Invited_talk abstract details

Strong gravitational lenses contain 2-4 multiple images of quasars. The lenses are galaxies or clusters of galaxies. The image separations are between 1 and 15 seconds of arc. In general, to derive models for the mass distribution of each lens, one requires Hubble Space Telescope images, such as those from the CASTLES catalogue. The fluxes of source quasars show variability that is measurable. The variations in fluxes appear in all the images, but with different phases for the different images. The resulting time delays between pairs of images have magnitudes that run from days to years. Measurements of these time delays allow us in principle to estimate cosmological parameters such as the Hubble constant $H_0$. Such measurements are complicated by the relative weakness of typical quasar fluctuations and by the possible degradation of measurements by noise under typical conditions over spans of years. Furthermore, telescope time for monitoring lenses comes at a high premium. In spite of the difficulties, we now have about 15 lens systems for which time delays have been measured. I describe recent interesting results. I also discuss the future, when monitoring telescopes such as Pan-STARSS and LSST are online. Such telescopes will resolve the temporal dimension with dense cadences and will yield thousands of new lens systems that will be useful for cosmology with new large samples that will represent a paradigm shift for lenses.