The Primordial Lithium Problem

TL;DR

The paper discusses the cosmic lithium problem, a significant discrepancy between predicted and observed lithium abundances from Big Bang nucleosynthesis, exploring astrophysical, nuclear, and new physics solutions.

Contribution

It reviews potential explanations for the lithium problem, including observational systematics, nuclear physics uncertainties, and physics beyond the Standard Model, highlighting future experimental prospects.

Findings

Deuterium and helium measurements agree with predictions.

Lithium observations are 3-4 times lower than expected.

Multiple possible solutions are discussed, including new physics and astrophysical effects.

Abstract

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 a factor 3-4 below the BBN+WMAP prediction. This 4-5\sigma\ mismatch constitutes the cosmic "lithium problem," with disparate solutions possible. (1) Astrophysical systematics in the observations could exist but are increasingly constrained. (2) Nuclear physics experiments provide a wealth of well-measured cross-section data, but 7Be destruction could be enhanced by unknown or poorly-measured resonances, such as 7Be + 3He -> 10C^* -> p + 9B. (3) Physics beyond the Standard Model can alter the 7Li abundance, though D and 4He must remain unperturbed; we discuss such scenarios, highlighting decaying Supersymmetric particles and time-varying fundamental constants. Present and planned experiments could reveal which (if any) of these is the solution to the problem.