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The standard theory of electromagnetic cascades onto a photon backgroundpredicts a quasi-universal shape for the resulting non-thermal photon spectrum.This has been applied to very disparate fields, including non-thermal big bangnucleosynthesis (BBN). However, once the energy of the injected photons fallsbelow the pair-production threshold the spectral shape is very different, afact that has been overlooked in past literature. This loophole may haveimportant phenomenological consequences, since it generically alters the BBNbounds on non-thermal relics: for instance it allows to re-open the possibilityof purely electromagnetic solutions to the so-called "cosmological lithiumproblem", which were thought to be excluded by other cosmological constraints.We show this with a proof-of-principle example and a simple particle physicsmodel, compared with previous literature.

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