BARYONIC DARK MATTER: THE RESULTS FROM MICROLENSING SURVEYS

Abstract

Baryonic material can exist in several dark forms: ``planets," brown dwarfs, very old degenerate dwarf stars, and neutron stars. (Black holes are frequently added to this list, even though a black hole has no baryon number.) These objects, most of which emit some light but at levels below present day detection thresholds, are collectively known as Machos. Several groups have exploited the gravitational microlens signature to search for Machos in the dark halo of the Milky Way. Over 200 microlensing events have been reported (most by the MACHO Project, which uses the Great Melbourne Telescope near this conference site), of which about 20 are toward the Magellanic Clouds. The most straightforward interpretation of the results is that Machos make up between 20% and 100% of the dark matter in the halo, and that these objects weigh about 0.5 msun. Objects of substellar mass do not comprise much of the dark matter. Many alternative interpretations of these results have been proposed. We will discuss strategies for resolving the differences among these competing explanations.