Big Bang Nucleosynthesis (in "The Review of Particle Properties" 2004)

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 helium-4 and lithium-7 is presently ~2 \sigma below the value implied by the abundance of deuterium. The latter value is also coincident with the independent determination of \eta from WMAP observations of CMB anisotropy. However taking systematic uncertainties in the abundance estimates into account, there is overall concordance in the range \eta = (3.4 - 6.9) x 10^{-10} @ 95% c.l. corresponding to a cosmological baryon density \Omega_B h^2 = 0.012 - 0.025. If the above discrepancy is due to a neutrino chemical potential, then upto 7.1 effective neutrino species are allowed by nucleosynthesis. Other constraints on new physics are briefly discussed.