Inside the Black Box of Big Bang Nucleosynthesis: A Comprehensive Sensitivity Atlas for the Precision Era

Abstract

In this study we present a comprehensive sensitivity atlas for Big Bang Nucleosynthesis (BBN) in which we quantify the dependence of the primordial abundances of helium-4, deuterium, and lithium-7 as well as $N_{\mathrm{eff}}$ on variations in 14 fundamental particle physics and cosmological parameters and 63 thermonuclear reaction rates. We use the publicly available BBN code \faGithub \href{https://github.com/vallima/PRyMordial}{\,\texttt{PRyMordial}} to compute each sensitivity using two nuclear reaction rate compilations and two weak-rate normalization schemes, and provide a model independent reference applicable to Beyond the Standard Model (BSM) models in which MeV scale physics is modified. In addition, we rank each parameter's contribution to the theoretical uncertainty budget. We compare our predictions against the latest observational determinations of the primordial abundances, including a recent LBT measurement of the helium-4 abundance \cite{Aver:2026dxv} which roughly halves the observational uncertainty relative to previous determinations. We present these results both fixing $\Delta N_{\mathrm{eff}}$ at its Standard Model (SM) value, and allowing it to be a free parameter using the latest uncertainty from the combined CMB+BAO+BBN 2026 value \cite{Goldstein:2026iuu}. When $\Delta N_{\mathrm{eff}}$ is allowed to be a free parameter, it dominates the theoretical uncertainty of the helium-4 abundance, highlighting the importance of upcoming observations from the Simons Observatory \cite{SimonsObservatory:2025wwn}. As illustrative applications, we examine the deuterium tension and the Lithium Problem in light of our sensitivity analysis. The full set of numerical results and figures is publicly available on GitHub \faGithub \href{https://github.com/Anne-KatherineBurns/bbn-sensitivity-atlas}{\,\texttt{bbn-sensitivity-atlas}}.

89 pages, 39 figures. Published version, with two additional parameters (alpha_EM and r_p) added to Table 4

References in corpus (23)

• Primordial Nucleosynthesis: from precision cosmology to fundamental physics
• The Primordial Lithium Problem
• Big Bang Nucleosynthesis as a Probe of New Physics
• Probing Physics Beyond the Standard Model: Limits from BBN and the CMB Independently and Combined
• A new tension in the cosmological model from primordial deuterium?
• Non-extensive Statistics Solution to the Cosmological Lithium Problem
• SPT-3G D1: CMB temperature and polarization power spectra and cosmology from 2019 and 2020 observations of the SPT-3G Main field
• Bayesian Estimation of Thermonuclear Reaction Rates
• Solving the cosmic lithium problems with primordial late-decaying particles
• Indications for a Nonzero Lepton Asymmetry from Extremely Metal-Poor Galaxies
• Primordial nucleosynthesis with varying fundamental constants: Improved constraints and a possible solution to the Lithium problem
• Primordial lepton asymmetries in the precision cosmology era: Current status and future sensitivities from BBN and the CMB
• Cosmology with varying fundamental constants from hyperlight, coupled scalars
• Bayesian estimation of thermonuclear reaction rates for deuterium+deuterium reactions
• Cosmological solutions to the Lithium problem: Big-bang nucleosynthesis with photon cooling, $X$-particle decay and a primordial magnetic field
• Primordial nucleosynthesis with varying fundamental constants: Solutions to the Lithium problem and the Deuterium discrepancy
• Constraints on primordial lepton asymmetries with full neutrino transport
• Resonance excitations in ${}^7$Be(d,p)${}^8$Be${}^{\ast }$ to address the cosmological lithium problem
• LINX: A Fast, Differentiable, and Extensible Big Bang Nucleosynthesis Package
• New Thermonuclear Rate of 7Li(d,n)24He relevant to the Cosmological Lithium Problem
• The cosmological lithium problem
• Cosmological Parameter Estimation with a Joint-Likelihood Analysis of the Cosmic Microwave Background and Big Bang Nucleosynthesis
• Primordial neutrinos fade to gray: constraints from cosmological observables