Incomplete neutrino decoupling effect on big bang nucleosynthesis

TL;DR

This paper investigates how incomplete neutrino decoupling in the early universe influences primordial nucleosynthesis, clarifying the expected sign of abundance variations and addressing discrepancies in previous literature.

Contribution

It provides a detailed review of neutrino decoupling effects on nucleosynthesis, predicting the sign of abundance changes and clarifying previous conflicting results.

Findings

Neutrino decoupling slightly before electron-positron annihilation increases neutrino energy density.

The sign of abundance variations due to neutrino decoupling is predicted and clarified.

Neutrino oscillations are neglected, with conjecture on their subdominant effect.

Abstract

In the primordial Universe, neutrino decoupling occurs only slightly before electron-positron annihilations, leading to an increased neutrino energy density with order $10^{-2}$ spectral distortions compared to the standard instantaneous decoupling approximation. However, there are discrepancies in the literature on the impact it has on the subsequent primordial nucleosynthesis, in terms of both the magnitude of the abundance modifications and their sign. We review how neutrino decoupling indirectly affects the various stages of nucleosynthesis, namely, the freezing out of neutron abundance, the duration of neutron beta decay, and nucleosynthesis itself. This allows to predict the sign of the abundance variations that are expected when the physics of neutrino decoupling is taken into account. For simplicity, we ignore neutrino oscillations, but we conjecture from the detailed interplay of neutrino temperature shifts and distortions that their effect on final light element abundances should be subdominant.