EQUATION OF STATE AND TEMPERATURE OF MASSIVE NONRELATIVISTIC BOSONS ARISING IN THE UNIVERSE AT THE FIRST STAGE OF REHEATING

We consider heavy nonrelativistic bosons with masses M~MGUT which arise in the Universe during phase transitions with spontaneous symmetry breaking at the grand unification scale. In the frame of statistical mechanics approach with model-independent description of cosmological background we show that the process of emerging of massive GUT bosons looks like evaporation of a Bose condensate. First massive nonrelativistic bosons behave like ideal quantum degenerate Bose gas which has the Gibbons-Hawking temperature due to the presence of the de Sitter event horizon.