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, $\eta$, corresponding to
the inferred primordial abundances of deuterium and helium-4 is consistent with
the independent determination of $\eta$ 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 $\eta = (5.7-6.7)\times 10^{-10}$ at 95% CL
(corresponding to a cosmological baryon density $\Omega_B h^2 = 0.021 -
0.025$). The D and He-4 abundances, when combined with the CMB determination of
$\eta$, provide the bound $N_\nu=3.28 \pm 0.28$ on the effective number of
neutrino species. Other constraints on new physics are discussed briefly.
