Editing Gravitational lens (section)

==Description==
[[File:Artist's impression of gravitational lensing of a distant merger.ogv|thumb|Gravitational lensing – intervening galaxy modifies appearance of a galaxy far behind it (video; artist's concept).]]
[[File:Gravitational lensing of distant star-forming galaxies (schematic) -vid-.webm|thumb|left|This schematic image shows how light from a distant galaxy is distorted by the gravitational effects of a foreground galaxy, which acts like a lens and makes the distant source appear distorted, but magnified, forming characteristic rings of light, known as Einstein rings.]]
[[File:Gravitational lensing of distant star-forming galaxies (schematic) 2.webm|thumb|An analysis of the distortion of SDP.81 caused by this effect has revealed star-forming clumps of matter.]]

Unlike an [[lens (optics)|optical lens]], a point-like gravitational lens produces a maximum deflection of light that passes closest to its center, and a minimum deflection of light that travels furthest from its center. Consequently, a gravitational lens has no single [[focus (optics)|focal point]], but a focal line.  The term "lens" in the context of gravitational light deflection was first used by O.&nbsp;J.&nbsp;Lodge, who remarked that it is "not permissible to say that the solar gravitational field acts like a lens, for it has no focal length".<ref>{{Cite journal |last=Lodge |first=Oliver J. |date=December 1919 |title=Gravitation and Light |journal=Nature |language=en |volume=104 |issue=2614 |pages=354 |doi=10.1038/104354a0 |bibcode=1919Natur.104..354L |s2cid=4157815 |issn=0028-0836|doi-access=free }}</ref> If the (light) source, the massive lensing object, and the observer lie in a straight line, the original light source will appear as a ring around the massive lensing object (provided the lens has circular symmetry). If there is any misalignment, the observer will see an arc segment instead. 

This phenomenon was first mentioned in 1924 by the [[St. Petersburg]] physicist [[Orest Khvolson]],<ref>{{cite web |url = http://www.abc.net.au/science/k2/moments/gmis9737.htm |title = Gravity Lens – Part 2 (Great Moments in Science, ABS Science) |website = [[Australian Broadcasting Corporation]] |date = 5 November 2001}}</ref> and quantified by [[Albert Einstein]] in 1936. It is usually referred to in the literature as an [[Einstein ring]], since Khvolson did not concern himself with the flux or radius of the ring image. More commonly, where the lensing mass is complex (such as a [[galaxy group]] or [[galaxy cluster|cluster]]) and does not cause a spherical distortion of spacetime, the source will resemble partial arcs scattered around the lens. The observer may then see multiple distorted images of the same source; the number and shape of these depending upon the relative positions of the source, lens, and observer, and the shape of the gravitational well of the lensing object.

There are three classes of gravitational lensing:<ref name="Schneider, Peter; Ehlers, Jürgen; Falco, Emilio E. 1992">{{cite book |author1=Schneider, Peter |title=Gravitational Lenses |author2=Ehlers, Jürgen |author3=Falco, Emilio E. |date=1992 |publisher=Springer-Verlag Berlin Heidelberg New York Press |isbn=978-3-540-97070-5}}</ref>{{rp|399-401}}<ref>{{cite book | title = The Galactic Supermassive Black Hole | author = Melia, Fulvio | author-link = Fulvio Melia | publisher = Princeton University Press | date = 2007 | isbn = 978-0-691-13129-0 | pages = 255–256}}</ref>

; [[Strong gravitational lensing|Strong lensing]]: Where there are easily visible distortions such as the formation of [[Einstein ring]]s, arcs, and multiple images. Despite being considered "strong", the effect is in general relatively small, such that even a galaxy with a mass more than 100 billion times [[solar mass|that of the Sun]] will produce multiple images separated by only a few [[arcsecond]]s. [[Galaxy cluster]]s can produce separations of several arcminutes. In both cases the galaxies and sources are quite distant, many hundreds of [[megaparsec]]s away from the Milky Way Galaxy.
; [[Weak gravitational lensing|Weak lensing]]: Where the distortions of background sources are much smaller and can only be detected by analyzing large numbers of sources in a statistical way to find coherent distortions of only a few percent. The lensing shows up statistically as a preferred stretching of the background objects perpendicular to the direction to the centre of the lens. By measuring the shapes and orientations of large numbers of distant galaxies, their orientations can be averaged to measure the [[Shear matrix|shear]] of the lensing field in any region. This, in turn, can be used to reconstruct the mass distribution in the area: in particular, the background distribution of [[dark matter]] can be reconstructed. Since galaxies are intrinsically elliptical and the weak gravitational lensing signal is small, a very large number of galaxies must be used in these surveys. These weak lensing surveys must carefully avoid a number of important sources of [[systematic error]]: the intrinsic shape of galaxies, the tendency of a camera's [[point spread function]] to distort the shape of a galaxy and the tendency of [[atmospheric seeing]] to distort images must be understood and carefully accounted for. The results of these surveys are important for cosmological parameter estimation, to better understand and improve upon the [[Lambda-CDM model]], and to provide a consistency check on other cosmological observations. They may also provide an important future constraint on [[dark energy]].
; [[Gravitational microlensing|Microlensing]]: Where no distortion in shape can be seen but the amount of light received from a background object changes in time. The lensing object may be stars in the [[Milky Way Galaxy|Milky Way]] in one typical case, with the background source being stars in a remote galaxy, or, in another case, an even more distant [[quasar]]. In extreme cases, a star in a distant galaxy can act as a microlens and magnify another star much farther away. The first example of this was the star [[MACS J1149 Lensed Star 1]] (also known as Icarus), thanks to the boost in flux due to the microlensing effect. 

Gravitational lenses act equally on all kinds of [[electromagnetic radiation]], not just visible light, and also in non-electromagnetic radiation, like gravitational waves. Weak lensing effects are being studied for the [[cosmic microwave background]] as well as [[galaxy survey]]s. Strong lenses have been observed in [[radio]] and [[x-ray]] regimes as well. If a strong lens produces multiple images, there will be a relative time delay between two paths: that is, in one image the lensed object will be observed before the other image.