, probably not.

(I am aware of informal comments which have raised questions about whether the disk is a singularity, which would destroy the possibility of solving an initial value problem, unlike already-in-use approaches. Additionally, the rotating disk developed in the paper is clearly not present in non-axisymmetric elliptical galaxies dominated by radial motion, and so cannot replace dark matter in them; the paper only deals with disk-like approximations of spiral and axially-rotating spheroidal galaxies. There are plenty of galaxies where there's no common rotational axis, but there's still a rotational curve problem for stars and hydrogen gas clouds moving inwards vs outwards. Lastly, the paper only claims to be a good approximation in the limit of weak gravitational fields, so very dense galaxy clusters (which will include the future collision of the Andromeda galaxy with our own) are not covered by the work in this paper: it makes no claim to be able to predict the outcome of that collision, which breaks the Vlasov condition. When you collide the dust and gas in these galaxies, or galaxies like them in our sky, you will see lots and lots of X-Rays and the like, while some fraction of actually collisionless matter would deform the galaxy and red/blueshift its component spectra.)

This is the work in question, fed into Google Scholar (sauce for the goose as for the gander: Springer adds Google Scholar links to each of the author's references) using the first URL you supply:

https://scholar.google.com/scholar?hl=en&as_sdt=0%2C5&q=http...

We then hit "Cited by 3"

https://scholar.google.com/scholar?cites=8505897096354068536...

Not much there, and two are author self-cites, with no citations on the newer papers, and no collaborators on these papers.

Glancing briefly through the two newer ones, I got distracted by one inconsistency which strikes me as glaring because the "novel form" (author's words) the author builds strongly hangs off it: I found "pseudo tensor", "pseudotensor" and "pseudo-tensor" at the very least, and promptly gave up reading more deeply. Choose just one, please, or don't try to use them at all [1].

Aside: EPJ+ charges authors a USD 3280 fee to publish each article. It's also not a journal working cosmologists or extragalactic astrophysicsts would follow closely.

Fortunately, readers without institutional access can find (again, via Google Scholar, at the link above) essentially the same material on researchgate (which says nothing either way about quality) so one can glance without handing Springer $30+ for the two newer articles. Even more fortunately, the link you supplied is open-access, and can be read there (the PDF is nicely formatted) without paying a fee.

Finally on this point, we can look the author up and see numerous papers with collaborators in (mainly terrestrial applications of) plasma physics, but the only papers on astrophysics (in the broadest sense) are those three most recent ones.

https://scholar.google.com/scholar?hl=en&as_sdt=0%2C5&q=%22G...

My comments above are nothing at all like "checking the work", but rather an excuse for why I personally would be in no rush to do so.

It was however momentarily amusing that a partial theory designed to eliminate a large amount of cold collisionless non-radiating matter in a galaxy (the dark matter component) models the entire galaxy, stars and all as a cold collisionless non-radiating dust (whose individual "motes" generate the Vlasov fields which govern the dust's motion), and that this idea has excited some dark matter self-styled sceptics.

Lastly, the idea is not stupid. If we try to represent gravitational interactions as particle exchanges, there will be charges and potentials, and that this can strongly resemble electromagnetism has been intriguing physicists for many decades. The author is exploring a flavour of which, where the sources and potentials are similar to what is familiar to a plasma physicist. However, the writeups of this idea, found at the links above, are simply not general enough for our diverse zoo of real galaxies. It is also not general in scale: it's not useful at solar system scales (we are probably eventually interetsed in the behaviour of dark matter in things like the Hulse-Taylor system, or black hole binaries, etc.), at galaxy cluster scales, or for understanding the small fluctuations in the cosmic microwave background and their apparent connection to large scale structure.

