Non-universal BBN bounds on electromagnetically decaying particles

TL;DR

This paper investigates how non-universal photon spectra from electromagnetic cascades affect big bang nucleosynthesis constraints, leading to tighter bounds on photon injection energies and implications for cosmological observations.

Contribution

It demonstrates that non-universality of photon spectra significantly alters BBN constraints, providing more stringent bounds than previous models and emphasizing the importance of energy-dependent cross-sections.

Findings

Tighter bounds on photon injection energies than previous literature.

Non-universality of spectra impacts photodissociation constraints.

Comparison with CMB spectral distortion bounds shows potential for future constraints.

Abstract

In Poulin and Serpico [Phys. Rev. Lett. 114, 091101 (2015)] we have recently argued that when the energy of a photon injected in the primordial plasma falls below the pair-production threshold, the universality of the non-thermal photon spectrum from the standard theory of electromagnetic cascades onto a photon background breaks down. We showed that this could reopen or widen the parameter space for an exotic solution to the 'lithium problem'. Here we discuss another application, namely the impact that this has on non-thermal big bang nucleosynthesis constraints from 4He, 3He and 2H, using the parametric example of monochromatic photon injection of different energies. Typically, we find tighter bounds than those existing in the literature, up to more than one order of magnitude. As a consequence of the non-universality of the spectrum, the energy-dependence of the photodissociation cross-sections is important. We also compare the constraints obtained with current level and future reach of cosmic microwave background spectral distortion bounds.