Constraints on $\Omega_b$ from nucleosynthesis of $^7$Li in the standard big bang model

We update Standard Big Bang Nucleosynthesis (SBBN) calculations on the basis of recent nuclear physics compilations (NACRE in particular), experimental and theoretical works. By a Monte--Carlo technique, we calculate the uncertainties on the light element yields (4He, D, 3He, 7Li) related to nuclear reactions. The results are compared to observations that are thought to be representative of the corresponding primordial abundances. It is found that 7Li could lead to more stringent constraints on the baryonic density of the universe (Omega_B) than deuterium, because of much higher observation statistics and an easier extrapolation to primordial values. The confrontation of SBBN results with 7Li observations is of special interest since other independent approaches have also recently provided Omega_B.h^2 values: i) the anisotropies of the Cosmic Microwave Background by the BOOMERANG, CBI, DASI and MAXIMA experiments and ii) the Lyman-alpha forest at high redshift. Comparison between these results obtained by different methods provides a test of their consistency and could provide a better determination of the baryonic density in the universe. However, the agreement between Omega_B.h^2 values deduced from SBBN calculation and 7Li observation on the one hand and CMB observations on the other hand is only marginal