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The possibilities for dark matter have shrunk

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Re: The possibilities for dark matter have shrunk

#31

Post by throwawaymaths »

dylan604 wrote: post #21
FactolSarin wrote: post #7

Earlier quoted context omitted.

We've also seen instances where visible "clumping" in a galactic disc correspond to differences in the rotation curves, which dark matter can't really explain. But this is still a topic of much debate. However, there's lots of good evidence for dark matter besides rotation curves of galaxies. For instance, models of galactic formation work a lot better with it than without it (without dark matter, as galaxies coalesc…

I wasn't familiar with the Bullet Cluster[0], but as you described it made it very interesting. I'm no astronomer, but from all of the examples of gravitational lensing[1] I'm familiar with have a very distinct look that I'm just not seeing with the Bullet Cluster. Where is the lensing effect occurring that is leading to this theory? [0] https://en.wikipedia.org/wiki/Bullet_Cluster [1] https://en.wikipedia.org/wiki/G…

It's "weak" lensing, which is statistical guesswork based on very faint distortions in images

Re: The possibilities for dark matter have shrunk

#32

Post by throwawaymaths »

marcosdumay wrote: post #25
slowmovintarget wrote: post #16

So this rules out many kinds of WIMPs, but not axions, or the more exotic white holes of loop quantum gravity. There's also the JWST observations which oddly seem to fit certain MOND scenarios better. Or Claudia DeRahm's massive gravity.

Shouldn't axions interact with atom nucleus? This rules out any interaction except for gravitational, right?

To a certain floor. Of course a particle which only interacts gravitationally is still possible but that's the cosmologist's nightmare. It'd be basically impossible to prove it's existence and anything could happen (like a small but fast fairly massive ~asteroid mass clump of dark matter explaining poltergeists)

Re: The possibilities for dark matter have shrunk

#33

Post by throwawaymaths »

AnimalMuppet wrote: post #15

Summary: An experiment that looked for WIMPs (weakly interacting massive particles) found none with masses above 9 GeV. So either they exist but are too rare for the experiment to see, or they are massive but not that massive*, or dark matter isn't WIMPs (or dark matter doesn't actually exist at all).

Dark matter could still be NIMPs

Re: The possibilities for dark matter have shrunk

#34

Post by empyrrhicist »

mppm wrote: post #4

By the way, apart from the fact that dark matter predictions have been contradicted by increasingly sensitive experiments over and over and over, there is another inconvenient piece of observational evidence: An analysis of stellar velocities at the edge of the Milky Way, published last year [1] shows that they are "Keplerian", i.e. following classical behavior without the influence of any purported dark matter. So e…

> apart from the fact that dark matter predictions have been contradicted by increasingly sensitive experiments over and over and over

My impression is that many physicists would disagree with this characterization entirely, and that they're eagerly working to constrain what it is or isn't. Ruling out big classes of phenomena that could be responsible for the things we observe isn't "contradicting" the predictions in the sense implied.

Re: The possibilities for dark matter have shrunk

#35

Post by Enginerrrd »

slowmovintarget wrote: post #16

So this rules out many kinds of WIMPs, but not axions, or the more exotic white holes of loop quantum gravity. There's also the JWST observations which oddly seem to fit certain MOND scenarios better. Or Claudia DeRahm's massive gravity.

I think white holes actually pop up from calculations outside of LQG as well.

Specifically I seem to recall there's something using Einstein-Cartan as a basis, written by someone of note which predicts white holes as a permanent endpoint of evaporation. Then again, maybe it was rovelli?

Alas, I can't seem to find the paper now. Google sucks and keeps dumbing down my query.

Re: The possibilities for dark matter have shrunk

#36

Post by SideQuark »

mppm wrote: post #4

By the way, apart from the fact that dark matter predictions have been contradicted by increasingly sensitive experiments over and over and over, there is another inconvenient piece of observational evidence: An analysis of stellar velocities at the edge of the Milky Way, published last year [1] shows that they are "Keplerian", i.e. following classical behavior without the influence of any purported dark matter. So e…

That link also claims that the results you're listing are not likely to hold up, including comments from an author of the paper used for the article. So if one of the paper;s own authors expects these claims to disappear once more data is gathered, then maybe it's too early to claim the end of dark matter.

After all, DM has been measured in many different ways. One surprising finding doesn't negate hundreds of surprising findings without significantly more evidence.

> shows that they are "Keplerian", i.e. following classical behavior without the influence of any purported dark matter.

That is NOT at all what the paper says [1]. if you actually read the paper, and don't interject your beliefs and reword it, in every instance they use the phrase "Keplerian decline," which does not AT ALL mean classical, which would break relativity and be a massive surprise. The phrase "Keplerian decline" means the measured items are less than expected for the previous model. The movement, even with this decline, are far beyond what Kepler's laws would imply (and even Kepler's laws are demonstrably wrong in our solar system - see the precession of Mercury for example).

[1] https://www.aanda.org/component/article?access=doi&doi=10.10...

Re: The possibilities for dark matter have shrunk

#37

Post by ajross »

throwawaymaths wrote: post #32
marcosdumay wrote: post #25

Earlier quoted context omitted.

Shouldn't axions interact with atom nucleus? This rules out any interaction except for gravitational, right?

To a certain floor. Of course a particle which only interacts gravitationally is still possible but that's the cosmologist's nightmare. It'd be basically impossible to prove it's existence and anything could happen (like a small but fast fairly massive ~asteroid mass clump of dark matter explaining poltergeists)

> like a small but fast fairly massive ~asteroid mass clump of dark matter explaining poltergeists

This is clever, but I have to believe the back of an envelope would show that a dark-matter scale distribution of "dark blobs" big enough to throw things around the room and frequent enough to be given names in human experience would be many, many orders of magnitude higher than the observed gravitational effects.

Re: The possibilities for dark matter have shrunk

#38

Post by api »

DebtDeflation wrote: post #22
api wrote: post #5

I’m betting on primordial black holes for at least some of it. If you consider early universe densities their formation seems almost certain. Most models predict that they exist but the unknowns are around how common they are and their mass ranges and therefore whether they could account for most or all dark matter. There’s a little bit of speculation that “planet nine” is a PBH. I really hope for that one. It would…

I think primordial black holes are the Occam's Razor solution too. No exotic new particles required, no modifications to gravity required, no new physics period. Observational results have ruled out certain masses of black holes but far from all.

AFAIK they are the only explanation that requires no new physics.

Re: The possibilities for dark matter have shrunk

#39

Post by AlbertCory »

Some day, "what is dark matter?" arguments will be recalled with the same amusement with which we view "what kind of substance is the ether?"

And then, people will say, "It was so obvious!" So let DM be a lesson the next time anyone claims that some breakthrough 50 years ago was obvious.

Re: The possibilities for dark matter have shrunk

#40

Post by naasking »

mppm wrote: post #4

By the way, apart from the fact that dark matter predictions have been contradicted by increasingly sensitive experiments over and over and over, there is another inconvenient piece of observational evidence: An analysis of stellar velocities at the edge of the Milky Way, published last year [1] shows that they are "Keplerian", i.e. following classical behavior without the influence of any purported dark matter. So e…

Even better recent evidence shows that rotation curves are flat up to a million light years, which no feasible particle dark matter halo could reproduce:

https://tritonstation.com/2024/06/18/rotation-curves-still-f...

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