Cosmological Cosmic Rays: Sharpening the Primordial Lithium Problem

TL;DR

This paper investigates how cosmic rays from structure formation influence primordial lithium measurements, constraining their contribution using 6Li observations, and discusses implications for the lithium problem and gamma-ray background.

Contribution

It introduces a novel constraint on pre-Galactic cosmic-ray lithium production using 6Li measurements, refining the primordial lithium abundance estimate and its cosmological implications.

Findings

SFCR-produced lithium accounts for about 15% of the observed lithium plateau.

The revised lithium discrepancy with BBN predictions is increased to a factor of 3.7.

Pre-Galactic SFCRs contribute less than 5% to the extragalactic gamma-ray background.

Abstract

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 \alpha+\alpha \to ^{6,7}Li. This pre-Galactic (but non-primordial) lithium should contribute to the primordial 7Li 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 6Li measurements can be used to place a strong constraint to the level of such contamination, because the exclusive astrophysical production of 6Li is from cosmic-ray interactions. We find that the putative 6Li plateau, if due to pre-Galactic cosmic-ray interactions, implies that SFCR-produced lithium represents Li_{SFCR}/Li_{plateau}\approx 15% of the observed elemental Li plateau. Taking the remaining plateau Li to be cosmological 7Li, we find a revised (and slightly worsened) discrepancy between the Li observations and Big Bang Nucleosynthesis predictions by a factor of ^7Li_{BBN}/^7Li_{plateau} \approx 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 6Li plateau limits the pre-Galactic (high-redshift) SFCR contribution to be at the level of I_{\pi_{\gamma}SFCR}/I_{EGRB} < 5% of the currently observed EGRB.