The Spectral Distortions of the Cosmic Microwave Background Radiation, LEP and Heavy Neutrinos

Abstract

In the early Universe, energy may have been released into the primordial plasma, generating spectral distortions of the cosmic background radiation (CBR) which, incidentally, would be today the precious signatures of the primeval epochs. In the first three sections, the delicate interplay between radiation and plasma is investigated. When thermal contact is established between photons and electrons, the radiation spectrum relaxes towards a Bose-Einstein distribution as a consequence of inverse Compton scatterings. The spectrum latterly achieves a Planck distribution as bremsstrahlung and double Compton emissions proceed. The µ and y spectral distortions are discussed. Then, the primordial behaviour of particles, such as the neutrino, is generically presented in order to analyse the implications of the recent LEP and COBE results as regards the cosmological rôle of heavy neutrinos. The latter are no longer viable candidates for cosmological dark matter. Finally, in the last section, the hypothetical 17 keV neutrino recently reported in some ,ß-decay experiments faces the COBE constraints on the CBR spectrum.