Speaker: Wyn Evans
Title: Microlensing and the Dark Matter Problem.
Affiliation: Theoretical Physics, University of Oxford.
Date: Tuesday, March 20, 2001.
Place and Time: Room 499 - CSIT, 3:45 pm.

Abstract. One of the major problems of modern astrophysics and cosmology is the nature of the dark matter. The evidence for dark matter in haloes surrounding galaxies like our own is extensive, but it is not clear whether the dark matter is baryonic or non-baryonic. Baryonic candidates include cold gas, brown dwarfs, low mass stars or supermassive black holes. Non-baryonic candidates include weakly interacting massive particles, such as axions or neutralinos, as well as macroscopic candidates like primordial black holes.     Microlensing has established itself as a powerful new method for the detection of compact dark matter. The theory of microlensing is sketched and its similarity with the optical effect of twinkling is explained. The results of the microlensing surveys towards two nearby satellite galaxies of the Milky Way, namely the Large and and the Small Magellanic Clouds, are discussed. Although about 15 examples of microlensing have been identified, the interpretation of the results is not clear-cut. The events may be caused by compact objects in the halo of the Milky Way, in which case part of the dark matter has been at long last identified. Or the events may be caused by dim stars in the satellite galaxies themselves, in which case the experiments are not teaching us anything fundamental about the nature of the dark matter.     A new experiment is described, the POINT-AGAPE collaboration (see "http://www.point-agape.org"), which has the potential to settle matters. It employs the Wide Field Camera on the 2.5 m Isaac Newton Telescope to carry out a microlensing survey towards the nearby large spiral galaxy in Andromeda (M31). The high inclination of M31's disk causes an asymmetry in the observed rate of microlensing by lenses in a halo. This is an unambiguous signature of dark objects in the halo as opposed to dim stellar lenses. The first results of this experiment are described and the first microlensing candidates will be presented.