Neutrino decoupling in the early Universe

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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 muon and tau neutrinos is 0.41%. This has the consequence of modifying the primordial ⁴He abundance by ΔY=+1.0× 10⁻⁴, and the cosmological mass limit on light neutrinos by 0.2--0.5 eV.