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

Abstract

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.

Observational signatures for depletion in the Spite plateau: solving the cosmological Li discrepancy?

“…It appears that the observed Li abundances in metal-poor stars can be reasonably well reconciled with the predictions from standard Big Bang nucleosynthesis (e.g. Cyburt et al 2008) by means of more realistic stellar evolution models that include Li depletion through diffusion and turbulent mixing (Richard et al 2005). We caution however, that, although encouraging, our results should not be viewed as proof of the correctness of the Richard et al models until the free parameters required for the stellar modeling are better understood from basic physical principles.…”

“…Using the theory of big bang nucleosynthesis (BBN) and the baryon density obtained from WMAP data, a primordial Li abundance of A Li = 2.72 +0.05 −0.06 is predicted (Cyburt et al 2008), which is a factor of 2-6 times higher than the Li abundance inferred from halo stars. There have been many theoretical studies on non-standard BBN trying to explain the cosmological Li discrepancy by exploring the frontiers of new physics (e.g.…”

Galactic evolution of D, 3He and 4He

“…(1) Cyburt et al (2008); (2) Pettini et al (2008); (3) Geiss & Gloeckler (1998); (4) Linsky et al (2006); (5) Hébrard et al (2005); (6) Prodanović et al (2009); (7) Bania et al (2002); (8) Gloeckler & Geiss (1996); (9) Peimbert et al (2007); (10) Steigman (2007); (11) Asplund et al (2009). can not be reproduced by the models, independently of how many low-mass stars burn their 3 He on the RGB.…”

“…The determination of the parameter η from WMAP data (see text by J. Dunkley, this volume) has allowed to fix, with unprecedented precision, the primordial abundances of the light elements in the framework of the SBBN model (see Cyburt et al 2008 for recent work). The primordial abundances of D, 3 He and 4 He determined indirectly from the CMB anisotropies agree very well with those inferred from recent, direct observations (see Fig.…”

Constraining the Astrophysical S Factor of the 3He(α,γ)7Be Reaction

“…Therefore, precise determination of the reaction rate is also crucial in the so called 7 Li problem, i.e. the discrepancy between the inferred primordial 7 Li abundance from direct astrophysical observations and that obtained using the Big Bang nucleosynthesis calculations [4].…”