Nonequilibrium Corrections to the Spectra of Massless Neutrinos in the Early Universe

Abstract

Distortion of the equilibrium spectra of cosmic neutrinos due to interaction with hotter electrons and positrons in the primeval cosmic plasma is considered. The set of integro-differential kinetic equations for neutrinos is accurately numerically solved. The relative corrections to neutrino energy densities are approximately 0.9% for $ν_e$ and 0.4% for $ν_μ$ and $ν_τ$. This effect results in $1.4 \cdot 10^{-4}$ increase in the primordial $^4 He$ abundance.