Gravitational lenses are accidental natural arrangements of gravity, light, and distant astronomical objects that create altered images of the those objects. Commonly, a lens is a piece of glass shaped so as to bend light passing through it. In the process, it alters the image of the light source as observed through the lens. A gravitational lens bends light using gravity rather than glass. Gravitational lensing is a useful tool for astronomers, allowing them to accurately determine the mass of distant galaxies and clusters of galaxies, including non-radiating (but gravitating) matter that cannot be observed directly.
Gravitational lensing is predicted by Einstein's theory of general relativity, which states that a gravitational field will bend the path of a ray of light. (Newton's older theory, according to which light is a stream of material particles, also predicted that light would be influenced by gravity; however, Einstein predicted a bending effect twice as great as Newton's, and has been confirmed by observation.) This bending effect is generally slight. Therefore, to produce significant lensing (image focusing) a comparatively large mass, such as a black hole, galaxy, cluster of galaxies, or the like, is required. What is more, gravitational lensing requires not only a lensing mass, but also a light source behind the lensing mass. Quasars, for example, are among the most distant objects in the Universe. If by chance a quasar is aligned with a galaxy (as seen from Earth), the galaxy may act as a gravitational lens and alter the image of the quasar.
General relativity was dramatically confirmed in 1919 when its prediction that starlight would be bent by passing near the Sun was verified. However, gravitational lensing of an entire image was not observed until 1979, when astronomers noticed that the two quasars, designated 0957 + 561A and 0957 + 561B, are unusually close together in the sky. (The designation numbers refer to the quasars' position in the sky, while the A and B distinguish the two nearby objects.) Investigating further, astronomers found that these quasars have nearly identical properties, as if they were a double image of the same quasar. Detailed photographs of the region revealed a fuzzy area near one of the quasar images. This fuzz, it turned out, was the faint image of an elliptical galaxy. This galaxy acts as a gravitational lens that bends the light from a single quasar, almost directly behind it as seen from Earth, to produce a double image. Since this initial discovery, dozens of other gravitational lenses have been discovered. Two of the most famous have been dubbed Einstein's Ring and Einstein's Cross. Ein stein's Ring is observed by radio telescopes to be a near perfect ring-image of a quasar. The Hubble Space Telescope reveals Einstein's Cross as four images of a quasar, arranged in a cross pattern around a central image of the lensing galaxy.
Objects other than single galaxies can also serve as gravitational lenses. Images of some clusters of galaxies show bright arcs in their vicinity, the gravitationally lensed images of more distant galaxies. By studying these arcs, astronomers can determine the total mass of the lensing cluster. It turns out that only 10% of the total mass of the cluster of galaxies can be accounted for by the visible galaxies in the cluster; the other 90% of the mass is unseen "dark matter," one of the standing mysteries of modern cosmology. Astronomers do not know what dark matter is, (or if it is matter at all, rather than "dark energy," a currently favored theory) but have observed that it seems to constitute 90% of the mass of the Universe.
One possible component of dark matter is massive compact halo objects (MACHOs). MACHOs are faint or nonradiating objects that may exist in large numbers in a spherical halo surrounding each galaxy (including ours). An otherwise invisible MACHO passing in front of a star in a nearby galaxy such as the Andromeda galaxy will produce a small gravitational-lens effect. Because MACHOs are in rapid motion relative to the Earth, such a microlensing event would produce a transient brightening of the distant star rather then a drastic, semipermanent distortion like that produced by a galactic lens. Numerous MACHO-type microlensing events have been observed, but their low rate shows that there are not enough MACHOs to account for the Universe's dark matter.
See also Gravity and gravitation.
Glanz, James, "In the Dark Matter Wars, Wimps Beat Machos." New York Times. (February 29, 2000).