Could MACHOS be Primordial Black Holes formed during the QCD Epoch?

Abstract

Observations by the MACHO collaboration indicate that a significant fraction of the galactic halo dark matter may be in form of compact objects with masses $M\sim 0.5M_{\odot }$. Identification of these objects as red or white dwarfs is problematic due to stringent observational upper limits on such dwarf populations. Primordial black hole (PBH) formation from pre-existing density fluctuations is facilitated during the cosmic QCD transition due to a significant decrease in pressure forces. For generic initial density perturbation spectra this implies that essentially all PBHs may form with masses close to the QCD-horizon scale, $M_h^{QCD}\sim 1M_{\odot }$. It is possible that such QCD PBHs contribute significantly to the closure density today. I discuss the status of theoretical predictions for the properties of QCD PBH dark matter. Observational signatures of and constraints on a cosmic solar mass PBH population are also discussed.

Comments: 4 pages, RevTex, to appear in proceedings of DM98, Los Angeles (ed D. Cline, Elsevier)