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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 was one of the predictions made by Einstein's general relativity.
In a gravitational lens, the gravity from the massive object bends light like a lens. As a result, the path of the light from the source is curved, distorting its image, and the apparent location of the source may be different from its actual position. In addition, the observer may see multiple images of a single source. If the source, massive object, and the observer lie on a straight line, the source will appear as a ring behind the massive object. This image is known as an Einstein ring. 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
- Micro-lensing: where no distortion in shape can be seen but the amount of light received from a background object can be changed.
Gravitational lenses act on all kinds of electromagnetic radiation, not just visible light. Weak lensing effects are being studied for the cosmic microwave background and strong lenses have been observed in radio and x-ray regimes as well.
According to general relativity, gravitational fields "warp" space-time 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.
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 (see below). 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.
In weak lensing large scale maps of dark matter distributions may be produced, and these techniques are particularly important to cosmology as they provide a measure of the mass directly, without relying on any assumptions about the link between distributions of dark matter and visible. Lensing therefore can give a way of constraining the amount of dark matter in the universe and the manner in which it clusters together.
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:
- the first is the obvious delay due to the difference in optical path length between the two rays.
- the second is a general relativistic effect, the Irwin Shapiro time-delay, that causes a change in the rate that clocks tick as they pass through a gravitational field.
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.
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 maybe even planets beyond the solar system.
References and external links
- Newbury, Pete, "Gravitational Lensing". Institute of Applied Mathematics, The University of British Columbia.
- Cohn, J., "Gravitational Lensing". Astron.berkeley.edu.
- "Q0957+561 Gravitational Lens". Harvard.edu.
- "Gravitational lensing". Gsfc.nasa.gov.
- Bridges, Andrew, "Most distant known object in universe discovered". Associated Press. February 15, 2004. (Farthest galaxy found by gravitational lensing, using Abell 2218 and Hubble Space Telescope.)
- "HST images of strong gravitational lenses". Harvard-Smithsonian Center for Astrophysics.
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