🚀 go-pugleaf

sichase@csa2.lbl.gov (SCOTT I CHASE) writes:

>In article <92Nov3.165317edt.629@neuron.ai.toronto.edu>, radford@cs.toronto.edu (Radford Neal) writes...

>>This relates to something that's bugged me for ages. In popular
>>accounts, neutrinos are mentioned as one possibility for the "missing
>>mass" needed to close the universe, PROVIDED THEY TURN OUT TO HAVE MASS.
>>This also seems wrong. Regardless of whether they have mass, they 
>>definitely have energy, and that's all that counts, right?

>Yes.  But the neutrinos have so little kinetic energy that even a small
>mass (a fraction of an eV) would dominate the total energy of the 
>cosmic neutrino background, which if I remember correctly, is at even
>lower temperature than the 3K photon background.

Also, the "yes" is not exactly right-- massless particles have a
different stress-energy tensor than massive ones, so their
gravitational effects are different.  A Robertson-Walker universe
filled primarily with EM radiation, for instance, evolves somewhat
differently from one filled primarily with massive stuff.  But this
is a nitpick.
-- 
Matt McIrvin