Precision calculation of neutrino evolution in the early Universe

Abstract

In the primordial Universe, neutrino decoupling occurs only slightly before electron-positron annihilations. This leads notably to an increased neutrino energy density compared to the standard instantaneous decoupling approximation, parametrized by the effective number of neutrino species $N_{\mathrm{eff}}$. A precise calculation of neutrino evolution is needed to assess its consequences during the later cosmological stages, and requires to take into account multiple effects such as neutrino oscillations, which represents a genuine numerical challenge. Recently, several key improvements have allowed such a precise numerical calculation, leading to the new reference value $N_{\mathrm{eff}}=3.0440$.

Comments: 5 pages, 2 figures. Based on arXiv:2008.01074. Proceedings for the 17th International Conference on Topics in Astroparticle and Underground Physics (TAUP2021)