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Is dark matter’s “nightmare scenario” true?

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Re: Is dark matter’s “nightmare scenario” true?

#131

Post by mhh__ »

choeger wrote: post #30

My intuitive objection against dark matter is that it's basically an infinite amount of additional parameters to be fitted to the observations. Galaxy behaves weirdly? Add dark matter! It behaves as expected? It obviously has lost its dark matter! It stands to reason that for every possible observation, a particular DM configuration can be calculated. I fail to see how dark matter could ever be falsified. The best po…

> infinite amount of additional parameters

Not in any technical sense I can think of.

Re: Is dark matter’s “nightmare scenario” true?

#132

Post by naasking »

sigmoid10 wrote: post #120
naasking wrote: post #92

Earlier quoted context omitted.

> However, while I'm only a cosmologist and not a particle physicist, I find the implied assertion by proponents of MoND, that the existence of non-interacting particles is impossible, questionable. I think this is a strawman. MOND proponents don't discount that some observations are better suited to a particle, they just assert that the particle dark matter sledgehammer is vastly overused, and that modified gravity…

>why did these falsifications fail to convince the majority? Because they are cherry picked. In the big picture, for every area where particle DM struggles, you will find a dozen datapoints where MOND struggles. Physicists are also wary of approaches like MOND+particle DM, because it just introduces more parameters which you can arbitrarily tune to explain data. It feels like every week someone comes with a new model…

> Because they are cherry picked.

Sure, as are the objections to MOND.

> Physicists are also wary of approaches like MOND+particle DM, because it just introduces more [...] You can imagine it like fitting a polynomial of degree N to N data points. Yes it will fit perfectly, but that doesn't mean it's a good model. This is just sophisticated curve fitting to generate papers and not good science.

That's exactly what's happening with cold dark matter models. I agree the state of things is unsatisfactory for either theory, we need bolder ideas. Superfluid dark matter was promising but doesn't currently match some observations. We need more of that.

Re: Is dark matter’s “nightmare scenario” true?

#134

Post by mr_mitm »

api wrote: post #124

I’m sure physicists have already thought about this but… Has anyone considered modeling the effect currently explained via dark matter as a new kind of force or field? Like a distinct force generated by mass and its relationship to space time? This wouldn’t be modified gravity but a proposal for an entirely new force whose effect is what we currently explain with the “fudge factor” of dark matter. From the rough pict…

Someone has: https://en.wikipedia.org/wiki/Self-interacting_dark_matter

They even think about dark chemistry: https://iopscience.iop.org/article/10.3847/1538-4357/ac75ef

Didn't have much support last time I checked.

Re: Is dark matter’s “nightmare scenario” true?

#135

Post by gpderetta »

treis wrote: post #119
panda-giddiness wrote: post #115

Earlier quoted context omitted.

On the contrary, if dark matter were to interact purely gravitationally, then there would be little friction to slow it down. This would prevent dark matter from clumping, which is in agreement with observations.

I don't think so. Say you had two stationary whatevers of dark matter 1 meter apart alone in the universe. At t=0 they accelerate towards each other. At some time they cross and occupy the exact same point. R becomes 0 and the force becomes infinite. They should be stuck together.

That would make DM collapse in a black hole. I don't think that DM black holes are considered impossible, just not likely.

Re: Is dark matter’s “nightmare scenario” true?

#136

Post by sega_sai »

naasking wrote: post #90
sega_sai wrote: post #84

Earlier quoted context omitted.

You don't seem to quite understand the current cosmological models. You don't need special cases for every galaxy. Instead you 'just' need to add a weakly interacting particle with the right density (30% of critical). That particle needs to be 'slow enough', i.e. non-relativistic. After that it turns out that the evolution of the universe basically reproduces the vast array of observations. * The right primordial abu…

> You don't seem to quite understand the current cosmological models. You don't need special cases for every galaxy. No, he's correct that each galaxy does need tuning. Specifically, the amount of DM and it's specific distribution is tuned to match each observation, which is what the OP said. > but it is also true that anything that has to do with galaxy/star formation is immensely complicated as it involves a lot of…

> No, he's correct that each galaxy does need tuning. Specifically, the amount of DM and it's specific distribution is tuned to match each observation, which is what the OP said.

Not true. The LCDM predicts the DM distribution with a typical DM profiles, and then the amount of baryons is heavily dependent on star formation, etc. Therefore each galaxy of the same luminosity will have somewhat different dark matter halo. Those are not tuning parameters at all.

