Gravitational Lensing

Gravitational lensing is the bending of light by massive objects, a prediction of general relativity confirmed by Arthur Eddington's 1919 solar eclipse expedition. When a massive object lies between a distant light source and an observer, it acts as a gravitational lens, distorting, magnifying, or multiplying the image of the background source. The effect is not a property of the light but of the spacetime geometry through which the light propagates.

Strong lensing produces multiple images or Einstein rings; weak lensing produces subtle distortions in the shapes of background galaxies, mapping the distribution of dark matter in galaxy clusters. Microlensing, produced by compact objects like planets or stellar-mass black holes, causes temporary brightening as the lens crosses the line of sight. Gravitational lensing has become a precision tool in observational cosmology, constraining the expansion history of the universe and the nature of dark energy.