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A gravitational lens is formed when the light from a very distant, bright source (such as a quasar) is "bent" around a massive object (such as a massive galaxy) between the source object and the observer. The process is known as gravitational lensing, and is one of the predictions of Albert Einstein's general relativity theory. It is sometimes known as the Einstein effect, although that is not the only meaning attributed to that term.
Description
Simulation To the right is a simulation of gravitational lensing caused by a Schwarzschild black hole passing in front of a background galaxy. A secondary image of the galaxy can be seen within the black hole's Einstein ring on the side opposite the galaxy. The secondary image grows (remaining within the Einstein ring) as the primary image approaches the black hole. The surface brightness of the two images remains constant, but their angular sizes vary, hence producing an amplification of the galaxy luminosity as seen by a distant observer. Maximum amplification occurs when the galaxy (or in this case a bright part of it) is exactly behind the black hole. History According to general relativity, mass "warps" space-time to create gravitational fields and therefore bend light as a result. This theory was confirmed in 1919 during a solar eclipse, when Arthur Eddington observed the light from stars passing close to the sun was slightly bent, so that stars appeared slightly out of position. Einstein realized that it was also possible for astronomical objects to bend light, and that under the correct conditions, one would observe multiple images of a single source, called a gravitational lens or sometimes a gravitational mirage. However, as he only considered gravitational lensing by single stars, he concluded that the phenomenon would most likely remain unobserved for foreseeable future. In 1937, Fritz Zwicky first considered the case where a galaxy could act as a lens, something that according to his calculations should be well within the reach of observations. It was not until 1979 that the first gravitational lens would be discovered. It became known as the "Twin Quasar" since it initially looked like two identical quasars; it is officially named Q0957+561. This gravitational lens was discovered accidentally by Dennis Walsh, Bob Carswell, and Ray Weymann using the Kitt Peak National Observatory 2.1 meter telescope. The study of gravitational lenses is an important part of the future of astronomy and astrophysics. Cosmological applications The most important application of gravitational lensing in cosmology is the weak lensing of galaxies. By measuring the shapes and orientations of large numbers of distant galaxies, their orientations can be averaged to measure the shear of the lensing field in any region. This, in turn, can be used to reconstruct the mass distribution in the area: in particular, the background distribution of dark matter can be reconstructed. Since galaxies are intrinsically elliptical and the weak gravitational lensing signal is small, a very large number of galaxies must be used in these surveys. These weak lensing surveys must carefully avoid a number of important sources of systematic error: the intrinsic shape of galaxies, the tendency of a camera's point spread function to distort the shape of a galaxy and the tendency of atmospheric seeing to distort images must be understood and carefully accounted for. The results of these surveys are important for cosmological parameter estimation, to better understand and improve upon the Lambda-CDM model, and to provide a consistency check on other cosmological observations. They may also provide an important future constraint on dark energy. Strong gravitational lenses may be used to examine objects at distances at which they would not normally be visible, providing information from further back in time than otherwise possible. Also, not just the object being lensed but the lens itself can provide useful information. By inverting the lens equations information can be gathered on the mass and distribution of the lensing body. The statistics of strong gravitational lenses can also be used to measure values of cosmological parameters such as the cosmological constant and the mean density of matter in the universe. Presently, the statistics do not place very strong limits on cosmological parameters, partly because the number of strong lenses found is relatively small (less than a hundred). Another parameter that may come out of the study of gravitational lenses is Hubble's constant which encodes the age and size of the universe. It can be determined, in theory, by measuring two quantities: the angular separation between two images, and the time delay between these images. There are two contributions to the time delay: Because the two rays travel through different parts of the potential well created by the deflector, the clocks carrying the source's signal will differ by a small amount. Astronomical applications
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