Big-Bang Nucleosynthesis

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, $η$, corresponding to the inferred primordial abundances of deuterium and helium-4 is consistent with the independent determination of $η$ from 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 η= (5.7-6.7)× 10⁻¹⁰ at 95% CL (corresponding to a cosmological baryon density ΩB h² = 0.021 - 0.025). The D and He-4 abundances, when combined with the CMB determination of $η$, provide the bound N_ν=3.28 ± 0.28 on the effective number of neutrino species. Other constraints on new physics are discussed briefly.