Neutrinos And Big Bang Nucleosynthesis

TL;DR

This review discusses how big bang nucleosynthesis and cosmic observations constrain neutrino properties, dark radiation, and baryon asymmetry, highlighting current agreements and unresolved issues like the lithium problem.

Contribution

It provides an updated review of BBN constraints on neutrinos, dark radiation, and baryon asymmetry, integrating recent cosmological data and addressing the lithium problem.

Findings

BBN constraints align with CMB results on dark radiation and baryon density.

Current data suggest possible presence of dark radiation but are inconclusive.

BBN predicts lithium abundance higher than observed by a factor of ~3.

Abstract

According to the standard models of particle physics and cosmology, there should be a background of cosmic neutrinos in the present Universe, similar to the cosmic microwave photon background. The weakness of the weak interactions renders this neutrino background undetectable with current technology. The cosmic neutrino background can, however, be probed indirectly through its cosmological effects on big bang nucleosynthesis (BBN) and the cosmic microwave background (CMB) radiation. In this BBN review, focused on neutrinos and, more generally on dark radiation, the BBN constraints on the number of "equivalent neutrinos" (dark radiation), on the baryon asymmetry (baryon density), and on a possible lepton asymmetry (neutrino degeneracy) are reviewed and updated. The BBN constraints on dark radiation and on the baryon density following from considerations of the primordial abundances of deuterium and helium-4 are in excellent agreement with the complementary results from the CMB, providing a suggestive, but currently inconclusive, hint of the presence of dark radiation and, they constrain any lepton asymmetry. For all the cases considered here there is a "lithium problem": the BBN-predicted lithium abundance exceeds the observationally inferred primordial value by a factor of ~3.