Neutrino Spectrum Distortion Due to Oscillations and its BBN Effect

We study the distortion of electron neutrino energy spectrum due to oscillations with the sterile neutrino nu_e <-> nu_s, and its influence on BBN for different initial populations of the sterile state delta N_s at the onset of oscillations. Only the case of an initially empty sterile state was studied in previous publications. We obtain the primordial abundance of He-4 in models with electron-sterile neutrino oscillations for all possible delta N_s: 0 <= delta N_s <= 1 and for the model of oscillations, effective after electron neutrino decoupling, for which the spectrum distortion effects on the neutron-proton transitions are the strongest. It is found that the spectrum distortion effect may be the dominant one not only for small delta N_s, but also for big initial population of the sterile state. It was found that in the resonant case it plays a considerable role even for very big delta N_s ~ 0.8. We discuss cosmological constraints on neutrino mixing for small delta N_s.