Cosmological constraints on exotic injection of electromagnetic energy

TL;DR

This paper develops a formalism to constrain exotic electromagnetic energy injections in the early universe using CMB anisotropies, complementing spectral distortion and nucleosynthesis bounds, with applications to primordial black holes, sterile neutrinos, and 21 cm signals.

Contribution

It introduces a novel approach to model electromagnetic energy injection effects on CMB power spectra as a function of injection epoch, including a new on-the-spot approximation, and applies it to various exotic scenarios.

Findings

Constraints on primordial black holes are comparable to gamma-ray background studies.

Sterile neutrino parameter space is constrained in a new mass-mixing range.

Forthcoming 21 cm experiments can improve constraints during the Cosmic Dawn epoch.

Abstract

We compute cosmic microwave background (CMB) anisotropy constraints on exotic forms of energy injection in electromagnetic (e.m.) channels over a large range of timescales. These constraints are very powerful around or just after recombination, although CMB keeps some sensitivity e.g. to decaying species with lifetimes as long as $10^{25}\,$s. We review here complementary with CMB spectral distortions and primordial nucleosynthesis bounds, which dominate at earlier timescales. For the first time, we describe the effects of the e.m. energy injection on the CMB power spectra as a function of the injection epoch, using the lifetime of a decaying particle as proxy. We identify a suitable on-the-spot approximation. Our results are of interest not only for early universe relics constituting (a fraction of) the dark matter, but also for other exotic injection of e.m. radiation. For illustration, we apply our formalism to: i) Primordial black holes of mass $\in [10^{13.5},10^{16.8}]$ g, showing that the constraints are comparable to the ones obtained from gamma-ray background studies and even dominate below $\sim 10^{14}$g. ii) To a peculiar mass-mixing range in the sterile neutrino parameter space, complementary to other astrophysical and laboratory probes. iii) Finally, we provide a first estimate of the room for improvement left for forthcoming 21 cm experiments, comparing it with the reach of proposed CMB spectral distortion (PiXiE) and CMB angular power spectrum (CORE) missions. We show that the best and most realistic opportunity to look for this signal (or to improve over current constraints) in the 21 cm probe is to focus on the Cosmic Dawn epoch, $15\lesssim z\lesssim30$, where the qualitatively unambiguous signature of a spectrum in emission can be expected for models that evade all current constraints.