Gravitational lensing as a powerful astrophysical tool: Multiple quasars, giant arcs and extrasolar planets

Abstract

In the 25 years since the discovery of the first double quasar Q0957+561, gravitational lensing has established itself as a valuable tool in many branches of astronomy. Fields as different as galactic structure, cosmology, or extrasolar planets benefit from the gravitational lensing effect. This article starts with a brief historic reflection, then the basics of light deflection are reviewed. Observable lensing effects and a few examples of strong lensing phenomena are shown. In the main part four applications of ''strong'' lensing will be presented and discussed: The determination of the Hubble constant from time delay measurements in multiple quasars; it is argued that this method of determining H0 is competitive with other methods by now. The lensing‐derived values of H0 are on the low side. Microlensing of quasars– the effects of compact stellar‐mass objects on the apparent brightness – allows us to constrain the quasar size and the occurrence of dark matter objects.The frequency of giant luminous arcs strongly depends on the high mass end of the galaxy cluster distribution. Recent investigations show that arc statistics is in agreement with the concordance cosmological model. Searching for extrasolar planets is one of the most recent applications of gravitational lensing. The first detection shows that the method works well. This planet‐search method is complementary to other programs and has the potential to detect exo‐planets with lower masses than other ground‐based techniques.An outlook is provided on the prospects of gravitational lensing in the next few years. In particular the magnification effect on faint high‐redshift sources will be used for the investigation of the early universe, and the detection of low‐mass extrasolar planets will provide a valuable sample for statistical evaluations of the frequency of exoplanets.