Primordial Nucleosynthesis: Theory and Observations

TL;DR

This paper assesses whether standard Big Bang Nucleosynthesis (BBN) consistently accounts for the primordial abundances of light elements and uses them to constrain the cosmic baryon density via the baryon-to-photon ratio $\\eta$ (with $\\eta_{10}=273\\Omega_B h^2$). It reviews the theoretical framework—Friedmann expansion, weak interaction rates, and nuclear reaction networks—and notes improvements to BBN codes that include radiative, finite-temperature, and non-equilibrium effects. Observational data on deuterium, helium-4, and lithium-7 are analyzed, highlighting two possible primordial deuterium scenarios (low-D and high-D) that map to $\\eta_{10} \\approx 4.2-6.3$ and $\\eta_{10} \\approx 1.2-2.8$, with corresponding baryon densities $\\Omega_B h^2 \\approx 0.015-0.023$ and $\\Omega_B h^2 \\approx 0.004-0.010$, respectively. The study derives limits on extra light particle degrees of freedom, obtaining $\\Delta N_\\nu < 0.3$ for the low-D case and $\\Delta N_\\nu < 1.8$ for the high-D case, demonstrating that BBN remains a powerful probe of early-Universe physics and beyond-standard-model scenarios.

Abstract

We review the Cosmology and Physics underlying Primordial Nucleosynthesis and survey current observational data in order to compare the predictions of Big Bang Nucleosynthesis with the inferred primordial abundances. From this comparison we report on the status of the consistency of the standard hot big bang model, we constrain the universal density of baryons (nucleons), and we set limits to the numbers and/or effective interactions of hypothetical new "light" particles (equivalent massless neutrinos).