Primordial Black Holes as Dark Matter

Abstract

Primordial black holes (PBHs) may readily form during the radiation dominated stages of the universe from the gravitational collapse of horizon-size energy density fluctuations of moderate amplitude. PBH formation is particularly efficient during a cosmic QCD transition, due to a generic decrease in pressure forces. This allows for a primordial black hole mass function which is peaked close to the QCD horizon scale, M _h ~ 2M _⊙(Tc/100MeV)^–2, where T _c is the cosmic QCD phase transition temperature. Such formed PBHs are a viable cold dark matter candidate and may offer an explanation for the results of microlensing surveys, as performed by the MACHO and EROS collaborations, that a fraction of the galactic halo dark matter may be in form of compact massive objects. It may well be that MACHO and EROS have only detected the low-mass tail of a PBH population comprising the dark matter with a mass function extending beyond M _bh > 2M _⊙. Observational signatures and constraints of cosmic solar mass PBH dark matter are discussed.