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The possibilities for dark matter have shrunk

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Re: The possibilities for dark matter have shrunk

#51

Post by physicsdude »

pfdietz wrote: post #49
physicsdude wrote: post #48

Earlier quoted context omitted.

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 )

This is absolutely correct, as there are no solar neutrinos with energy above ~20 MeV (but below that, the solars dominate). Thanks for the clarification.

Re: The possibilities for dark matter have shrunk

#52

Post by bunderbunder »

If you've got the time, these YouTube videos are a nice counterpoint to a lot of popular communication and discussion on dark matter:

Dark matter is not a theory: https://www.youtube.com/watch?v=PbmJkMhmrVI

That dark matter video aged like milk: https://www.youtube.com/watch?v=qS34oV-jv_A

the tl;dr is that "dark matter" is a bit of a misnomer. (But also a fun name so it's probably here to stay through sheer memetic momentum.) Dark matter is not actually a hypothesized kind of matter. Dark matter is just an unresolved discrepancy between theory and observation. I'll say that again: "dark matter" is not an attempt at explaining the discrepancy; dark matter is the discrepancy.

There are some hypotheses that postulate some kind of as-yet-unobserved matter that could resolve the discrepancy, but these hypotheses are not the actual problem that we call "dark matter". And not all hypotheses postulate unobserved or exotic matter. Some of them suggest alternative physics, others suggest measurement error.

Re: The possibilities for dark matter have shrunk

#53

Post by physicsdude »

mppm wrote: post #44
empyrrhicist wrote: post #34

Earlier quoted context omitted.

> 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…

Reprioritization of direct experimental searches for dark matter is already happening. WIMPs are by no means being abandoned, but because we are closing in on the neutrino fog background (which is mentioned in another comment), it's been recognized that to myopically cling to the same kind of experiment which dominated the 2000s and 2010s is not a strategic move (both from the "we expect to see something" and the "responsible use of tax dollars" perspectives).

For example: axions, an alternative DM candidate mentioned in another comment, have seen a significant growth in attention in recent years, and the usual detector technology for axion searches is currently being refined and scaled up, from benchtop-scale, dedicated experiments to lab-scale, wide searches.

At the same time, different groups which have developed past WIMP detectors are merging to collaborate on the larger, next-generation detectors. And there is R&D and prototyping happening to create detectors which, although looking for WIMPs, are sensitive in entirely different mass ranges than those of yesteryear.

Re: The possibilities for dark matter have shrunk

#54

Post by rbanffy »

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.

Considering time dilation, a detector inside a relativistic spacecraft will be hit by a lot of neutrinos in the apparent short duration of the mission.

I wonder what is the speed at which interstellar neutrino bombardment becomes lethal to a crew considering it’s essentially impossible to shield against them. You can put a mountain in front of the ship, but neutrinos will just pass through it like is a cloud.

OTOH if the ship is so fast neutrinos are a problem to the crew, that mountain will be suffering some intense ablation from everything including neutrinos that’ll hit it.

Re: The possibilities for dark matter have shrunk

#55

Post by physicsdude »

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?

Axions can seem a bit goofy. One thing to know is that when talking about particle physics, we like to talk about particles, but "particles" are really a concept from classical physics. When doing quantum physics, one is fundamentally concerned with waves. If you've read any about quantum mechanics, you've heard of "wave/particle duality," which is something of a connection between the two pictures. Another thing to know is that axions should interact electromagnetically.

The bottom line is that because axions would necessarily be very "light" (that is, very much not-massive), it is misleading to picture them as "particles" and better to picture them as "waves." So while it's true that axions would feel the electromagnetic field of an atom's nucleus, that's really just because it's an electromagnetic field. So to make an experiment which is sensitive by modern standards, you say "forget individual atoms, I'm just going to make a cavity and crank it up to large electromagnetic field." And that's exactly what is done in practice.

Re: The possibilities for dark matter have shrunk

#56

Post by mr_mitm »

naasking wrote: post #41
throwawaymaths wrote: post #30

Earlier quoted context omitted.

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.

How is it not clear? GR reduces to Newtonian gravity in the weak field limit, which MOND is explicely not. If MOND is true, then GR is wrong.

Re: The possibilities for dark matter have shrunk

#58

Post by throwawaymaths »

mr_mitm wrote: post #56
naasking wrote: post #41

Earlier quoted context omitted.

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.

How is it not clear? GR reduces to Newtonian gravity in the weak field limit, which MOND is explicely not. If MOND is true, then GR is wrong.

Have some nuance. GR is already known to be "wrong" since it isn't reconcilable with QM.

If any given MOND is true, GR needs some (possibly minor) changes in the limit, just as GR needs (possibly minor) changes to accommodate QM

Re: The possibilities for dark matter have shrunk

#59

Post by kouru225 »

AlbertCory wrote: post #39

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.

What’s always crazy to me is that so many people know the words “dark matter” and don’t realize it’s basically a placeholder phrase.

Re: The possibilities for dark matter have shrunk

#60

Post by throwawaymaths »

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…

> DM has been measured in many different ways

That is begging the question. Claiming that DM has been measured many ways presupposes that the different ways that "it has been measured" are have the same cause, which itself is the dark matter hypothesis.

Until we get a multimodal observation of DM itself we can't claim that DM has been measured many ways. We are very much still in the "guessing that these things are DM" phase.

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