The primordial lithium problem

Big bang nucleosynthesis (BBN) theory, together with the precise WMAP cosmic baryon density, makes tight predictions for the abundances of the lightest elements. Deuterium and ⁴He measurements agree well with expectations, but ⁷Li observations lie below the BBN+WMAP prediction by a factor of three to four. This 45-σ mismatch constitutes the so-called cosmic lithium problem; disparate solutions are possible. First, astrophysical systematics in the observations could exist but are increasingly constrained. Second, nuclear physics experiments provide a wealth of well-measured cross-section data, but ⁷Be destruction could be enhanced by unknown or poorly measured resonances. Third, physics beyond the Standard Model could alter the ⁷Li abundance, although deuterium and ⁴He must remain unperturbed. In this review, we discuss such scenarios, highlighting decaying supersymmetric particles and time-varying fundamental constants. Present and planned experiments could reveal which (if any) of these proposed solutions is correct.