Primordial nucleosynthesis redux

The latest nuclear reaction cross sections (including the most recent determinations of the neutron lifetime) are used to recalculate the abundances of deuterium, ^3^He, ^4^He, and ^7^Li within the framework of primordial nucleosynthesis in the standard (homogeneous and isotropic) hot, big bang model. The observational data leading to estimates of (or bounds to) the primordial abundances of the light elements is reviewed with an emphasis on ^7^Li and ^4^He. A comparison between theory and observation reveals the consistency of the predictions of the standard model and leads to bounds to the nucleon-to-photon ratio, 2.8 <=n_10_<=4.0 (n_10_= 10^10^n_B_/n_y_), which constrains the baryon density parameter, {OMEGA}_b_h^2^_50_= 0.05 +/- 0.01 (the Hubble parameter is H_0_= 50h_50_ km s^-1^ Mpc). These bounds imply that the bulk of the baryons in the universe are dark if {OMEGA}_TOT_ = 1 and would require that the universe be dominated by nonbaryonic matter. An upper bound to the primordial mass fraction of ^4^He, Y_p_<=0.240, constrains the number of light (equivalent) neutrinos to N_v_<=3.3 in excellent agreement with the LEP and SLC collider results. Alternatively, for N_y_ = 3, we bound the predicted primordial abundance of ^4^He: 0.236 <= Y_p_ <= 0.243 (for 882 <= T_n_ <= 896 s).