Primordial Nucleosynthesis

TL;DR

Primordial nucleosynthesis explains the early universe's element formation, matching observations for most isotopes but showing a persistent lithium discrepancy, and serves as a tool to test cosmological theories.

Contribution

This review comprehensively discusses nuclear physics aspects of Big-Bang Nucleosynthesis, including isotopes beyond helium and lithium, and explores its role in probing early universe physics.

Findings

Good agreement between observed and calculated abundances for most isotopes.

Persistent lithium abundance discrepancy remains unresolved.

BBN remains a valuable tool for testing cosmological models.

Abstract

Primordial nucleosynthesis, or Big-Bang Nucleosynthesis (BBN), is one of the three evidences for the Big-Bang model, together with the expansion of the Universe and the Cosmic Microwave Background. There is a good global agreement over a range of nine orders of magnitude between abundances of 4He, D, 3He and 7Li deduced from observations, and calculated in primordial nucleosynthesis. This comparison was used to determine the baryonic density of the Universe. For this purpose, it is now superseded by the analysis of the Cosmic Microwave Background (CMB) radiation anisotropies. However, there remain, a yet unexplained, discrepancy of a factor 3-5, between the calculated and observed lithium primordial abundances, that has not been reduced, neither by recent nuclear physics experiments, nor by new observations. We review here the nuclear physics aspects of BBN for the production of 4He, D, 3He and 7Li, but also 6Li, 9Be, 11B and up to CNO isotopes. These are, for instance, important for the initial composition of the matter at the origin of the first stars. Big-Bang nucleosynthesis, that has been used, to first constrain the baryonic density, and the number of neutrino families, remains, a valuable tool to probe the physics of the early Universe, like variation of "constants" or alternative theories of gravity.