Gravitational lensing

In general relativity, mass and energy curve spacetime, and light follows paths through that curved spacetime. When light from a distant object passes near a massive foreground object, such as a galaxy or galaxy cluster, its path is deflected. The foreground mass acts as a gravitational lens. Unlike an ordinary optical lens, it does not need a glass surface: its effect arises from gravity. The apparent image depends on the masses and their distribution, the distances involved, and the alignment of the source, lens and observer.

Lensing can be strong or weak, depending on the geometry and the effect on the observed light. Strong lensing can create several images of a background source, stretched arcs, or, with close alignment, a ring-like image called an Einstein ring. Weak lensing produces smaller distortions that are difficult to discern in a single galaxy but can be measured statistically across many background galaxies. Microlensing occurs when a relatively compact object causes a temporary change in the apparent brightness of a more distant source; its separate images are generally too close together to resolve with ordinary telescopes. These forms describe different observational regimes rather than different underlying laws of gravity.

Gravitational lenses magnify background sources, making some distant and faint objects easier to observe. Multiple images and distortions also encode information about the mass distribution in the foreground lens. Astronomers use these effects to estimate masses of galaxies and clusters, examine the structure of distant galaxies, and map mass that emits little or no light. The NASA Hubble resource describes how lensing observations help trace the distribution of dark matter in galaxy clusters. Measurements of time delays between images of a variable source can also provide information about the geometry of the lens system and cosmic distances, though such analyses require detailed models and data.

A well-known early test of gravitational deflection was the 1919 total solar eclipse expeditions led by Arthur Eddington, Frank Dyson and their collaborators, who compared star positions near the Sun with reference photographs. NASA's account notes that the early measurements were debated, while subsequent observations have repeatedly confirmed gravitational deflection predicted by general relativity. Today, telescopes such as Hubble use gravitational lensing across a wide range of scales, from microlensing by individual stars to arcs and multiple images produced by massive galaxy clusters.

A lensed image is not a direct photograph of the foreground mass or an undistorted view of the background source. Light can travel along several paths to the observer, and the resulting images may differ in brightness, shape and arrival time. Interpreting them requires models of the lens and careful separation of foreground and background light. Magnification can extend a telescope's reach, but it does not remove observational uncertainty; the inferred source properties depend on how well the lens geometry and mass distribution are understood.