Gravitational lensing is a distortion of light from a distant object caused by the gravitational pull of an intervening object. This effect is called lensing because the gravitational pull bends the light somewhat as a lens would.
All objects with mass “warp” or “bend” space because of their gravity. With small objects, this effect cannot be detected. However, massive objects can distort space enough to noticeably bend the light passing through it. For example, consider a distant bright object, such as a bright galaxy. Imagine that there is a massive object, such as a cluster of galaxies, between that distant galaxy and an observer. The light from that distant galaxy will be distorted or bent by the cluster of galaxies before reaching the observer. The German-born physicist Albert Einstein predicted this bending or warping of light, based on his general theory of relativity.
Gravitational lensing is often divided into three categories. They are: (1) strong lensing, (2) weak lensing, and (3) microlensing.
Strong lensing
results in easily visible distortions. If the distant object, the lensing object, and the observer all lie in a straight line, the light from the distant object will appear as a ring around the lensing object. Such a ring is often called an Einstein ring. If the two objects and the observer are not properly aligned, the observer may see arcs (partial rings) or multiple images of the distant object. By examining the distortion, astronomers can use strong lensing to map in detail the distribution of matter in the lensing object. In some cases, strong lensing can also magnify the distant object. Astronomers can use this magnification to detect distant objects too faint to be seen without lensing.
Weak lensing.
In weak lensing, the bending is much smaller. Astronomers can only detect it by observing subtle distortions in the shape of the distant object. Astronomers can use weak lensing to map the general distribution of matter in clusters of galaxies. This mapping can include dark matter, a mysterious substance thought to make up the majority of matter in the universe (see Dark matter ).
Microlensing.
In microlensing, no distortion in shape can be seen in the distant object. However, the amount of light received from the object is slightly changed. Microlensing can be used to search for planets crossing in front of distant stars. Although a planet itself may be too faint to detect, its gravitational field slightly focuses the star’s light. This focusing alerts astronomers to the planet’s presence. Astronomers also use microlensing to seek out a type of dark matter called massive compact halo objects (MACHO’s).