- --

[1] Tensors (not pseudo-tensors) are enormously useful in General Relativity, because a tensor solution solved in one set of coordinates is solved in all systems of coordinates (including no coordinates at all). Matter should be specified as tensors. This has been done (e.g. the (Faraday) https://en.wikipedia.org/wiki/Electromagnetic_tensor). This lets us arrive at an understanding that works for you standing on your part of Earth using local notions of up, down, left, right, forward backward; and me standing somewhere else on Earth using my local notions of the same directions. It also works for people in the ISS who have to pick a conventional up/down, and whose local clocks tick fast (from our standing-on-Earth's-surface perspective). It also works for coordinates covering the solar system, the Milky way, or the entire cosmos. But not all physical systems have fully developed tensor theories. For them we may choose to introduce a pseudo-tensor, which are valid only for certain coordinate systems. A solution in one of those will work for some other systems of coordinates but not all, and if one is not careful in choosing coordinates, one can get things spectacularly wrong, like wildly wrong recoveries of the components of the energy-momentum 4-vector ("In some coordinate systems you think there is energy present when there is no energy in other coordinate systems" is a common symptom, and in this context means "you think you do not need a generator of curvature beyond the dust encoded in the pseudo-tensor"). If one is inconsistent about the spelling of pseudo-tensor, I think that's a bad sign about the extreme care in keeping different coordinate systems mutually and fully consistent when using them.

One runs into pseudo-tensor inconsistencies fairly often in cosmological contexts, so much so that it was dealt with in a USENET sci.physics FAQ entry: https://math.ucr.edu/home//baez/physics/Relativity/GR/energy...

[deleted]5 tahun yang lalu

miohtama5 tahun yang lalu

What is the projection used to display ”the Universe” maps? What is the left edge, the right edge, etc. They seem to be similar in all articles but I have no idea how map relates to e.g. a night sky?

Sharlin5 tahun yang lalu

The illustration in the article ([1]) seems to be a (slightly cropped) Mollweide projection [2], in familiar equatorial coordinates such that the north pole is top center, south pole is bottom center, and the equator is a (imaginary) horizontal line in the middle. The region of acquired dark matter data is located in the southern hemisphere sky, which makes sense given that the observatories used were in Chile. At the bottom you can see the Magellanic Clouds, satellite galaxies of the Milky Way which can only ever be seen from the southern hemisphere.

[1] https://ichef.bbci.co.uk/news/976/cpsprodpb/F39E/production/...

[2] https://en.wikipedia.org/wiki/Mollweide_projection

tshaddox5 tahun yang lalu

I believe that image in the news article is cropped from this: https://www.darkenergysurvey.org/wp-content/uploads/2021/05/...

I found that image on this page, which seems to the be web page created by this research group about this project: https://www.darkenergysurvey.org/des-year-3-cosmology-result...

From what I can tell, it's a projection of a view from the night sky from one particular telescope on Earth. They sampled a bunch of galaxies in an area about one eighth of the night sky.

Sharlin5 tahun yang lalu

The background starfield appears, in fact, to be a projection of the entire celestial sphere, all 360°×180° of it, so only a half of it at most can be seen at a time from any point on Earth.

motloch5 tahun yang lalu

Check out Fig 10 in https://www.darkenergysurvey.org/wp-content/uploads/2021/05/...

mellosouls5 tahun yang lalu

Mostly mysterious here is the detail from unconnected parties as to the significance of the supposed deviation.

It reads mostly like a puff piece from an academia PR person. Way too vague in the article itself.

_rpd5 tahun yang lalu

To be fair, there were 30 new papers published:

https://www.darkenergysurvey.org/des-year-3-cosmology-result...

The overview paper appears to be this one:

https://www.darkenergysurvey.org/wp-content/uploads/2021/05/...

(the first published paper at the bottom of the page. I'd have posted the title and abstract, but they've disabled copy-paste.)

motloch5 tahun yang lalu

Yeah, as a cosmologist my reading of the DES results is exactly the opposite than what the article mentions - new DES results seem to show that one mystery (low values of S_8 in the low redshift measurements) was actually resolved..

