Re: Temp of Universe (was Definitions of Temperature)

["Paul Camp" <pjcamp@coastal.edu>]

> The main thing that makes me queasy about all this is that while the
> photons can exchange energy with other objects, they don't regain
> equilibrium among themselves after doing so.  Thus we can look at
> how readily the gas of photons will give up or accept energy, but
> I can't think of a meaningful way to ask how the entropy of the
> photon gas changes after such an exchange, since afterwards it will
> no longer be in internal equilibrium.  I guess the reason that this
> isn't a problem operationally is that the photon reservoir is so
> huge, we can't really disturb it significantly no matter how much
> energy we add or remove.
>
> Well, those are my thoughts.  What do you think?
>
> Dan

You make basically the standard cosmological choice for "temperature 
of the universe." I would just add that you last point here should 
basically resolve all your queasiness. The background photons do not 
interact significantly with matter since most of the matter has no 
charge. Further, the entropy of the universe (which is mostly the 
entropy of the background radiation) is ridiculously large -- in 
energy units of GeV, the entropy per comoving volume is of order 
10^87. It would be tough to change that significantly.

Some relatively easily accessible references: Blau & Guth in 300 
Years Of Gravitation (ed Hawking & Israel). Also Kold & Turner's 
The Early Universe. Both are somewhat heavy going but the former 
is a bit less formidable.

Paul J. Camp                   "The Beauty of the Universe
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