Oscillation effects on neutrino decoupling in the early universe

In the early universe, neutrinos decouple from equilibrium with the electromagnetic plasma at a temperature which is only slightly higher than the temperature where electrons and positrons annihilate. Therefore neutrinos to some extent share in the entropy transfer from e⁺e^- to other species, and their final temperature is slightly higher than the canonical value T_ν=(4/11)1/3T_γ. We study neutrino decoupling in the early universe with effects of neutrino oscillations included, and find that the change in neutrino energy density from e⁺e^- annihilations can be about 2--3 % higher if oscillations are included. The primordial helium abundance can be changed by as much as 1.5×10^-4 by neutrino oscillations.