  Consideration of the adiabatic character of radiation expansion in early
Universe leads to the conclusion that equilibrium distribution of the
primordial radiation in the presence of charged particles could be different
from the Planck distribution in some regions of the spectrum. The equilibrium
distribution of electromagnetic radiation (the black body radiation) is
generalized for the system containing an extremely dense fully ionized plasma.
The conditions of the adiabatic expansion of radiation for the model of the
early Universe are found.
