Comment by knzhou | Hacker News Reader

Comment by knzhou | Hacker News Reader

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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.

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