The cosmology of black hole relics

The cosmological implications of the possibility that the end point of black hole evaporation is a stable relic state are investigated. We provide a toy model in which black holes evaporate to leave a relic, and address the question of nucleation of such objects in a thermal bath of gravitons at high temperature. Two options are chosen for a primordial black hole spectrum. In the first, appropriate to black holes formed during phase transitions, black holes form over a limited mass range; in this case one can obtain a substantial relic density and the relics may provide the necessary dark matter to close the Universe. In the second case, appropriate to formation from primordial density perturbations, black holes form over an extended mass range with a power-law spectrum. In this case the evidence is against the relics providing a critical density. In any event, the existence of relics would place strong constraints on cosmology.