Some cosmological consequences of primordial black-hole evaporations

Abstract

According to Hawking, primordial black holes of less than 10/sup 15/ g would have evaporated by now. This paper examines the way in which small primordial black holes could thereby have contributed to the background density of photons, nucleons, neutrinos, electrons, and gravitons in the universe. Any photons emitted late enough should maintain their emission temperature apart from a redshift effect: it is shown that the biggest contribution should come from primordial black holes of about 10/sup 15/ g, which evaporate in the present era, and it is argued that observations of the ..gamma..-ray background indicate that primordial black holes of this size must have a mean density less than 10/sup -8/ times the critical density. Photons which were emitted sufficiently early to be thermalized could, in principle, have generated the 3 K background in an initially cold universe, but only if the density fluctuations in the early universe had a particular form and did not extend up to a mass scale of 10/sup 15/ g. Primordial black holes of less than 10/sup 14/ g should emit nucleons: it is shown that such nucleons could not contribute appreciably to the cosmic-ray background. However, nucleon emission could have generated the observedmore » number density of baryons in an initially baryon-symmetric universe, provided some CP-violating process operates in black hole evaporations such that more baryons are always produced than antibaryons. We predict the spectrum of neutrinos, electrons, and gravitons which should result from primordial black-hole evaporations and show that the observational limits on the background electron flux might place a stronger limitation on the number of 10/sup 15/ g primordial black holes than the ..gamma..-ray observations. Finally, we examine the limits that various observations place on the strength of any long-range baryonic field whose existence might be hypothesized as a means of preserving baryon number in black-hole evaporations. (AIP)« less