Regarding the linked article, it an article written by proponents of MOND and is not objective. That is fine, but at the moment I don't believe MOND is a serious alternative, and the majority of the astro community would agree. Just the fact that MOND still requires dark matter anyway to reproduce CMB is major problem for it. But if someone develops the version of MOND that has equal predictive power to LCDM, I'll be happy to accept it, but we are not there.

Re: Is dark matter’s “nightmare scenario” true?

#137

Post by throwawaymaths »

fsh wrote: post #33
habibur wrote: post #21

Dark matter and dark energy are like what if there was no Einstein to discover relativity in early 1900s. We later continue to discover that gravity can't explain planetary motions correctly and therefore search for any explanation. There are more laws of Physics waiting to be discovered. At least two big ones from what we currently observe.

You are cherry-picking one particular example. The neutrino and the planet Neptune were "dark matter" for decades until better detectors became available.

Ironically Pluto was found the same way that Neptune was, but it turned out to be a calculation error.

Re: Is dark matter’s “nightmare scenario” true?

#138

Post by throwawaymaths »

superjan wrote: post #28
habibur wrote: post #21

Dark matter and dark energy are like what if there was no Einstein to discover relativity in early 1900s. We later continue to discover that gravity can't explain planetary motions correctly and therefore search for any explanation. There are more laws of Physics waiting to be discovered. At least two big ones from what we currently observe.

There is certainly new physics involved, the issue is more that for dark matter, the evidence points to matter that hardly, or perhaps not at all, interacts* with the stuff we are familiar with (standard model). It might be that the correct theory of dark matter is already found and debated, we just have no way to tell it from the false ones, as for all that we know, dark matter might be a shadow world of spaghetti m…

> the evidence points to matter that hardly, or perhaps not at all, interacts* with the stuff we are familiar with

I think this is why people get frustrated with the DM story. It's not the evidence that points to that. It's the lack of evidence that points to that. Specifically speaking, we keep hypothesizing limits on interaction in the hopes of finding it below some threshold but keep pushing those limits back. A multimodal observation would be clutch, but we keep confirming that a multimodal observation can't happen.

Re: Is dark matter’s “nightmare scenario” true?

#139

Post by exmadscientist »

AnIdiotOnTheNet wrote: post #125
plasticchris wrote: post #121

Earlier quoted context omitted.

It feels like this is implying that questioning is unscientific? I mean, I get being annoyed with laypeople but if you didn’t question the scientific consensus the historic record and the philosophy of science are against you.

It's more the level of confidence they seem to have in their take, given their relative lack of understanding of the subject.

For me it's the fact that so many of these commenters seem to think they've come up with something special, found some secret idea. That's completely false. Most physicists aren't very happy with dark matter as a theory (especially since the failure of the big direct particle searches). So the commenters are trying to convince the professionals of something most of them already believe. It's silly.

(My credentials: my fingerprints are all over one of those (pilot) experiments.)

Re: Is dark matter’s “nightmare scenario” true?

#140

Post by naasking »

AnIdiotOnTheNet wrote: post #129
naasking wrote: post #100

Earlier quoted context omitted.

> ...and? If we look at the way a star wobbles, then tune the amount and distribution of planets around it to fit, isn't that just science? That is science because gravity has been extensively tested and confirmed in that regime. Gravity on the scales where dark matter is relevant has not been tested to this extent, so we're just assuming our theory is correct in this new regime and thus inferring the DM distribution…

> Gravity on the scales where dark matter is relevant has not been tested to this extent, so we're just assuming our theory is correct in this new regime Couldn't we just as easily say the same about EM or any of the other forces? The reason we think it is some kind of matter is because we know that matter interacts with gravity in ways that would explain our observations, as long as we assume that matter isn't visib…

> Couldn't we just as easily say the same about EM or any of the other forces?

We've tested other forces in more regimes than gravity because gravity is so weak, but in principle, if predictions of those theories mismatched observations to the same extent as gravity, then we absolutely should be questioning those theories too.

> They keep having to add parameters to MOND to make it fit what we see and currently attribute to dark matter, like the bullet cluster.

I don't know why people keep saying this like cold dark matter model doesn't do this. LCDM has been refuted many times if this is your criterion. The review paper I cited shows numerous examples of parameterization and curve fitting for LCDM.

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