🚀 go-pugleaf

In article <92Nov4.112758edt.486@neuron.ai.toronto.edu>, radford@cs.toronto.edu (Radford Neal) writes...
>In article <3NOV199215121855@csa2.lbl.gov> sichase@csa2.lbl.gov (SCOTT I CHASE) writes:
> 
>>> Regardless of whether [ neutrinos ] 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.  Even if this is not 
>>correct, you can easily see that a reasonable mass would dominate if 
>>you remember that room temperature corresponds to kT = 1/40 eV.
> 
>I'm a bit surprised that the temperatures are different.  I can believe
>that present photon/neutrino interactions are not sufficient to produce 
>equilibrium, but wouldn't it be sufficient for equilibrium to have been
>present in the early universe?  I'd think that both backgrounds would then
>cool at the same rate, as the universe expands.  Hmmm... maybe the two 
>backgrounds both cease to have significant interactions with anything 
>else at some point, but the points are different for the two...?

Oh, boy.  Here goes nothing:

In order to understand this, you have to follow the history of the Universe
in more detail.  At a times much earlier than radiation-matter decoupling time,
the neutrinos and photons were part of the same thermal equilibrium, along
with heavier particles like electrons.  Weak nuclear processes involving
leptons and neutrinos provided channels for equilibration.  But when 
the temperature became significantly less than the electron mass, i.e,
kT < M(electron), these processes slowed down, while at the same time
most of the existing electrons and positrons annihilated into photons, 
reheating the photon radiation relative to the neutrino radiation.

This is very qualitative, and may also not be accurate.  I'm really reaching
here.  I.e., I am not a cosmologist, nor do I play one on TV.  Perhaps
someone else would be kind enough to provide a better explanation.

-Scott
--------------------
Scott I. Chase			"It is not a simple life to be a single cell,
SICHASE@CSA2.LBL.GOV		 although I have no right to say so, having
				 been a single cell so long ago myself that I 
				 have no memory at all of that stage of my 
				 life." - Lewis Thomas