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]
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 first two of these surveys will provide excellent data of weak gravitational lensing, which is one of the key probes to test models for the cosmic acceleration.
Through a process called "weak gravitational lensing," the distortions in the B-mode polarization pattern, they hoped, would allow astronomers to map regions of the universe filled with invisible "dark matter" and "dark energy" and well as provide a test for general relativity on cosmological scales.
But masses calculated using X-rays and weak gravitational lensing are consistent within about 10 to 15% of each other, Benson says.
Observations of weak gravitational lensing and statistical tomographic techniques have revealed that galaxies have formed along filaments, essentially one-dimensional lines or strings [1], see Fig.1.
These 37 papers and analyses of papers, along with the full collection of posters, were presented at the conference of June 2006, and concentrate largely on methodological issues peculiar to current research, including cosmology (including statistical challenges of weak gravitational lensing), small-N problems (including Bayesian methods in particle physics), astronomical surveys (including photometric calibration), planetary systems (including Bayesian model selection), periodic variability (including nonparametric estimation of dark matter distributions), developments in statistics and cross-disciplinary perspectives from particle physics, statistics and astronomy.
Objective: Weak gravitational lensing is a very powerful tool for investigating the relation between galaxies and their host dark matter halos.thanks to current photometric surveys such as kids, Des and hsc, The quality of the data available for weak lensing studies is rapidly improving.however, There has not yet been a corresponding improvement in the methods used for the analysis of this data: Most weak lensing studies are still carried out by stacking the signal in bins, Resulting in a net loss of an alternative to stacking, I have recently developed a new method to infer the distribution of halo masses of a population of galaxies with weak lensing data.
Andisheh Mahdavi (San Francisco State University) first mapped the cluster's dark matter distribution with weak gravitational lensing in 2007.
Weak gravitational lensing uses measurements of the deflection of light from distant galaxies to infer the intervening distribution of matter, and thus allows us to discriminate between acceleration models.
Wide-field surveys such as the Sloan Digital Sky Survey (page 20) are measuring the universe's mass distribution using the phenomenon known as weak gravitational lensing. Much like Eddington's experiment, some of the most evocative studies involve the microlensing of relatively nearby objects.
Objective: Weak gravitational lensing is one of the most promising techniques for probing and mapping the dark matter distribution within our Universe.
Objective: "Weak gravitational lensing has emerged as a powerful tool to probe the distribution and nature of the dark components of our Universe, and thus to better constrain our cosmological model.