Strong gravitational lensing is a gravitational lensing effect that is strong enough to produce multiple images, arcs, or even Einstein rings. 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 The odd number theorem is a theorem in strong gravitational lensing which comes directly from differential topology. The theorem states that ''the number of multiple images produced by a bounded transparent lens must be odd''. Formulation The ...

. Strong lensing was predicted by Albert Einstein's general theory of relativity and observationally discovered by Dennis Walsh, Bob Carswell, and Ray Weymann in 1979. They determined that the

Twin Quasar The Twin Quasar (also known as Twin QSO, Double Quasar, SBS 0957+561, TXS 0957+561, Q0957+561 or QSO 0957+561 A/B), was discovered in 1979 and was the first identified gravitationally lensed object. It is a quasar that appears as two images, a r ...

Q0957+561A comprises two images of the same object.

Observations

Most strong gravitational lenses are detected by large-scale

galaxy survey In astronomy, a redshift survey is a survey of a section of the sky to measure the redshift of astronomical objects: usually galaxies, but sometimes other objects such as galaxy clusters or quasars. Using Hubble's law, the redshift can be use ...

Galaxy lensing

The foreground lens is a

galaxy A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ...

. When the background source is a quasar or unresolved

jet Jet, Jets, or The Jet(s) may refer to: Aerospace * Jet aircraft, an aircraft propelled by jet engines ** Jet airliner ** Jet engine ** Jet fuel * Jet Airways, an Indian airline * Wind Jet (ICAO: JET), an Italian airline * Journey to Enceladus a ...

, 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 The Vera C. Rubin Observatory, previously referred to as the Large Synoptic Survey Telescope (LSST), is an astronomical observatory currently under construction in Chile. Its main task will be carrying out a synoptic astronomical survey, the L ...

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

Cluster lensing

The foreground lens is a galaxy cluster. 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, it can be used to constrain the mass 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 halo According to modern models of physical cosmology, a dark matter halo is a basic unit of cosmological structure. It is a hypothetical region that has decoupled from cosmic expansion and contains gravitationally bound matter. A single dark matte ...

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 f ...

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