An update on the big bang nucleosynthesis prediction for ⁷Li: The problem worsens

The lithium problem arises from the significant discrepancy between the primordial ⁷Li abundance as predicted by big bang nucleosynthesis (BBN) theory and the Wilkinson Microwave Anisotropy Probe (WMAP) baryon density, and the pre-Galactic lithium abundance inferred from observations of metal-poor (Population II) stars. This problem has loomed for the past decade, with a persistent discrepancy of a factor of 2-3 in ⁷Li/H. Recent developments have sharpened all aspects of the Li problem. Namely: (1)BBN theory predictions have sharpened due to new nuclear data; in particular, the uncertainty on the reaction rate for³He(α,γ) ⁷Be has reduced to 7.4%, nearly a factor of 2 tighter than previous determinations. (2)The WMAP five-year data set now yields a cosmic baryon density with an uncertainty reduced to 2.7%. (3)Observations of metal-poor stars have tested for systematic effects. With these, we now find that the BBN+WMAP predicts⁷Li/H = (5.24_-0.67^+0.71) × 10^-10. The central value represents an increase by 23%, most of which is due to the upward shift in the³He(α,γ)⁷Be rate. More significant is the reduction in the⁷Li/H uncertainty by almost a factor of 2, tracking the reduction in the³He(α, γ)⁷Be error bar. These changes exacerbate the Li problem; the discrepancy is now a factor 2.4 or 4.2σ (from globular cluster stars) to 4.3 or 5.3σ (from halo field stars). Possible resolutions to the lithium problem are briefly reviewed, and key experimental and astronomical measurements highlighted.