The Lithium Problem(s)

Lithium measured in low-metallicity halo-stars shows a so called "Spite plateau", which is thus believed to be the evidence of primordial lithium. This is a reasonable conclusion being that lithium is very fragile and the Big Bang nucleosynthesis is the only mechanism able to produce lithium abundance close to the observed plateau. However, recent WMAP measurements of baryon density result in a primordial ⁷Li abundance that is about two times higher then the "Spite plateau", hence "the lithium problem". Moreover, any pre-galactic source of lithium would contribute to the value observed in low-metallicity halo-stars, and will have to be corrected for, which amplifies the lithium problem.
Besides in the Big Bang, lithium is also produced in cosmic-ray interactions. A cosmological cosmic-ray population that arises from the process of large-scale structure formation will be the source of pre-Galactic lithium production, thus making the lithium problem worse. We have taken a model-independent approach by using the lithium-gamma-ray connection in order to constrain lithium production by structure-formation cosmic-rays. We found that present limits on the extragalactic gamma-ray background allow for significant production of pre-galactic lithium by SFCRs.

Unlike ⁷Li, the rare isotope ⁶Li is mainly produced in cosmic-ray interactions. The standard picture is that the observed ⁶Li is produced by galactic cosmic-rays accelerated in supernova remnants. Using our lithium-gamma-ray connection we tested this assumption and came to a surprising and alarming conclusion: extragalactic gamma-ray background allows for only ~50% of solar lithium-6 abundance to be produced by Galactic Cosmic Rays (in the standard picture). Our calculation uses a carefully propagated cosmic-ray spectrum and accounts for 6Li production from both fusion reactions ($\alpha \alpha \to ^6Li$) as well as from spallation channels (${p,\alpha+CNO \to ^6Li$). We find that although extreme assumptions yield a consistent picture, more realistic ones indicate that solar 6Li cannot be produced by standard GCRs alone without overproducing the hadronic gamma rays. Could this be another lithium problem?

Reference: Fields, B. D., & Prodanović, T. 2005, ApJ 623, 877 (get from astro-ph/)
                   Prodanović, T. & B. D. Fields 2006, ApJL accepted (get from astro-ph/)