The 2024 BBN baryon abundance update

Abstract

We revisit the state of the light element abundances from big bang nucleosynthesis in early 2024 with particular focus on the derived baryon abundance. We find that the largest differences between the final baryon abundances are typically driven by the assumed Deuterium burning rates, characterized in this work by the underlying code. The rates from theoretical ab-initio calculations favor smaller baryon abundances, while experimentally-determined rates prefer higher abundances. Through robust marginalization over a wide range of nuclear rates, the recently released $\mathtt{PRyMordial}$ code allows for a conservative estimate of the baryon abundance at $\Omega_b h^2 = 0.02218 \pm 0.00055$ (using PDG-recommended light element abundances) in $\Lambda$CDM and $\Omega_b h^2 = 0.02196 \pm 0.00063$ when additional ultra-relativistic relics are considered ($\Lambda$CDM + $N_\mathrm{eff}$). These additional relics themselves are constrained to $\Delta N_\mathrm{eff} = -0.14 \pm 0.21$ by light element abundances alone.