Gravitational Lenses
Most everyone is familiar with lenses. They are found in eyeglasses and other optical instruments. When light hits the curved glass in a lens, its path is bent and the image of the light source changes.
Einstein's general relativity theory predicts that strong gravitational fields will bend the path of nearby light rays. As a consequence of this theory, a very large mass can bend light rays and in a sense become a lens. The lens is a gravitational field rather than a piece of glass. It is called a gravitational lens.
A gravitational lens requires a very large mass, such as the mass of an entire galaxy. It also requires a very distant light source behind it. Quasars are among the most distant objects in the universe. Both are common enough that occasionally, by chance, a distant quasar should be aligned perfectly behind a galaxy. The galaxy will act as a gravitational lens for the quasar and alter its image. Do these gravitational lenses predicted from Einstein's theory actually exist?
Discovery
The answer came in 1979 when astronomers found two quasars, 0957+561A and 0957+561B, that are extremely close together in the sky. The numbers in their names refer to the position in the sky, as is common in naming quasars. Because they are both so close to the same position the A and B were added to distiguish the two quasars.
Further study revealed that these two quasars, like identical twins, are so similar that it seemed they were a double image of the same quasar. Detailed photographs of the region, made under the best sky conditions possible prior to the Hubble Space Telescope, revealed a faint fuzzy region near one of the quasars. This fuzz is a faint elliptical galaxy between the quasars and us.
The galaxy is a gravitational lens. It bends the light from the distant quasar to produce a double image of a quasar that is almost directly behind it. This discovery was the first confirmation of this prediction from Einstein's general relativity.
Since then many more gravitational have been discovered. Einstein's Ring and Einstein's Cross are perhaps the most interesting. Radio telescopes reveal Einstein's ring as a near perfect ring like image of the quasar. Einstein's Cross was found with the Hubble Space Telescope, and is four images of the quasar around the central lensing galaxy. Both of these effects require that the quasar be aligned nearly perfectly behind the lensing galaxy. Hence they are rare.
Gravitational Lenses and Dark Matter
Clusters of galaxies can also act as gravitational lenses. Images of some clusters of galaxies show many blue arcs which are the gravitationally lensed images of more distant background galaxies. Detailed study of these arcs allows astronomers to measure the total mass of the cluster of galaxies. We see only about 10% of the total mass of the clusters in the form of the individual galaxies in the clusters. The remaining 90% is dark matter. It seems that we can not find 90% of the matter in the universe.
In May 2007, astronomers announced the first direct detection of dark matter revealed by a gravitational lensing effect.
Small microlensing events also provide a way to search for dark matter. MACHOs (Massive Compact Halo Objects) are one possible component of dark matter. These objects, possibly including black holes, brown dwarfs, and white dwarfs, are collapsed stars that are too small and faint to be seen directly. Such objects orbiting our galaxy in the halo might occasionally pass in front of other nearby galaxies, such as the Magellanic Clouds. Doing so would produce a transient microlensing event. The random and transient nature of these microlensing events makes it difficult to detect MACHOs with this technique, but astronomers are conducting a number of searches.
Gravitational lenses provide experimental evidence supporting general relativity and help in the search for dark matter.
Paul A. Heckert - I have a Ph.D. in astrophysics, over 30 years experience teaching physics and astronomy, and over 60 published research articles.
