Big Bang nucleosynthesis: the Standard Model and alternatives

Abstract

Big bang nucleosynthesis provides (with the microwave background radiation) one of the two quantitative experimental tests of the big bang cosmological model. This paper reviews the standard homogeneous-isotropic calculation and shows how it fits the light element abundances ranging from 4He at 24% by mass through 2H and 3He at parts in 105 down to 7Li at parts in 1010. Furthermore, the recent LEP (and SLC) results on the number of neutrinos are discussed as a positive laboratory test of the standard scenario. Discussion is presented on the improved observational data as well as the improved neutron lifetime data. Alternate scenarios of decaying matter or of quark-hadron induced inhomogeneities are discussed. It is shown that when these scenarios are made to fit the observed abundances accurately, the resulting conclusions on the baryonic density relative to the critical density, Ωb, remain approximately the same as in the standard homogeneous case, thus, adding to the robustness of the conclusion that Ωb ≃ 0.06. This latter point is the driving force behind the need for non-baryonic dark matter (assuming Ωtotal = 1) and the need for dark baryonic matter, since Ωvisible < Ωb.