Strong gravitational lensing is a

gravitational lensing A gravitational lens is a distribution of matter (such as a cluster of galaxies) between a distant light source and an observer that is capable of bending the light from the source as the light travels toward the observer. This effect is known ...

effect that is strong enough to produce multiple images, arcs, or even

Einstein ring An Einstein ring, also known as an Einstein–Chwolson ring or Chwolson ring (named for Orest Chwolson), is created when light from a galaxy or star passes by a massive object en route to the Earth. Due to gravitational lensing, the light is dive ...

s. Generally, the strong lensing effect requires the projected lens mass density 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 1879 – 18 April 1955) was a German-born theoretical physicist, widely acknowledged to be one of the greatest and most influential physicists of all time. Einstein is best known for developing the theory ...

general theory of relativity General relativity, also known as the general theory of relativity and Einstein's theory of gravity, is the differential geometry, geometric scientific theory, theory of gravitation published by Albert Einstein in 1915 and is the current descr ...

and observationally discovered by

Dennis Walsh Dennis Walsh (12 June 1933 – 1 June 2005) was an English astronomer. He was an early radio astronomer, as well as an optical astronomer. He was best known for his discovery in 1979 of the first example of a gravitational lens, B0957+561, using ...

, Bob Carswell, and

Ray Weymann Ray Weymann is a retired astronomer and astrophysicist, associated with the Carnegie Institution of Washington. His PhD is from Princeton University. He is a founder of the Climate Science Rapid Response Team, a member National Academy of Sci ...

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

The foreground lens is a galaxy. When the background source is a quasar 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 (; grc-gre, Εὐκλείδης; 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 ...

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. They are the second-lar ...

. 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

Since gravitational lensing is an effect only depending on

gravitational potential In classical mechanics, the gravitational potential at a location is equal to the work (energy transferred) per unit mass that would be needed to move an object to that location from a fixed reference location. It is analogous to the electric ...

, it can be used to constrain the

mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different eleme ...

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. 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 they are, the faster they are moving ...

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