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 <=n₁0_<=4. 0 (n₁0_= 10¹0ⁿB_/nᵧ_), which constrains the baryon density parameter, OMEGAbₕ²^₅0_= 0. 05 +/- 0. 01 (the Hubble parameter is H₀_= 50h₅0_ km s^-1^ Mpc). These bounds imply that the bulk of the baryons in the universe are dark if OMEGATOT_ = 1 and would require that the universe be dominated by nonbaryonic matter. An upper bound to the primordial mass fraction of ⁴He, Yₚ_<=0. 240, constrains the number of light (equivalent) neutrinos to Nᵥ_<=3. 3 in excellent agreement with the LEP and SLC collider results. Alternatively, for Nᵧ_ = 3, we bound the predicted primordial abundance of ⁴He: 0. 236 <= Yₚ_ <= 0. 243 (for 882 <= Tₙ_ <= 896 s).