A Bitter Pill? The Primordial Lithium Problem

Abstract

Big-bang nucleosynthesis (BBN) describes the production of the lightest elements during the first three minutes of cosmic time, and represents our earliest reliable probe of the universe. BBN has stood as both cornerstone of modern cosmology and particle astrophysics, and Keith Olive has made fundamental contributions to BBN over a span of four decades. I will review BBN and Keith's hand in it, emphasizing the transformative influence of cosmic microwave background experiments in precisely determining the cosmic baryon density. Standard BBN combines this with the Standard Model of particle physics to make tight predictions for the primordial light element abundances. Deuterium observations match these predictions spectacularly, helium observations are in good agreement, but lithium observations (in metal-poor halo stars) are significantly discrepant-–this is the ”lithium problem.” Over the past decade, the lithium discrepancy has become more severe, and very recently the solution space has shrunk dramatically, with all resolutions of the problem facing stringent constraints. Future observations will either confirm surprises in stellar astrophysics, or most intriguingly, could reveal new physics at play in the early universe.