If a CMB theorist gloats that he or she can measure the neutrino density,
make sure to point out that galaxy surveys are much more sensitive.
As you can see, the CMB power spectrum (top) changes only
very weakly as you replace cold dark matter by hot. This is because
the neutrinos were already quite cold (nonrelativistic) the time the
CMB fluctuations formed. The effect on the galaxy power spectrum
(bottom) is seen to be much more dramatic, effectively erasing
small-scale power. This will happen even if the neutrinos are
fairly cold (moving much slower than the speed of light).
A speed of a few hundred kilometers mer second is sufficient
to escape from a Galaxy halo, thereby erasing structure on that scales.