Strong gravitational lensing
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 . For point-like background sources, there will be multiple images; for extended background emissions, there can be arcs or rings. Topologically, the multiple image production is governed by the odd number theorem.
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 Q0957+561A comprises two images of the same object.
Most strong gravitational lenses are detected by large-scale galaxy surveys.
The foreground lens is a galaxy. When the background source is a quasar or resolved 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 Large Synoptic Survey Telescope and Euclid surveys are expected to discover more than 100,000 such objects.
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).
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 currently interests lensing astronomers are the central mass distribution and dark matter halos.
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, the cosmological constant like Hubble constant can be inferred.
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