Constraints on the Injection of Radiation in the Early Universe

TL;DR

This paper investigates the effects of radiation injection in the early universe, focusing on constraints from baryon-to-entropy ratio dilution and effective neutrino number, finding tight limits on extra radiation.

Contribution

It provides a numerical analysis of how electromagnetic and dark radiation injections impact early universe parameters, establishing bounds on additional radiation.

Findings

Extra radiation can be up to 25% more than dark radiation alone.

Electromagnetic radiation injection dilutes baryon-to-entropy ratio.

Constraints are tight due to effects on BBN and recombination.

Abstract

We consider the generic injection of radiation (both dark and electromagnetic) during the epoch between big bang nucleosynthesis (BBN) and recombination. The contribution of the additional radiation to the number of effective neutrinos may be quite small in this scenario, since dark radiation and electromagnetic radiation provide contributions of opposite sign. However, the injection of electromagnetic radiation dilutes the baryon-to-entropy ratio, which is measured both at BBN and at recombination. As a result, this scenario is expected to be tightly constrained. Indeed, performing a numerical study, we find that the allowed amount of extra radiation may be no more than $\sim 25\%$ greater than in the case where it is assumed to be entirely dark radiation.