NEUTRINO OSCILLATIONS IN THE EARLY UNIVERSE

Numerical studies are performed for neutrino oscillations for t > 1/3 seconds after the big bang. The effects of electron, positron and neut rino backgrounds are properly included. Flavor evolution of the neutri no background is dominantly smooth. However, in a certain parameter re gion, coherent undamped oscillations of the neutrino background are po ssible. In addition, neutrino CP asymmetry is found to be considerably smaller than expected. Reasons are provided for the above effects and some physical implications are discussed.