Primordial nucleosynthesis with varying fundamental constants: Solutions to the Lithium problem and the Deuterium discrepancy

Abstract: The success of primordial nucleosynthesis has been limited by the long-standing Lithium problem. We use a self-consistent perturbative analysis of the effects of relevant theoretical parameters on primordial nucleosynthesis, including variations of nature's fundamental constants, to explore the problem and its possible solutions, in the context of the latest observations and theoretical modeling. We quantify the amount of depletion needed to solve the Lithium problem, and show that transport processes of chemical elements in stars are able to account for it. Specifically, the combination of atomic diffusion, rotation and penetrative convection allows us to reproduce the lithium surface abundances of Population II stars, starting from the primordial Lithium abundance. We also show that even with this depletion factor there is a preference for a value of the fine-structure constant at this epoch that is larger than the current laboratory one by a few parts per million of relative variation, at the two to three standard deviations level of statistical significance. This preference is driven by the recently noticed discrepancy between the best-fit values for the baryon-to-photon ratio (or equivalently the Deuterium abundance) inferred from cosmic microwave background and primordial nucleosynthesis analyses, and is largely insensitive to the Helium-4 abundance. We thus conclude that the Lithium problem most likely has an astrophysical solution, while the Deuterium discrepancy provides a possible hint of new physics.