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, ³He, ⁴He, and ⁷Li 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 ⁷Li and ⁴He. 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 ≤ η₁₀ ≤ 4.0 (η₁₀ ≡ 10¹⁰n_B/n_γ), which constrains the baryon density parameter, Ω_Bh₅₀² = 0.05 ± 0.01 (the Hubble parameter is H₀ = 50h₅₀ km s^-1 Mpc^-1). These bounds imply that the bulk of the baryons in the universe are dark if Ω_TOT = 1 and would require that the universe be dominated by nonbaryonic matter. An upper bound to the primordial mass fraction of ⁴He, 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_v = 3, we bound the predicted primordial abundance of ⁴He: 0.236 ≤ Y_p ≤ 0.243 (for 882 ≤ τ_n ≤ 896 s).