Non-equilibrium Neutrino in the Early Universe Plasma

We discuss the evolution of cosmic neutrinos, participating in neutrino oscillations and interacting with the fermions of the hot plasma during pre-BBN and BBN epoch. The neutrino evolution and the neutrino oscillations effects depend on the type of oscillations: oscillation channels, the degree of equilibrium of oscillating neutrinos and on the plasma characteristics. Neutrino spectrum distortion by neutrino oscillations in medium is discussed in detail. Non-equilibrium decays and their effect on electron neutrino spectrum distortion and nucleons kinetics during pre-BBN epoch are briefly discussed. BBN model with such decays and active-inert neutrino oscillations may resolve the tension between BBN and LSS preferred numbers of neutrino types. Cosmological constraints on neutrino characteristics are presented.