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 ?
Since then, astronomers have used gravitational lenses in many ways, including studying dark matter and as "Nature's Telescope" to investigate galaxies in the distant universe
The imprint of their extremely high gravity was expected to be seen in the cosmic microwave background - the radiation left over from the big bang - or as gravitational lenses that bend distant light towards us.
According to Einstein's theory of general relativity, massive objects bend light, acting as gravitational lenses.
None reveals major new discoveries, but the telescope's unusual optics - a mosaic of 36 mirrors that acts as a single 10-meter mirror- have probed gravitational lenses, clistant galaxies, and quasars in unprecedented detail, says Jerry Nelson, director of the Keck Observatory.
Using Sloan data, the team picked out large, elliptical galaxies capable of acting as gravitational lenses.
The Adler's next citizen science project is Space Warps, which asks for the public's help in finding gravitational lenses in deep space.
They act as gravitational lenses with a somewhat distorted focus, creating a diamond-shaped region wherein objects produce three distinct but distorted images.
The theory of gravitational lenses says that if the lensed object, the lens and the observer are perfectly in line, the lens should make an image in the form of a circular ring.
The notion that not only luminous galaxies but also invisible matter could act as gravitational lenses has prompted a number of searches for multiple images and for unusually bright quasars.
More than 100 gravitational lenses have been discovered so far.
Astronomers from the CLASH team and the Supernova Cosmology Project are using these supernovae in a new method for measuring the magnification, or prescription, of the gravitational lenses.
Astronomers already use gravitational lenses to identify objects that would otherwise be too far or too faint to detect; a new Hubble Space Telescope survey is using the technique to find galaxies that formed a mere several hundred million years after the Big Bang.