- Concentration of matter dense enough to refract light into an image. The idea of gravitational lensing arises from General Relativity, with its proof that the straight-line paths of light can be bent by gravitation. Many cases of small galaxies and other objects performing gravitational lensing have been observed by astronomers and the method has been used as a technique for hunting missing mass in the universe. If such mass is in compact form, it will lens the light from distant galaxies. Lenses form elaborate images of distant objects in the universe, sometimes forming several images of the same object and generating distorted, arc-shaped images of distant galaxies called Einstein rings. Many have now been observed.
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