Cosmic Rays during BBN as Origin of Lithium Problem

TL;DR

This paper explores whether non-thermal cosmic rays during big-bang nucleosynthesis can explain the lithium abundance discrepancy, finding they can account for $^7$Li but not fully for $^6$Li, with the acceleration mechanism still uncertain.

Contribution

It demonstrates that BBNCRs with a specific spectrum can partially resolve the lithium problem, highlighting the need for an early universe acceleration mechanism.

Findings

BBNCRs can account for $^7$Li abundance.

$^6$Li abundance remains underpredicted.

Deuteron abundance is highly sensitive to BBNCR spectrum.

Abstract

There may be non-thermal cosmic rays during big-bang nucleosynthesis (BBN) epoch (dubbed as BBNCRs). This paper investigated whether such BBNCRs can be the origin of Lithium problem or not. It can be expected that BBNCRs flux will be small in order to keep the success of standard BBN (SBBN). With favorable assumptions on the BBNCR spectrum between 0.09 -- 4 MeV, our numerical calculation showed that extra contributions from BBNCRs can account for the $^7$Li abundance successfully. However $^6$Li abundance is only lifted an order of magnitude, which is still much lower than the observed value. As the deuteron abundance is very sensitive to the spectrum choice of BBNCRs, the allowed parameter space for the spectrum is strictly constrained. We should emphasize that the acceleration mechanism for BBNCRs in the early universe is still an open question. For example, strong turbulent magnetic field is probably the solution to the problem. Whether such a mechanism can provide the required spectrum deserves further studies.