Digression: Stretching Plasma II

Primordial plasma is a very funny thing
Density so low Bremsstrahlung insignificant
Cannot make/destroy photons

Electrons/Protons cool as scale factor squared
So photons must pump energy into e/p to maintain equal temp...
But since photon number cannot change must become non-Plankian... but, 
Does happen but tiny effect
Kinetic energy density of e/p at combination billionth of photon energy density