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 ${e}^{+}{e}^{\ensuremath{-}}$ to other species, and their final temperature is slightly higher than the canonical value ${T}_{\ensuremath{\nu}}{=(4/11)}^{1/3}{T}_{\ensuremath{\gamma}}.$ We study neutrino decoupling in the early universe with effects of neutrino oscillations included, and find that the change in neutrino energy density from ${e}^{+}{e}^{\ensuremath{-}}$ annihilations can be about 2\char21{}3 % higher if oscillations are included. The primordial helium abundance can be changed by as much as $1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ by neutrino oscillations.