The Cosmology of Hawking Axions

Abstract

Since primordial black holes (PBH) are always non-relativistic, their cosmological energy fraction grows relative to that of radiation. Thus, for even a tiny initial PBH abundance, the universe generically enters an epoch of early matter domination, which ends when the PBH evaporate to restore the hot big bang. We consider the possibility that a PBH dominated era yields a population of boosted axion-like particles (ALPs) produced from Hawking evaporation. Eventually these ``Hawking axions" become non-relativistic and decay to photons, which can affect various cosmological and astrophysical observables. Here we place new constraints on these particles through their impact on big bang nucleosynthesis, cosmic microwave background, large-scale structure, and X-ray observables. Furthermore, if Hawking axions remain relativistic through recombination, then depending on the progenitor PBH mass, they contribute $ΔN_{\rm eff}\simeq0.05$-$0.11$, which may be observable with future CMB probes.