Gravitational lensing is a phenomenon where the gravitational field of a massive object, such as a galaxy cluster or a black hole, bends the light from a more distant background object. This effect acts like a lens, magnifying and distorting the image of the background source. It allows astronomers to observe objects that would otherwise be too faint or distant to be seen and provides a unique tool to map the distribution of dark matter in the universe.
The theoretical foundation for gravitational lensing comes from Albert Einstein's General Theory of Relativity, published in 1915, which predicted that massive objects curve spacetime and thus bend light. The first experimental confirmation occurred during the 1919 solar eclipse. However, the first observation of a gravitational lens in deep space wasn't made until 1979, with the discovery of the 'Twin Quasar' Q0957+561.
Gravitational lensing has become an indispensable tool in modern astrophysics. It is used to discover exoplanets (microlensing), estimate the mass of galaxy clusters, and study the properties of dark matter and dark energy. Major observatories like the Hubble Space Telescope and the James Webb Space Telescope frequently utilize this phenomenon to peer into the early universe.