A Bitter Pill: The Primordial Lithium Problem Worsens

TL;DR

Recent advances in cosmological and nuclear data have sharpened the primordial lithium discrepancy, revealing it to be more severe than previously thought, with potential implications for cosmology and particle physics.

Contribution

This paper updates the lithium problem with new nuclear, observational, and cosmological data, emphasizing that the discrepancy has worsened despite improved measurements.

Findings

BBN predictions now more precise with reduced nuclear uncertainties.

WMAP data constrains cosmic baryon density with 2.7% uncertainty.

Discrepancy between predicted and observed lithium abundance increased to over 4 sigma.

Abstract

The lithium problem arises from the significant discrepancy between the primordial 7Li abundance as predicted by BBN theory and the 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, particularly the uncertainty on 3He(alpha,gamma)7Be, has reduced to 7.4%, and with a central value shift of ~ +0.04 keV barn. (2) The WMAP 5-year data now yields a cosmic baryon density with an uncertainty reduced to 2.7%. (3) Observations of metal-poor stars have tested for systematic effects, and have reaped new lithium isotopic data. With these, we now find that the BBN+WMAP predicts 7Li/H = (5.24+0.71-0.67) 10^{-10}. The Li problem remains and indeed is exacerbated; the discrepancy is now a factor 2.4--4.3 or 4.2sigma (from globular cluster stars) to 5.3sigma (from halo field stars). Possible resolutions to the lithium problem are briefly reviewed, and key nuclear, particle, and astronomical measurements highlighted.