Evaporating primordial black holes, the string axiverse, and hot dark radiation
Abstract
We show that primordial black holes (PBHs) develop nonnegligible spins through Hawking emission of the large number of axionlike particles generically present in string theory compactifications. This is because scalars can be emitted in the monopole mode ($l=0$), where no angular momentum is removed from the BH, so a sufficiently large number of scalars can compensate for the spindown produced by fermion, gauge boson, and graviton emission. The resulting characteristic spin distributions for $10^8$$10^{12}$ kg PBHs could potentially be measured by future gammaray observatories, provided that 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 local energy density of hot, MeVTeV, axions produced by this Hawking emission can possibly exceed $\rho_{\rm CMB}$. Evaporation constraints on PBHs are also somewhat weakened.
 Publication:

arXiv eprints
 Pub Date:
 October 2021
 arXiv:
 arXiv:2110.13602
 Bibcode:
 2021arXiv211013602C
 Keywords:

 Astrophysics  Cosmology and Nongalactic Astrophysics;
 General Relativity and Quantum Cosmology;
 High Energy Physics  Phenomenology
 EPrint:
 5 pages, 3 figures