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 {μ} and {τ} neutrinos is 0.41%. This has the consequence of modifying the primordial ⁴He abundance by {Δ}Y=+1.0×{}10^-4, and the cosmological mass limit on light neutrinos by 0.2--0.5 eV.