> It's funny, I almost posted this as a question, i.e. was the early
> undifferentiated plasma state, considered in thermodynamic
> equilibrium?
its often assumed to be nearly so. the argument goes crudely like this:
1.) cosmic microwave background fairly uniform over space and
matches amazingly well the theoretical black-body curve for 2.73 K.
this curve derives from a theory of thermal equilibrium of
electro-magnetic radiation field.
2.) extrapolate redshift backwards in time, get high density, high
temperature "early universe".
3.) since time needed to reach equilbrium from non-equilibrium
states goes like the inverse of density (higher density means faster
rate of interactions between fields/particles so faster approach to
equilibrium), then early "tiny" universe could equilibrate "super"
fast.
of course, one could always ask, well, what about the characteristics of the non-equilibria in really really early times? and thats where the hard work comes in. for example, density variations which grow to create gross structures like galaxies and inhomogeneities in CMB, etc., i.e. on scales smaller than universal scale.
one nicety of inflationary theory is to obtain consistent results without explicit dependence on specific initial conditions of the big bang universe. check out Guth's "The Inflationary Universe" if you havent already.
on the other hand, only if we could now somehow see the end results of those initial conditions could we CHECK the results of such a theory. so if we suspect that universe equilibrates on largest scales quickly, we shall be prevented from knowing exactly what went down in the very earliest universe, for sufficiently well defined "knowing" and "earliest".
this ought to give you some feel for why assumption of thermal equilibrium isn't a bad place to start.
les schaffer