PRIMORDIAL BLACK-HOLE FORMATION DURING THE QCD EPOCH

We consider the formation of horizon-size primordial black holes (PBH' s) from pre-existing density fluctuations during cosmic phase transiti ons. It is pointed out that the formation of PBH's should be particula rly efficient during the QCD epoch due to a substantial reduction of p ressure forces during adiabatic collapse, or equivalently, a significa nt decrease in the effective speed of sound during the color-confineme nt transition. Our considerations imply that for generic initial densi ty perturbation spectra PBH mass functions are expected to exhibit a p ronounced peak on the QCD-horizon mass scale similar to 1 M.. This mas s scale is roughly coincident with the estimated masses for compact ob jects recently observed in our galactic halo by the MACHO Collaboratio n. Black holes formed during the QCD epoch may offer an attractive exp lanation for the origin of halo dark matter evading possibly problemat ic nucleosynthesis and luminosity bounds on baryonic halo dark matter.