Neutrinos in the early Universe

The effect of neutrino oscillations on the processes in the primeval plasma is discussed. A kinetic equation for oscillating neutrinos is obtained. The manner in which the primeval nucleosynthesis is affected by the additional degrees of freedom due to the massiveness of the neutrinos is considered. It is shown that the simultaneous presence of Dirac and Majorana neutrino masses would lead to a contradiction with the observed abundance of helium in the Universe for deltam2>10-6 eV2. Bounds on the coupling constants of right-handed currents and on the masses of right-handed Majorana neutrinos which follow from cosmological data are presented