Gamma rays from primordial black holes

Abstract

This paper examines the possibilities of detecting hard gamma-rays produced by the quantum-mechanical decay of small black holes created by inhomogeneities in the early universe. Observations of the isotropic gamma-ray background around 100 MeV place an upper limit of 10^4 pc^-3 on the average number density of primordial black holes with initial masses around 10^15 g. The local number density could be greater than this by a factor of up to 10^6 if the black holes were clustered in the halos of galaxies. The best prospect for detecting a primordial black hole seems to be to look for the burst of hard gamma-rays that would be expected in the final stages of the evaporation of the black hole. Such observations would be a great confirmation of general relativity and quantum theory and would provide information about the early universe and about strong-interaction physics.