Cosmological cosmic rays: Sharpening the primordial lithium problem

Cosmic structure formation leads to large-scale shocked baryonic flows which are expected to produce a cosmological population of structure-formation cosmic rays (SFCRs). Interactions between SFCRs and ambient baryons will produce lithium isotopes via α+α→Li6,7. This pre-galactic (but nonprimordial) lithium should contribute to the primordial Li7 measured in halo stars and must be subtracted in order to arrive to the true observed primordial lithium abundance. In this paper we point out that the recent halo star Li6 measurements can be used to place a strong constraint to the level of such contamination, because the exclusive astrophysical production of Li6 is from cosmic-ray interactions. We find that the putative Li6 plateau, if due to pre-galactic cosmic-ray interactions, implies that SFCR-produced lithium represents LiSFCR/Liplateau ≈15% of the observed elemental Li plateau. Taking the remaining plateau Li to be cosmological Li7, we find a revised (and slightly worsened) discrepancy between the Li observations and big bang nucleosynthesis predictions by a factor of LiBBN7/Liplateau7 ≈3.7. Moreover, SFCRs would also contribute to the extragalactic gamma-ray background (EGRB) through neutral pion production. This gamma-ray production is tightly related to the amount of lithium produced by the same cosmic rays; the Li6 plateau limits the pre-galactic (high-redshift) SFCR contribution to be at the level of IγπSFCR/IEGRB 5% of the currently observed EGRB.