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.
Click on the images for an enlarged view.
Images of Einstein rings
Images taken by the Hubble Space Telescope.
![Einstein rings](/applied-mathematics/sites/default/files/uploads/images/erings.jpg)
Einstein cross
![Einstein Cross](/applied-mathematics/sites/default/files/uploads/images/ecross.jpg)
Image taken by the Hubble Space Telescope.
Simulated gravitational lensing
![Black Hole Lensing (animated gif)](/applied-mathematics/sites/default/files/uploads/images/blackholelensing.gif)
Black hole going past a background galaxy.