See the bottom of their Fig 14 in https://www.darkenergysurvey.org/wp-content/uploads/2021/05/...

The problem was that the green (CMB/Planck) and gray (weak lensing) curves in the past were not overlapping as well as they are now. To me the tension seems to mostly go away now.

tooltower5 tahun yang lalu

Another article in the Guardian: https://www.theguardian.com/science/2021/may/27/astronomers-...

grenoire5 tahun yang lalu

It looks fascinatingly close to Perlin noise in its cloudiness.

mturk5 tahun yang lalu

When generating cosmological initial conditions for use in simulations of large-scale structure, one common method is to utilize random numbers (distributed about a known power spectrum) in k-space and then transforming back to real space. Some details can be found here: https://enzo.readthedocs.io/en/enzo-2.3/_downloads/makeics.p...

aaroninsf5 tahun yang lalu

Pet theory: "dark matter" is gravitation leaking from adjacent universes, in specific, from the juxtaposition of many divergent universes.

Most diverge in trivial ways, so dark matter concentration is correlated with where matter is in our universe.

Some relatively few diverge more drastically, so there is a background.

As someone outside the field I remain curious if there are reasons this is inconsistent with observations to date, for some models of the multi-world hypothesis.

hef198985 tahun yang lalu

Flying through hyperspace is not like flying over fields. Said one famous pilot doing the Kessel Run in less than 12 parsecs. Maybe Dark Matter is the real space shadow of stuff in hyperspace the same way real space stuff has shadows in hyperspace.

Seriously, I cannot judge this level of physics, or just understand them. It would make a nice addition to Star Wars lore so.

jfengel5 tahun yang lalu

Yes, it's inconsistent with observation.

1. It would tend to imply dark matter having the same shape as the matter in galaxies. It doesn't. So it doesn't explain the galactic rotation curves, which is the whole reason we hypothesized dark matter in the first place.

2. It doesn't explain the Bullet Cluster.

3. It doesn't match why some galaxies have more dark matter than others.

4. It doesn't explain Baryon Acoustic Oscillations.

pdonis5 tahun yang lalu

> for some models of the multi-world hypothesis

What "multi-world hypothesis" are you talking about?

gtsop5 tahun yang lalu

The only mysteries here are:

1. The certainty that an infinite entity (universe) can be determined to have a very specific percentage (80%) of something (dark matter)

2. The fact that the big bang is still being considered and cited as the beginning (?) of the universe

3. That we have accepted terms such as "dark" matter and "dark" energy in science as if some witchcrafty comes into play, as if lord voldemort created the cosmos.

Analog245 tahun yang lalu

In cosmology, the "Universe" typically refers to the observable universe which is far from infinite. There is nothing to "accept" about the terms "dark matter" and "dark energy", they are colloquial terms that describe two unrelated phenomena that we have yet to explain (hence the "dark" part). Just because you have an incorrect assumption about what the informal names imply doesn't mean others do as well.

modshatereality5 tahun yang lalu

unobserved hypothetical variables used to explain a big assumption.

Analog245 tahun yang lalu

What are you talking about? "Dark Matter" and "Dark Energy" are terms used to describe to _obeserved_ phenomona that we can not explain. And I'm not even sure what a "hypothetical" variable is.

modshatereality5 tahun yang lalu

They have observed discrepencies in the calculations, meaning something is either wrong with the formula, or something unobservable actually does exist and manipulates the universe in such an (edit: so far) unexplainable way.

gtsop5 tahun yang lalu

> Just because you have an incorrect assumption about what the informal names imply doesn't mean others do as well.

