Precision calculation of neutrino evolution in the early Universe

TL;DR

This paper presents a highly precise numerical calculation of neutrino evolution in the early Universe, accounting for oscillations and other effects, refining the effective number of neutrino species to 3.0440.

Contribution

It introduces advanced computational methods to accurately model neutrino decoupling, improving the precision of $N_{\rm eff}$ beyond previous approximations.

Findings

Refined $N_{\rm eff}$ value of 3.0440

Enhanced understanding of neutrino decoupling effects

Implications for cosmological models

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 $N_{\rm eff}$. 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 $N_{\rm eff}=3.0440$.