Evaporating Primordial Black Holes, the String Axiverse, and Hot Dark Radiation

The search for primordial black holes (PBHs) with masses M≪M_{⊙} is motivated by natural early-Universe production mechanisms and that PBHs can be dark matter. For M≲10^{14}  kg, the PBH density is constrained by null searches for their expected Hawking emission (HE), the characteristics of which are, however, sensitive to new states beyond the standard model. If there exists a large number of spin-0 particles in nature, PBHs can, through HE, develop and maintain non-negligible spins, modifying the visible HE. Taking account of the string axiverse of spin-0 axions that are expected to be present in string theory, we study in detail the resulting PBH characteristics, finding that for 10^{8}≲M≲10^{12}  kg evaporation constraints on PBHs are somewhat weakened, and the spin distributions could potentially be measured by future gamma-ray observatories if the PBH abundance is not too small. This yields a unique probe of the total number of light scalars in the fundamental theory, independent of how weakly they interact with known matter. The present energy density of hot, MeV-TeV axions produced by HE can exceed ρ_{CMB}.