Big-Bang nucleosynthesis (Particle Data Group mini-review)

TL;DR

This review assesses the current understanding of Big-Bang nucleosynthesis within the Standard Model, highlighting concordance with cosmic microwave background data and discussing discrepancies like lithium-7 abundance.

Contribution

It provides a comprehensive update on primordial element abundances, their consistency with cosmological observations, and constraints on new physics such as neutrino species.

Findings

Primordial deuterium and helium-4 abundances align with WMAP data.

Lithium-7 abundance is significantly lower than expected.

Constraints on the effective number of neutrino species are derived.

Abstract

A critical review is given of the current status of cosmological nucleosynthesis. In the framework of the Standard Model with 3 types of relativistic neutrinos, the baryon-to-photon ratio, $\eta$, corresponding to the inferred primordial abundances of deuterium and helium-4 is consistent with the independent determination of $\eta$ from WMAP observations of anisotropies in the cosmic microwave background. However the primordial abundance of lithium-7 inferred from observations is significantly below its expected value. Taking systematic uncertainties in the abundance estimates into account, there is overall concordance in the range $\eta = (4.7 - 6.5) x 10^{-10}$ @ 95% c.l. (corresponding to a cosmological baryon density $\Omega_B h^2$ = 0.017 - 0.024). The D and He-4 abundances, together with the CMB determination of $\eta$, provide the bound $N_\nu = 3.24 \pm 1.2$ @ 95% c.l. on the effective number of neutrino species. Other constraints on new physics are discussed briefly.