Overproduction of primordial helium-4 in the presence of neutrino oscillations

The production of helium-4 during the cosmological nucleosynthesis in the presence of active--sterile neutrino oscillations, efficient after decoupling of electron neutrino, is analyzed. The oscillation effects on primordial nucleosynthesis, namely: neutrino spectrum distortion, depletion of electron neutrino number density and generation of neutrino-antineutrino asymmetry, are precisely taken into account. Primordially produced He-4 abundance is calculated, in a self-consistent study of the kinetics of the nucleons and the oscillating neutrinos, for the full range of parameters of the discussed oscillation model. A considerable relative increase of helium-4, up to 14% for non-resonant oscillations and up to 32% for resonant ones is registered. Cosmological constraints on oscillation parameters are discussed.