Strong gravitational lensing is a

gravitational lensing A gravitational lens is matter, such as a galaxy cluster, cluster of galaxies or a point particle, that bends light from a distant source as it travels toward an observer. The amount of gravitational lensing is described by Albert Einstein's Ge ...

effect that is strong enough to produce

multiple images Multiple may refer to: Economics *Multiple finance, a method used to analyze stock prices *Multiples of the price-to-earnings ratio *Chain stores, are also referred to as 'Multiples' *Box office multiple, the ratio of a film's total gross to tha ...

, arcs, or Einstein rings. Generally, for strong lensing to occur, the projected lens mass density must be greater than the '' critical density'', that is

\Sigma_

. For point-like background sources, there will be multiple images; for extended background emissions, there can be arcs or rings. Topologically, multiple image production is governed by the odd number theorem. Strong lensing was predicted by

Albert Einstein Albert Einstein (14 March 187918 April 1955) was a German-born theoretical physicist who is best known for developing the theory of relativity. Einstein also made important contributions to quantum mechanics. His mass–energy equivalence f ...

's general theory of relativity and observationally discovered by Dennis Walsh, Bob Carswell, and Ray Weymann in 1979. They determined that the Twin Quasar Q0957+561A comprises two images of the same object.

Observations

Most strong gravitational lenses are detected by large-scale galaxy surveys.

Galaxy lensing

A remote light source passing behind a gravitational lens

The foreground lens is a

galaxy A galaxy is a Physical system, system of stars, stellar remnants, interstellar medium, interstellar gas, cosmic dust, dust, and dark matter bound together by gravity. The word is derived from the Ancient Greek, Greek ' (), literally 'milky', ...

. When the background source is a

quasar A quasar ( ) is an extremely Luminosity, luminous active galactic nucleus (AGN). It is sometimes known as a quasi-stellar object, abbreviated QSO. The emission from an AGN is powered by accretion onto a supermassive black hole with a mass rangi ...

or unresolved jet, the strong lensed images are usually point-like multiple images; When the background source is a galaxy or extended jet emission, the strong lensed images can be arcs or rings. As of 2017, several hundred galaxy-galaxy (g-g) strong lenses have been observed. The upcoming Vera C. Rubin Observatory and

Euclid Euclid (; ; BC) was an ancient Greek mathematician active as a geometer and logician. Considered the "father of geometry", he is chiefly known for the '' Elements'' treatise, which established the foundations of geometry that largely domina ...

surveys are expected to discover more than 100,000 such objects.

Cluster lensing

The foreground lens is a

galaxy cluster A galaxy cluster, or a cluster of galaxies, is a structure that consists of anywhere from hundreds to thousands of galaxies that are bound together by gravity, with typical masses ranging from 1014 to 1015 solar masses. Clusters consist of galax ...

. In this case, the lens is usually powerful enough to produce noticeable both strong lensing (multiple images, arcs or rings) and weak lensing effects (ellipticity distortions). The lensing nicknamed the "Molten Ring" is an example.

Astrophysical applications

Mass profiles

Because the strong lensing of a background source depends only on the

gravitational potential In classical mechanics, the gravitational potential is a scalar potential associating with each point in space the work (energy transferred) per unit mass that would be needed to move an object to that point from a fixed reference point in the ...

of the foreground mass, this phenomenon can be used to constrain the

mass Mass is an Intrinsic and extrinsic properties, intrinsic property of a physical body, body. It was traditionally believed to be related to the physical quantity, quantity of matter in a body, until the discovery of the atom and particle physi ...

model of lenses. With the constraints from multiple images or arcs, a proposed mass model can be optimized to fit to the observables. The subgalactic structures that currently interest lensing astronomers are the central mass distribution and dark matter halos.

Time delays

Since the light rays go through different paths to produce multiple images, they will get delayed by local potentials along the light paths. The time delay differences from different images can be determined by the mass model and the

cosmological model Physical cosmology is a branch of cosmology concerned with the study of cosmological models. A cosmological model, or simply cosmology, provides a description of the largest-scale structures and dynamics of the universe and allows study of fu ...

. Thus, with observed time delays and constrained mass model, cosmological constants such as the

Hubble constant Hubble's law, also known as the Hubble–Lemaître law, is the observation in physical cosmology that galaxies are moving away from Earth at speeds proportional to their distance. In other words, the farther a galaxy is from the Earth, the faste ...

can be inferred.

Gallery

Peering into the past SDSS J0928+2031.jpg, Gravitational lensing system SDSS J0928+2031. Awesome gravity SDSS J1138+2754.jpg, SDSS J1138+2754 taken by Hubble's WFC3 camera. Rings of Relativity.jpg, Dubbed the "Molten Ring", Hubble sees strong lensing around GAL-CLUS-022058s.

References

{{reflist, 60em Gravitational lensing