gravitational lens

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Gravitational lensing

gravitational lens

A concept arising from the fact that a gravitational field bends light, and hence a concentration of mass can focus light rays in a manner similar to that of a lens. In the illustration, the observer at O sees two apparent images S′ of the background source S caused by lensing effects of the intervening galaxy. The theory of gravitational lensing was discussed by both Einstein and Lodge in 1919, and its applications to cosmology realized by Zwicky in 1937, but the first known gravitational lens (the double quasar) was not discovered until 1979. Lensing by a smooth mass distribution such as a galaxy or a cluster of galaxies is known as macrolensing, and can occur in several forms.

The simplest form of gravitational lensing is where a pointlike background source, usually a quasar, is split into multiple images, the location and number of which are dependent on the relative geometry of the source and lens. The lens will distort and concentrate the original path of the light, so that an image will also appear brighter, or magnified. Different images forming a multiple system may have their luminosities magnified by different factors. Cases of double, triple and even quadruple lensing have been found (e.g. the Cloverleaf and the Einstein cross). In most cases the lensing galaxy is not observed. Theoretical models of gravitational lensing predict that there should always be an odd number of images so both the double and quadruple systems are expected to have a central image that is too faint to be detected.

If the background object is a distant galaxy that is itself extended, the lensed images are smeared out into long luminous arcs several arc seconds long. Such arcs are commonly observed in the core of rich clusters of galaxies, usually elongated tangentially to the cluster center and bluer in color than the cluster member galaxies. In several clusters many tens of smaller arclets are seen, which originate from weak lensing of background galaxies that are not so strongly magnified. The most extreme case of gravitational lensing is observed when an extended background source is exactly aligned with a symmetrical lens. The lensed image takes the form of an Einstein ring.

The alteration in the light path to the quasar will result in different times of flight for each image. If the quasar itself is variable, then a corresponding time delay for the brightening to be seen in each component of the image may be measured. The difference in the light travel time is related to the inverse of the Hubble constant, so it is theoretically possible to estimate H 0 from such time delays. In practice, precise modeling of the lens geometry is required before H 0 can be well constrained.

It is possible that individual stars in a lensing galaxy can cross the light path to the quasar and cause fluctuations in image brightness known as microlensing. This effect can also be seen when objects known as MACHOs in the galactic halo lens the light from an extragalactic star to cause a large amplification in its brightness, although such events are very rare.

gravitational lens

[‚grav·ə′tā·shən·əl ′lenz]
(astronomy)
A massive galaxy or other massive object whose gravitational field focuses light from a distant quasar near or along its line of sight, giving a double or multiple image of the quasar.
References in periodicals archive ?
The supermassive black hole at its center wields an enormous amount of gravity, which causes gravitational lensing.
Gravitational lensing magnifies them so that you can see small details that otherwise are unimaginable.
Gravitational lensing occurs because of galaxies with extreme gravitational power, which magnifies and warps light travelling from the fringes.
It's an amazing example of gravitational lensing," Lotz says.
Team members Kenneth Wong and Sherry Suyu of Academia Sinica Institute of Astronomy and Astrophysics (ASI) in Taipei, Taiwan, used the gravitational lensing from the chance alignment to measure the giant galaxy's total mass, including the amount of dark matter, by gauging the intensity of its lensing effects on the background galaxy's light.
Outlining the principle of gravitational lensing, Dr Bridle said that when light travelled past massive objects such as galaxies, its path was deflected by the object's gravitational field.
1] claim that the analysis of gravitational lensing data from the HST observations of the galaxy cluster CL 0024+17 demonstrates the existence of a "dark matter ring".
By observing a rare collision of galaxies, Marusa Bradac of the Kavli Institute for Particle Astrophysics and Cosmology at Stanford, along with colleagues, have made the first direct detection of "dark matter" by exploiting one of its few visible effects: gravitational lensing.
The scientists used a special method called gravitational lensing.
To obtain the dark matter distribution, David Wittman, Anthony Tyson and David Kirkman of Bell Labs, along with two collaborators, Ian Dell'Antonio of both the National Optical Astronomy Observatory as well as Brown University and Gary Bernstein of the University of Michigan at Ann Arbor, used a method known as weak gravitational lensing, in which they analyzed the light from 145,000 very distant galaxies for evidence of distortions produced by dark matter that lay in its path.
The event itself was spotted by the Optical Gravitational Lensing Experiment (OGLE) collaboration, a Polish astronomical project based at the University of Warsaw which utilizes the 1.
Monica Young replies: I agree that gravitational lensing should be a great way to detect dark matter clumps.