Gravitational lensing is an effect of the General Theory of Relativity. This is the bending of a distant source light in the gravitational field of a massive body; this bending, of course, is valid not only for light, but for the whole electromagnetic spectrum radiation. The massive body causing the light bending is called gravitational lens. Stars, galaxies and clusters of galaxies can act as gravitational lenses while the source is a high redshift galaxy or a quasar. As a result of gravitational lensing the parameters (position, brightness, shape) of the source are changed. Therefore, we have to consider rather the image of the source created by a gravitational lens than the source itself. Under certain conditions the image of the source could be split into several distinct images each with characteristics that differ from those of the source - this is the strong lensing. The weak lensing changes the source parameters, but does not split the source image. The changes of the source parameters induced by a gravitational lens help us to study the gravitational lens (light and dark) mass distribution. If the source is placed right behind the lens, then a ring of light, the so called Einstein ring, will result. The ring radius is tightly constrained by the mass projected inside the ring. If the source is variable and strong lensing takes place, there will be a time delay between the variability patterns of the distinct source images. We can estimate the Hubble parameter value measuring the time delay and building the lens mass model. This provides to us an independent Hubble parameter estimate.
Note, that our Sun also acts as a gravitational lens shifting the position of the stars projected close to the solar limb. This was observed firstly by A. Eddington during the solar eclipse in 1919 and was used as an observational confirmation of the General Theory of Relativity. The real development in the area of gravitational lensing started in 1979 when the first multiply imaged quasar (0957+561A & B) was discovered by Walsh, Carswell and Weymann.
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Among the various astrophysical applications of gravitational lensing we are interested mainly in studying the galaxies mass distribution and Hubble parameter estimation. The main outgoing or finished research subjects are listed below:
Written by B. Mihov, 2005