  We studied nonequilibrium oscillations between left-handed electron neutrinos
and nonthermalized sterile neutrinos. The exact kinetic equations for
neutrinos, written in terms of neutrino density matrix in momentum space were
analyzed. The evolution of neutrino density matrix was numerically calculated.
This allowed to study precisely the evolution of the neutrino number densities,
energy spectrum distortion and the asymmetry between neutrinos and
antineutrinos for each momentum mode. Both effects of distortion and asymmetry,
which cannot be accounted for correctly when working in terms of particle
densities and mean energies, and the depletion of electron neutrino state have
been proved considerable for a certain range of oscillation parameters. The
influence of nonequilibrium oscillations on primordial nucleosynthesis was
calculated. Cosmologically excluded regions for oscillation parameters were
obtained.
