Gamma rays from primordial black holes.

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 10,000 per cu pc on the average number density of primordial black holes with initial masses around 10 to the 15th power g. The local number density could be greater than this by a factor of up to 1 million 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.