The Spectrum of Dark Radiation as a Probe of Reheating

TL;DR

Measuring the spectrum of dark radiation produced during reheating can reveal details about the Universe's thermal history and the timing of key events like particle decay.

Contribution

This work proposes using dark radiation spectra as a novel probe to understand the reheating phase after inflation, highlighting how spectral features relate to early Universe conditions.

Findings

Dark radiation spectrum depends on whether heavy particles dominated the Universe.

Spectral features can indicate the temperature at decay of heavy particles.

Reheating details can be inferred from relativistic particle spectra.

Abstract

After inflation the Universe presumably undergoes a phase of reheating which in effect starts the thermal big bang cosmology. However, so far we have very little direct experimental or observational evidence of this important phase of the Universe. In this letter, we argue that measuring the spectrum of freely propagating relativistic particles, i.e. dark radiation, produced during reheating may provide us with powerful information on the reheating phase. To demonstrate this possibility we consider a situation where the dark radiation is produced in the decays of heavy, non-relativistic particles. We show that the spectrum crucially depends on whether the heavy particle once dominated the Universe or not. Characteristic features caused by the dependence on the number of the relativistic degrees of freedom may even allow to infer the temperature when the decay of the heavy particle occurred.