Neutrino decoupling in the early Universe

A calculation of neutrino decoupling in the early Universe, including full Fermi-Dirac statistics and electron mass dependence in the weak reaction rates, is presented. We find that after decoupling the electron neutrinos contribute 0. 83% more to the relativistic energy density than in the standard scenario, where neutrinos are assumed not to share the heating from e^ annihilation. The corresponding number for and neutrinos is 0. 41%. This has the consequence of modifying the primordial ^4He abundance by =+1. 010^-4, and the cosmological mass limit on light neutrinos by 0. 2--0. 5 eV.