One dramatic prediction of General Relativity is that gravity bends the path followed by a light signal. In cosmology this leads to the phenomenon of gravitational lensing.

Suppose that light from a very distant bright object that we observe, for example a quasar, passes through a compact cluster of galaxies. The strong gravitational field of the cluster will bend the light, thereby distorting the image into an arc or a circle (also called an Einstein ring), or possibly creating multiple images (for example, the Einstein cross, a four-fold image of a distant quasar). In this way the cluster of galaxies acts as a gravitational lens.

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Images of Einstein rings

Images taken by the Hubble Space Telescope.

Einstein rings

Einstein cross

Einstein CrossImage taken by the Hubble Space Telescope.

Simulated gravitational lensing

Black Hole Lensing (animated gif)Black hole going past a background galaxy.