Cosmological Microlensing

Variability in gravitationally lensed quasars can be due to intrinsic fluctuations of the quasar or due to " microlensing " by compact objects along the line of sight. If disentangled from each other, microlens-induced variability can be used to study two cosmological issues of great interest, the size and brightness profile of quasars on one hand, and the distribution of compact (dark) matter along the line of sight. In particular, multi-waveband observations are useful for this goal. In this review recent theoretical progress as well as observational evidence for quasar microlensing over the last few years will be summarized. Comparison with numerical simulations will show " where we stand ". Particular emphasis will be given to the questions microlensing can address regarding the search for dark matter, both in the halos of lensing galaxies and in a cosmologically distributed form. A discussion of desired observations and required theoretical studies will be given as a conclusion/outlook. 1. What is cosmological microlensing? 1.1. Mass, length and time scales The lensing effects of cosmologically distant compact objects in the mass range 10 −6 ≤ m/M ⊙ ≤ 10 6 on background objects is usually called " cosmological microlensing ". The " source " is typically a background quasar, but in principle other distant source can be microlensed as well, i.e. distant supernovae or gamma-ray bursters. The only " condition " is that the source size is comparable to or smaller than the Einstein radius of the respective lenses. The microlenses can be ordinary stars, brown dwarfs, planets, black holes, molecular clouds, or other compact mass concentrations (as long as their physical size is smaller than the Einstein radius). In most practical cases, the micro-lenses are part of a galaxy which acts as the main (macro-) lens. However, microlenses could also be located in, say, clusters of galaxies or they could even be imagined " free floating " and filling intergalactic space.