Gravity bends light. That is a prediction of Einstein's theory of general relativity, and observations bear it out — most obviously, in gravitational lenses, where a distant galaxy's image is stretched or multiplied, or both, by an intervening mass. In the latest lens to be discovered, the image forms a complete ring (an Einstein ring') around the lensing galaxy. With a large enough sample of lenses, one can set constraints on the geometry of the Universe.
