Gravitatielensing

Gravitatielensing, usually referred to as gravitational lensing, is the bending of light by gravity as predicted by general relativity. When light from a distant source passes near a massive foreground object, its path is deflected, leading to distorted, amplified, or multiple images.

Lensing is categorized by strength. Strong lensing produces easily resolvable multiple images, elongated arcs, or complete Einstein rings when the source, lens, and observer are well aligned. Weak lensing induces small distortions in the shapes of many background galaxies, detectable statistically. Microlensing involves compact objects, such as stars or planets, causing temporary brightness changes without resolving separate images.

The lensing effect is described by the lens equation beta = theta - (D_ls/D_s) alpha(theta), where beta is the source position, theta the image position, and alpha the deflection angle. The projected mass density, represented by kappa, and the shear gamma describe the local amplification and distortion; the magnification mu depends on (1 - kappa)^2 - gamma^2.

Gravitational lensing provides a tool to study mass distributions, including dark matter, in galaxies and clusters, and to constrain cosmology. In time-delay strong lensing, differences in light travel time between images can yield the Hubble constant. Weak lensing surveys map the growth of structure through cosmic shear, while microlensing searches can reveal exoplanets and compact objects.

Historically, gravitational lensing was predicted by Einstein in 1915 and confirmed during a solar eclipse expedition in 1919. The first confirmed gravitationally lensed quasar, Q0957+561, was discovered in 1979. Since then, lensing has become a central tool in astrophysics and cosmology.