Editing General relativity (section)

=== Gravitational lensing ===
{{Main|Gravitational lensing}}
[[File:Einstein cross (cropped).jpg|thumb|upright=0.88|[[Einstein cross]]: four images of the same astronomical object, produced by a gravitational lens]]
The deflection of light by gravity is responsible for a new class of astronomical phenomena. If a massive object is situated between the astronomer and a distant target object with appropriate mass and relative distances, the astronomer will see multiple distorted images of the target. Such effects are known as gravitational lensing.<ref>For overviews of gravitational lensing and its applications, see {{Harvnb|Ehlers|Falco|Schneider|1992}} and {{Harvnb|Wambsganss|1998}}</ref> Depending on the configuration, scale, and mass distribution, there can be two or more images, a bright ring known as an [[Einstein ring]], or partial rings called arcs.<ref>For a simple derivation, see {{Harvnb|Schutz|2003|loc=ch. 23}}; cf. {{Harvnb|Narayan|Bartelmann|1997|loc=sec. 3}}</ref>
The [[Twin Quasar|earliest example]] was discovered in 1979;<ref>{{Harvnb|Walsh|Carswell|Weymann|1979}}</ref> since then, more than a hundred gravitational lenses have been observed.<ref>Images of all the known lenses can be found on the pages of the CASTLES project, {{Harvnb|Kochanek|Falco|Impey|Lehar|2007}}</ref> Even if the multiple images are too close to each other to be resolved, the effect can still be measured, e.g., as an overall brightening of the target object; a number of such "[[microlensing]] events" have been observed.<ref>{{Harvnb|Roulet|Mollerach|1997}}</ref>

Gravitational lensing has developed into a tool of [[observational astronomy]]. It is used to detect the presence and distribution of [[dark matter]], provide a "natural telescope" for observing distant galaxies, and to obtain an independent estimate of the [[Hubble constant]]. Statistical evaluations of lensing data provide valuable insight into the structural evolution of [[galaxy|galaxies]].<ref>{{Harvnb|Narayan|Bartelmann|1997|loc=sec. 3.7}}</ref>