A loophole to the universal photon spectrum in electromagnetic cascades: application to the "cosmological lithium problem"

TL;DR

This paper reveals a previously overlooked spectral shape in electromagnetic cascades below the pair-production threshold, which can impact cosmological constraints and offers new solutions to the lithium problem.

Contribution

It identifies a loophole in the universal photon spectrum theory, demonstrating its implications for Big Bang nucleosynthesis and the cosmological lithium problem.

Findings

Spectral shape differs below pair-production threshold

Reopens electromagnetic solutions to lithium problem

Impacts BBN bounds on non-thermal relics

Abstract

The standard theory of electromagnetic cascades onto a photon background predicts a quasi-universal shape for the resulting non-thermal photon spectrum. This has been applied to very disparate fields, including non-thermal big bang nucleosynthesis (BBN). However, once the energy of the injected photons falls below the pair-production threshold the spectral shape is very different, a fact that has been overlooked in past literature. This loophole may have important phenomenological consequences, since it generically alters the BBN bounds on non-thermal relics: for instance it allows to re-open the possibility of purely electromagnetic solutions to the so-called "cosmological lithium problem", which were thought to be excluded by other cosmological constraints. We show this with a proof-of-principle example and a simple particle physics model, compared with previous literature.