Big Bang Nucleosynthesis: Nuclear Physics in the Early Universe

We summarize the status of big bang nucleosynthesis (BBN), which describes the production of the lightest nuclides during the first 3 min of cosmic time. We emphasize the transformational influence of cosmic microwave background (CMB) experiments culminating today with Planck, which pins down the cosmic baryon density to exquisite precision. Standard BBN combines this with the standard model of particle physics and with nuclear cross section measurements – notably recent precision measurements of d(p, γ)³He by the LUNA collaboration. These allow BBN to make tight predictions for the primordial light-element abundances, with the result that deuterium observations agree spectacularly with theoretical expectations, and helium observations are in good agreement. This CMB/BBN concordance marks a profound success of the hot big bang, and BBN and the CMB together now sharply probe cosmology, neutrino physics, and dark matter physics back to times of around 1 s. But this success is tempered by lithium observations (in metal-poor halo stars) that are significantly discrepant with BBN+CMB predictions. Recent work strengthens the case that the resolution of this “lithium problem” could well lie in the stellar astrophysics of lithium depletion, while new physics solutions are possible but becoming ever more tightly constrained. We conclude with an outlook for how future CMB, astronomical, and laboratory measurements can better probe new physics and bring into focus the solution to the lithium problem.