All kinds of names are given by people to reflect the essence of the entity being named. The name I give to something reflects what I think of it or something about that thing. The use of the word "dark" here is being used to reflect (as you said) our inability to yet explain the described phenomena which, to me, sounds like medieval people talking of witches. These phenomena could have easily be named "Analog24's energy" or "gtsop's matter" like a thousand other forces, particles, laws. Hence, what we have to "accept" is a term that reflects the inability of the human intellect to discover something. It is just silly and I think it shows something about the mentality of contemporary physics

I hope my point is clearer now

junon5 tahun yang lalu

The real mystery here is your point.

gtsop5 tahun yang lalu

My point is that contemporary physics is disoriented by hooking onto unftuitful notions

junon5 tahun yang lalu

That's a rather broad assertion. Can you elaborate?

gtsop5 tahun yang lalu

Late reply, but just in case:

It is indeed a broad assertion. If I could summarize it, I would say that physicists in general (from the prism of the public view) is oriented towards "initial" and "final" states of time and space rather than infinite processes. E.g "big bang" and "heat death" as the start and end of everything. At the same time physics has given an insane amount of effort on the gravitative qualities of the universe but not the repulsion ones. We've known for centuries that for each - there is a +. We've spent so much time on figuring out mass/gravity, the (+) . Where is the - in the universe? We detect a gravitational effect of unknown source: let's throw in there some imaginary dark matter (since we're so familiar with it's gravitational attributes) instead of making a hypothesis that an opposite force might come in play here.

You get my point? Finate, measurable, terminal states and one-sided forces instead of infinite processes and interraction of completely opposite forces: that's the general oriantation of where i see physics going despite the myriads of hints the universe gives us to search in the opposite way.

chadcmulligan5 tahun yang lalu

My theory is dark matter is the shadow of angels, there's more of it where new galaxies are forming because there's more angels there building the galaxies :-).

Here's a movie of it in progress https://youtu.be/1x3RRrqJWKA

I know this will get downvoted, but wouldn't it be funny if it turned out to be angels and we're looking at heaven. It's possible.

bpiche5 tahun yang lalu

I upvoted you. It's almost more plausible than the existence of a ton of tiny leptons that don't interact with the rest of the universe at all..

ccvannorman5 tahun yang lalu

:D :D T_T

jjcon5 tahun yang lalu

I wish I could be as open minded as you, maybe one day

chadcmulligan5 tahun yang lalu

Well thank you, I keep my more wacky theories to myself :-)

EthanHeilman5 tahun yang lalu

I know you got downvoted into oblivion but I enjoyed that short movie. It changed the way I look at the moon. Thanks for sharing.

chadcmulligan5 tahun yang lalu

Glad you liked it, here's another in a similar vein - Abiogenesis - https://www.richardmans.com/film. The amount of work that goes into these for no chance of profit always amazes me.

OldGoodNewBad5 tahun yang lalu

Dark matter is the name for compound mathematical errors based upon poor assumptions. We can't re-examine fundamental constants or established theories, so we'll be kludging around forever with this.

It's not really important though because the practical application of cosmology is nil.

Analog245 tahun yang lalu

This is incredibly ignorant and pretty arrogant to actually think that you know better than thousands of researchers who presumably know far more about this than you. There are numerous independent and unrelated pieces of evidence that point to the existence of dark matter. You honestly think they're all the result of mathematical errors that just so happen to all point to the same conclusion? That would be quite a coincidence.

EthanHeilman5 tahun yang lalu

Dark matter could be due to some fundamental mathematical or measurement error that is deep enough to cross multiple independent research efforts. However given the lack of evidence for it, such an error seems unlikely but not unthinkable.

Analog245 tahun yang lalu

As I said, it would be a huge coincidence. And no, this could not be due to a measurement error for the reason I already states: there are numerouse indpenedent observations, of completley different consequences of dark matter that all point to the same thing. At this point you can safely say it's statistically impossible to chalk this up to measurement error. I will concede that is possible that it could be due to an error in our understanding of the laws of physics at such large scales but the current evidence does not favor this conclusion.

ggggtez5 tahun yang lalu

> practical applications are nil

It's 80% of mass on the universe. You don't think that has applications in space travel? Maybe it doesn't effect your day to day life, but it's rather ridiculous to say there is no applications at all.