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

The lithium problem arises from the significant discrepancy between the primordial Li-7 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 Li-7/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 for He-3(alpha, gamma)Be-7 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 predicts Li-7/H = (5.24(-0.67)(+ 0.71)) x 10-(10). The central value represents an increase by 23%, most of which is due to the upward shift in the He-3(alpha, gamma)Be-7 rate. More significant is the reduction in the Li-7/H uncertainty by almost a factor of 2, tracking the reduction in the 3He(a,.) 7Be error bar. These changes exacerbate the Li problem; the discrepancy is now a factor 2.4 or 4.2 sigma (from globular cluster stars) to 4.3 or 5.3 sigma (from halo field stars). Possible resolutions to the lithium problem are briefly reviewed, and key experimental and astronomical measurements highlighted.