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Comment by mehrdadn | Hacker News Reader

Question: as a layman, knowing nothing about the quantitative nature of the evidence, neutrinos seem like the perfect candidates for dark matter, since we've already detected them and we know interact so poorly with anything. Why are they not the obvious candidate for dark matter? Is there any evidence against them?

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That's a perfectly good question, and that's exactly why for a time in the 90s they were one of the leading candidates for dark matter. Unfortunately it fell apart when we got a better understanding of structure formation, which determines the large-scale distribution of matter in the universe today. If the 3 neutrinos have masses around where we think, then they'd be too light and move too fast in the early universe, smoothing the distribution out and thus suppressing small-scale clumping, contrary to observations. On the other hand, if the 3 neutrinos were a lot heavier (possible we can only detect their mass differences, not the absolute masses) they would screw up the CMB. The qualitative case was great, but quantitatively it just didn't work out.

I'm not an astrophysicist and well outside my depth here, but couldn't observed clumping with low-mass neutrinos suppressing clumping be explained by an older universe as well? Put another way: if clumping is suppressed, that doesn't mean it doesn't happen, it just means it happens slower than initially thought, no?

Also a good idea, but the details don't work out. Specifically, early on the Fourier components of the matter density distribution grow independently. Light neutrino DM would suppress the small scale clumping too much. Making the whole process last longer would fix this, but it would also enhance large scale clumping, contrary to observation.

It might be possible to make it all work out by adding a third effect (and maybe somebody has already done that!), but there's also a danger of constructing a Rube Goldberg machine -- if there are too many gadgets needed to get the structure right it starts looking like epicycles. The great thing about traditional dark matter is that it doesn't require any cosmological complications, it just works.

So lemme check my understanding here: are you saying that clumping has an observed size distribution, and if dark matter were just neutrinos, there isn't a timescale which would produce the clumping size distribution we see: if the timescale were short then we would see less small clumping than we see, if the timescale were longer then we would see more large clumping than we see?

Neat.

We know how many neutrinos should be there, and it's not enough to provide a dark matter candidate. Additionally, neutrinos would be hot dark matter, that is they would move fast, and therefore not cluster nicely. That would lead to galaxies that are more spread out than we observe.

(That is, as long as we assume standard neutrinos, there is actually a theoretical mechanism to get heavy right handed neutrinos, while leaving the left handed ones we observe light, and that could be kinda sort of a dark matter candidate. The most obvious way to generate them is as far as I know ruled out, but I am sure people work on more complex models.)

Neutrinos are the obvious candidate. To make long story short, the evidence against is that numbers don't work out. I know it is unsatisfying to those who can't derive numbers themselves, but that's what it is.

Neutrinos move too fast. Dark matter ought to move at nonrelativistic speeds to generate the observed effects. Another way to put this is that neutrinos are "hot" dark matter and we expect "cold" dark matter.

they are obvious. The evidence against it is that there are not that many of them (or we would have detected more), and while we aren't sure how much they all weigh we're pretty sure it's orders of magnitude less than what would be required for dark matter.

I believe I heard on a podcast that there's a theory that dark matter might actually be multiple things, and that neutrinos could just account for a small percentage of that. Is that correct?

That's correct. In the standard ΛCDM cosmology, neutrinos would provide about 0.5% of the required matter. See the Particle Data Group's dark matter review: http://pdg.lbl.gov/2020/reviews/rpp2020-rev-dark-matter.pdf

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