Condensation of photons in the hot universe and longitudinal relict radiation
Kuz'min V.A., Shaposhnikov M.E.
Assuming that the photon mass differs from zero, a hypothesis is advanced that a photon condensate existed in the universe at earlier expansion stages. The evaporation of the condensate as a result of interaction with charged particles has led to the formation of longitudinal relict radiation that constitutes possibly the bulk of the mass of the universe.