The possibilities for dark matter have shrunk • Hacker News Board

Re: The possibilities for dark matter have shrunk

#21

Post by dylan604 » Tue, Aug 27, 2024, 1:31 PM UTC

FactolSarin wrote: post #7

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…

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/Gravitational_lens

Re: The possibilities for dark matter have shrunk

#22

Post by DebtDeflation » Tue, Aug 27, 2024, 1:33 PM UTC

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.

Re: The possibilities for dark matter have shrunk

#23

Post by pfortuny » Tue, Aug 27, 2024, 1:43 PM UTC

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…

Aaaaah, the ether… Always coming back from death. What will it become next?

Re: The possibilities for dark matter have shrunk

#24

Post by mapt » Tue, Aug 27, 2024, 2:05 PM UTC

Notably, the possibilities are quasi-finite because of the solar neutrino noise floor. When we reach that, we stop needing to scale to 10x larger detectors for a 90% reduction in reaction cross section, and they start to need to be 100x larger, in my understanding.

Even in a world with WIMPs, we aren't necessarily privileged to have them show up above the solar neutrino noise floor.

It's going to be funny if the reason we get a mission to Neptune's moon Triton a century from now is to house a dark matter search experiment, once the cost for continuing to scale Earth's search becomes dramatically worse.

Re: The possibilities for dark matter have shrunk

#25

Post by marcosdumay » Tue, Aug 27, 2024, 2:06 PM UTC

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?

Re: The possibilities for dark matter have shrunk

#26

Post by ajross » Tue, Aug 27, 2024, 2:20 PM UTC

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…

> maybe the discrepancy that dark matter was invented to explain will simply disappear when more accurate astronomical data including the fainter stars becomes available

FWIW, that would be a much more surprising result than anything to do with dark matter. We can see "faint stars" just fine in the near field. What you're positing here is that somehow "faint" stars in the farther universe behave in notably different ways than they do in the Milky Way, which is exactly the kind of theory you're arguing against.

No, a few confusing results isn't going to throw dark matter out. It's too strong a signal, and too hard to explain via classical means (I mean, it's not like astronomers haven't tried!).

Re: The possibilities for dark matter have shrunk

#27

Post by hnuser123456 » Tue, Aug 27, 2024, 2:23 PM UTC

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…

https://digital.library.unt.edu/ark:/67531/metadc877627/m1/4...

Re: The possibilities for dark matter have shrunk

#28

Post by throwawaymaths » Tue, Aug 27, 2024, 2:26 PM UTC

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…

> maybe the discrepancy that dark matter was invented to explain will simply disappear when more accurate astronomical data including the fainter stars becomes available.

That's unlikely. Measuring the milky way galaxy's rotation curve is bitchingly hard because we're inside it. Curves of other galaxies are much, much easier to measure and for example Andromeda is likely to be correct.

Moreover galaxies with exceptionally large satellite galaxies are known to have keplerian decline, so this could have been predicted.

The more interesting recent results is that for some galaxies the rotation curve is flat very very far out (those measurements might not be as accurate ofc) which would imply much larger dark matter halos which are inconsistent with e.g. cmb ringing

Re: The possibilities for dark matter have shrunk

#29

Post by Hikikomori » Tue, Aug 27, 2024, 2:26 PM UTC

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…

Gravitational lensing doesn't have to be apparent to the eye as those extreme examples on the wiki page are. The second image on the bullet page shows matter in pink and measured lensing in blue.

Re: The possibilities for dark matter have shrunk

#30

Post by throwawaymaths » Tue, Aug 27, 2024, 2:30 PM UTC

CorrectHorseBat wrote: post #19

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 remember reading/hearing somewhere that the bullet cluster actually fits MOND better than dark matter. I think it was a video by Sabine Hossenfelder but I can't find it right now and I'm not qualified to say whether it made sense or not.

Bullet cluster is inconsistent with LCDM on the grounds that given the apparent dark matter:matter ratio a collision between clusters of that size is something like unlikely to the tune of one in a trillion? IIRC universes. (Aka the collision should not have happened in the first place).

The thing about lensing is that we don't have a solution in (GR would need to be tweaked if MOND is true -- the math is much harder!!) so we can't really say what the lensing would look like in any given MOND-like theory yet. Seems weird to declare that MOND can't explain lensing. It's should be more qualified: "we don't think MOND can explain the lensing"