Cosmological Implications of Light Element Abundances: Theory

Primordial nucleosynthesis provides (with the microwave background radiation) one of the two quantitative experimental tests of the hot Big Bang cosmological model (versus alternative explanations for the observed Hubble expansion). The standard homogeneous-isotropic calculation fits the light element abundances ranging from ^1H at 76% and ^4He at 24% by mass through ^2H and ^3He at parts in 10^5 down to ^7Li at parts in 10¹⁰. It is also noted how the recent Large Electron Positron Collider (and Stanford Linear Collider) results on the number of neutrinos (N_ν) are a positive laboratory test of this standard Big Bang scenario. The possible alternate scenario of quark-hadron-induced inhomogeneities is also discussed. It is shown that when this alternative scenario is made to fit the observed abundances accurately, the resulting conclusions on the baryonic density relative to the critical density (Ω_b) remain approximately the same as in the standard homogeneous case, thus adding to the robustness of the standard model and the conclusion that Ω_b ≈ 0.06. This latter point is the driving force behind the need for nonbaryonic dark matter (assuming total density Ω_total = 1) and the need for dark baryonic matter, since the density of visible matter Ω_visible < Ω_b. The recent Population II B and Be observations are also discussed and shown to be a consequence of cosmic ray spallation processes rather than primordial nucleosynthesis. The light elements and N_ν successfully probe the cosmological model at times as early as 1 sec and a temperature (T) of ≈ 10¹⁰ K (≈1 MeV). Thus, they provided the first quantitative arguments that led to the connections of cosmology to nuclear and particle physics.