Cosmological constraints on exotic injection of electromagnetic energy

Abstract

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