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The possibilities for dark matter have shrunk

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Re: The possibilities for dark matter have shrunk

#41

Post by naasking »

throwawaymaths wrote: post #30
CorrectHorseBat wrote: post #19

Earlier quoted context omitted.

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…

It's not clear that GR would need to be tweaked to match MOND. The GR solutions we currently use in LCDM are based on the FLRW metric, but that metric could just be the wrong fit for our universe.

Re: The possibilities for dark matter have shrunk

#42

Post by naasking »

api wrote: post #38
DebtDeflation wrote: post #22

Earlier quoted context omitted.

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.

Can they explain flat rotation curves out to a million light years though?

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

Re: The possibilities for dark matter have shrunk

#43

Post by pfdietz »

mapt wrote: post #24

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 reas…

Is it the solar neutrino noise floor, or distant supernova neutrino noise floor?

Maybe the justification for interstellar colonization will be so we can run a DM experiment on the near-relativistic ship during the trip. Boost those dark matter particles to some good fraction of c for easier detection.

Re: The possibilities for dark matter have shrunk

#44

Post by mppm »

empyrrhicist wrote: post #34
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…

I'd normally agree with you, but in the particular case of dark matter particles something still smells fishy. The theories that the predicted cross-sections are based on are just too flexible and numerous. I'm not sure how much we gain from ruling out yet more. What if there are in fact no weakly interacting particles? At what point do we decide that enough has been ruled out to start looking elsewhere? Like plasma dynamics or something (please don't shoot me if this is too silly to even contemplate).

Re: The possibilities for dark matter have shrunk

#45

Post by mr_mitm »

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.

Clearly you mean galactic rotation curves. However, your statement seems to ignore the numerous, independent probes of dark matter which we have seen since then, most of which are much stronger evidence, most notably the shape of the CMB angular powerspectrum.

https://en.m.wikipedia.org/wiki/Dark_matter#Observational_ev...

Re: The possibilities for dark matter have shrunk

#46

Post by mppm »

SideQuark wrote: post #36
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 surpri…

Thank you, I stand corrected.

Re: The possibilities for dark matter have shrunk

#47

Post by physicsdude »

empyrrhicist wrote: post #34
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…

Particle physicist here. I've worked on direct detection DM experiments in the past, and personally know some folks who work on the LZ experiment. That direct detection experiments, such as LZ, have not detected a signal does not contradict any predictions.

Indeed, relevant to what an experiment like LZ might see, there really isn't much in the way of "predictions" which can be "contradicted." What we have at this point are mechanisms to calculate the interaction rate _given at least one free parameter_. If we were to detect a non-zero rate, then we would "know" the free parameter of a single-parameter theory underlying that calculation. If we were to continue to detect a non-zero rate, then we would try to do so using different materials, and look at the time dependence of the rate (or, really, the dependence of the rate on the Earth's direction of travel in our local galaxy). That would help us choose between different theories, pin down the free parameters, and confirm that what we're seeing is consistent with "heavy stuff just sitting out in the universe."

But, from a particle physics perspective, right now there are no predictions to contradict - just an opportunity to detect something.

Re: The possibilities for dark matter have shrunk

#48

Post by physicsdude »

pfdietz wrote: post #43
mapt wrote: post #24

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 reas…

Is it the solar neutrino noise floor, or distant supernova neutrino noise floor? Maybe the justification for interstellar colonization will be so we can run a DM experiment on the near-relativistic ship during the trip. Boost those dark matter particles to some good fraction of c for easier detection.

The "floor" is due to solar neutrinos - the sun is a continuous source of neutrinos and is located much closer to us than virtually every past supernova was. It is also a soft floor, as there are analysis tricks that can be played to distinguish between interactions from neutrinos and WIMPs at the statistical level. For this reason, the current fashion is to refer to a "neutrino fog," which can be entered into to some depth, as opposed to a "neutrino floor."

Re: The possibilities for dark matter have shrunk

#49

Post by pfdietz »

physicsdude wrote: post #48
pfdietz wrote: post #43

Earlier quoted context omitted.

Is it the solar neutrino noise floor, or distant supernova neutrino noise floor? Maybe the justification for interstellar colonization will be so we can run a DM experiment on the near-relativistic ship during the trip. Boost those dark matter particles to some good fraction of c for easier detection.

The "floor" is due to solar neutrinos - the sun is a continuous source of neutrinos and is located much closer to us than virtually every past supernova was. It is also a soft floor, as there are analysis tricks that can be played to distinguish between interactions from neutrinos and WIMPs at the statistical level. For this reason, the current fashion is to refer to a "neutrino fog," which can be entered into to som…

It's actually due to both (for different DM particle masses).

https://physics.aps.org/assets/0ce5ba95-0367-46ce-890b-de3a4...

(which is from https://physics.aps.org/articles/v14/96)

Re: The possibilities for dark matter have shrunk

#50

Post by physicsdude »

api wrote: post #38
DebtDeflation wrote: post #22

Earlier quoted context omitted.

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.

Primordial black holes do not require new fundamental physics, but for them to constitute the primary component of dark matter would require a revision, at some level, of our narrative of the history of the universe. This gets a bit outside of my core knowledge, but as it stands there aren't any super solid mechanisms for generating black holes of a plausible mass distribution, early in the universe, such that we would see what we see today.

But, IMO, this is worthy of more study both theoretically and experimentally. An update to the evolution of the universe would be awesome!

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