Big-Bang Nucleosynthesis

TLDR

Cosmological nucleosynthesis is reviewed, summarizing its current status and noting brief constraints on new physics. Within the Standard Model with three neutrino species, the baryon‑to‑photon ratio inferred from deuterium and helium‑4 agrees with CMB determinations, yet the lithium‑7 abundance is lower than expected; overall concordance holds for η≈(5.7–6.7)×10⁻¹⁰ (Ω_B h²=0.021–0.025), yielding N_ν=3.28±0.28.

Abstract

A critical review is given of the current status of cosmological nucleosynthesis. In the framework of the Standard Model with 3 types of relativistic neutrinos, the baryon-to-photon ratio, $\eta$, corresponding to the inferred primordial abundances of deuterium and helium-4 is consistent with the independent determination of $\eta$ from observations of anisotropies in the cosmic microwave background. However the primordial abundance of lithium-7 inferred from observations is significantly below its expected value. Taking systematic uncertainties in the abundance estimates into account, there is overall concordance in the range $\eta = (5.7-6.7)\times 10^{-10}$ at 95% CL (corresponding to a cosmological baryon density $\Omega_B h^2 = 0.021 - 0.025$). The D and He-4 abundances, when combined with the CMB determination of $\eta$, provide the bound $N_\nu=3.28 \pm 0.28$ on the effective number of neutrino species. Other constraints on new physics are discussed briefly.