REFRACTION AND OSCILLATIONS OF NEUTRINOS IN THE EARLY UNIVERSE

We have investigated the behaviour of the electron neutrinos upsilon-e and antineutrinos upsilonbar-e in the heat bath of the early Universe. We have determined the effective energies of upsilon-e and upsilonbar-e as functions of the temperature of the Universe. We assume that upsilon-e and upsilonbar-e mix with inert states upsilon-x and upsilonbar-x, respectively, and consider the upsilon-e <--> upsilon-x and upsilonbar-e <--> upsilonbar-x oscillations. We have paid special attention to the adiabaticity and coherence of these transitions and have found that for a large region of the parameter space the system will evolve through a Mikheyev-Smirnov resonance. The numerical study of the combined neutrino and antineutrino systems shows that the dynamical lepton asymmetry is driven to zero. Thus no significant upsilon-e - upsilonbar-e asymmetry can be created. If a resonance exists, both neutrinos and antineutrinos will go through it simultaneously. A large conversion of the upsilon-e and upsilonbar-e populations in the early Universe into the inert states upsilon-x and upsilonbar-x is thus possible.