Oscillation effects on neutrino decoupling in the early universe

Abstract

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_nu = (4/11)^{1/3} T_gamma. 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 oscillation are included. The primordial helium abundance can be changed by as much as 1.5 x 10^-4 by neutrino oscillations.