1979

Gravitational Lensing

One of the fundamental features of physicist Albert Einstein’s early-twentieth-century theory of general relativity is that space and time are curved near extremely massive objects. The curvature of space-time led Einstein and others to predict that light from distant objects would be bent by the gravitational field of massive foreground objects. The prediction was verified in 1919 by the British astrophysicist Arthur Stanley Eddington, who noticed that stars observed near the Sun during a solar eclipse were slightly out of position. Einstein continued to study this effect in the 1930s, and he and others, including the Swiss-American astronomer Fritz Zwicky, speculated that more massive objects, such as galaxies and clusters of galaxies, could bend and amplify light from distant objects almost as a lens bends and magnifies normal light.

It took many decades for astronomers to find observational evidence of such gravitational lensing, however. The first example was discovered in 1979 by astronomers at the Kitt Peak National Observatory in Arizona, who found an example of what appeared to be twin quasars—two active galactic nuclei very close to each other in the sky. The two quasars were shown to actually be a single object whose light was bent and split into two parts by the strong gravitational field of a foreground galaxy.

Since then, many more examples of gravitational lensing have been found, and the effect seems to occur in three ways: strong lensing is when distinct multiple or partial (usually arc-like) images are formed; weak lensing has been detected by observing small and subtle shifts in star or galaxy positions over large regions; and microlensing events have been detected when random distant stars (or even planets) have their brightness temporarily amplified by the gravitational lensing effect of a large foreground mass, such as another star or galaxy.

Gravitational lenses were initially discovered and studied as accidental, serendipitous events. Recently, however, a number of astronomical surveys have been conducted to intentionally search for gravitational lensing events, in order to obtain unique measurements of the properties of distant galaxies that would not be visible without the amplification from the lens, as well as the properties (such as mass) of the lensing galaxies and clusters themselves.

The thin arcs seen here are gravitationally lensed galaxies in the galactic cluster Abell 2218, photographed by the Hubble Space Telescope in 1999. These so-called Einstein rings are the smeared-out light from distant galaxies being bent by a massive foreground galaxy.