A phenomenon resulting from Einstein's general theory of relativity. The theory showed that light follows a curved path when it passes close to massive objects, and thus presented the possibility that galaxies can focus the light of more distant objects along the same line of sight. This lensing effect has been observed around certain massive galaxies and clusters of galaxies.
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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.
Description
In a gravitational lens, the gravity from the massive object bends light like a lens. More commonly, the massive galaxy is off-center, creating a number of images according to the relative positions of the source, lens, and observer, and the shape of the gravitational well of the lensing galaxy.
There are three classes of gravitational lensing:
Strong lensing: where there are easily visible distortions such as the formation of Einstein rings, arcs, and multiple images. Weak lensing: where the distortions of background objects are much smaller and can only be detected by analysing large numbers of objects to find distortions of only a few percent. The lensing shows up statistically as a preferred stretching of the background objects perpendicular to the direction to the center of the lens.The effect is small, such that (in the case of strong lensing) a galaxy having a mass of over 100 billion solar masses will produce multiple images separated by only a few arcseconds.
Gravitational lenses act equally on all kinds of electromagnetic radiation, not just visible light. Weak lensing effects are being studied for the cosmic microwave background as well as galaxy surveys. If a strong lens produces multiple images, there will be a relative time delay between two paths: that is, in one image the lensed object will be observed before the other image.
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.
History
According to general relativity, mass "warps" space-time to create gravitational fields and therefore bend light as a result.
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.
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. 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.
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.
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. the second is a general relativistic effect,
the Shapiro effect, that describes light rays as taking longer to traverse a region of stronger gravitation, (see: gravity well, gravitational time dilation).
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
Gravitational lenses can be used as gravitational telescopes, because they magnify objects seen behind them. Researchers at Caltech have used the gravitational lensing afforded by the Abell 2218 cluster of galaxies to detect the most distant galaxy known (February 15, 2004) through imaging with the Hubble Space Telescope.
Gravitational microlensing can provide information on comparatively small astronomical objects, such as MACHOs within our own galaxy, or extrasolar planets (planets beyond the solar system).
Gravitational lensing can be used to calculate an estimate of the amount of dark matter contained in the lensing body. "The Origin of Gravitational Lensing: A Postscript to Einstein's 1936 Science paper". A Computer Program to visualize Gravitational Lenses, Francisco Frutos-Alfaro "G-LenS". (Farthest galaxy found by gravitational lensing, using Abell 2218 and Hubble Space Telescope.) Analyzing Corporations ... and the Cosmos An unusual career path in gravitational lensing. Gravitational lensing on arxiv.org Wikimedia Commons has media related to: Gravitational lens
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