Some cosmological consequences of primordial black-hole evaporations

Abstract

The paper examines how the evaporation of primordial black holes (PBHs) could have contributed to the background density of photons, nucleons, neutrinos, electrons, and gravitons in the universe. Major uncertainties in the analysis are discussed, a Friedmann model is adopted for the early universe, the important features of Hawking's (1974) particle-emission process are summarized, and analytical limits are placed on the PBH mass spectrum. The main conclusions are that: (1) any photons emitted late enough should maintain their emission temperature, apart from a redshift effect; (2) the greatest contribution should come from PBHs of about 10 to the 15th power g, which evaporate in the present era; (3) photons emitted sufficiently early to be thermalized could have generated the 3-K background, but only if density fluctuations in the early universe had a particular form and did not exceed a certain mass scale; (4) PBHs of less than 10 to the 14th power g should emit nucleons, but such nucleons could not contribute appreciably to the cosmic-ray background; and (5) nucleon emission could have produced the observed number density of baryons in an initially baryon-symmetric universe, provided more baryons than antibaryons are emitted in PBH evaporations.