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A gravitational lens is a distribution of matter (such as a
cluster of galaxies 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 ...
) 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 as gravitational lensing, and the amount of bending is one of the predictions of
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 theor ...
's
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 ...
. Treating light as corpuscles travelling at the
speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit fo ...
,
Newtonian physics Classical mechanics is a physical theory describing the motion of macroscopic objects, from projectiles to parts of machinery, and astronomical objects, such as spacecraft, planets, stars, and galaxies. For objects governed by classical mec ...
also predicts the bending of light, but only half of that predicted by general relativity. Although Einstein made unpublished calculations on the subject in 1912,
Orest Khvolson Orest Danilovich Khvolson or Chwolson (russian: Орест Данилович Хвольсон) (November 22 (New Style, N.S. December 4), 1852 – May 11, 1934) was a Russian physicist and honorary member of the Soviet Academy of Sciences ( ...
(1924) and Frantisek Link (1936) are generally credited with being the first to discuss the effect in print. However, this effect is more commonly associated with Einstein, who published an article on the subject in 1936.
Fritz Zwicky Fritz Zwicky (; ; February 14, 1898 – February 8, 1974) was a Swiss astronomer. He worked most of his life at the California Institute of Technology in the United States of America, where he made many important contributions in theoretical an ...
posited in 1937 that the effect could allow galaxy clusters to act as gravitational lenses. It was not until 1979 that this effect was confirmed by observation of the Twin QSO SBS 0957+561.


Description

Unlike an
optical lens A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements ...
, 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
focal point Focal point may refer to: * Focus (optics) * Focus (geometry) * Conjugate points, also called focal points * Focal point (game theory) * Unicom Focal Point UNICOM Focal Point is a portfolio management and decision analysis tool used by the p ...
, but a focal line. The term "lens" in the context of gravitational light deflection was first used by O.J. 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". 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 Saint Petersburg ( rus, links=no, Санкт-Петербург, a=Ru-Sankt Peterburg Leningrad Petrograd Piter.ogg, r=Sankt-Peterburg, p=ˈsankt pʲɪtʲɪrˈburk), formerly known as Petrograd (1914–1924) and later Leningrad (1924–1991), i ...
physicist
Orest Khvolson Orest Danilovich Khvolson or Chwolson (russian: Орест Данилович Хвольсон) (November 22 (New Style, N.S. December 4), 1852 – May 11, 1934) was a Russian physicist and honorary member of the Soviet Academy of Sciences ( ...
, and quantified 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 theor ...
in 1936. It is usually referred to in the literature as an
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 ...
, 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 A galaxy group or group of galaxies (GrG) is an aggregation of galaxies comprising about 50 or fewer gravitationally bound members, each at least as luminous as the Milky Way (about 1010 times the luminosity of the Sun); collections of galaxie ...
or
cluster may refer to: Science and technology Astronomy * Cluster (spacecraft), constellation of four European Space Agency spacecraft * Asteroid cluster, a small asteroid family * Cluster II (spacecraft), a European Space Agency mission to study th ...
) 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: ; Strong lensing: Where there are easily visible distortions such as the formation of
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, 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 that of the Sun will produce multiple images separated by only a few
arcsecond A minute of arc, arcminute (arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of one degree. Since one degree is of a turn (or complete rotation), one minute of arc is of a turn. The n ...
s.
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 ...
s can produce separations of several arcminutes. In both cases the galaxies and sources are quite distant, many hundreds of
megaparsec The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (au), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, and ...
s away from our Galaxy. ; 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 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 Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not ...
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 Observational error (or measurement error) is the difference between a measured value of a quantity and its true value.Dodge, Y. (2003) ''The Oxford Dictionary of Statistical Terms'', OUP. In statistics, an error is not necessarily a " mistak ...
: 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 In astronomy, seeing is the degradation of the image of an astronomical object due to turbulence in the atmosphere of Earth that may become visible as blurring, twinkling or variable distortion. The origin of this effect are rapidly changing var ...
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 The ΛCDM (Lambda cold dark matter) or Lambda-CDM model is a parameterization of the Big Bang cosmological model in which the universe contains three major components: first, a cosmological constant denoted by Lambda ( Greek Λ) associated ...
, and to provide a consistency check on other cosmological observations. They may also provide an important future constraint on
dark energy In physical cosmology and astronomy, dark energy is an unknown form of energy that affects the universe on the largest scales. The first observational evidence for its existence came from measurements of supernovas, which showed that the univ ...
. ;
Microlensing Gravitational microlensing is an astronomical phenomenon due to the gravitational lens effect. It can be used to detect objects that range from the mass of a planet to the mass of a star, regardless of the light they emit. Typically, astronomers ...
: 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 The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...
in one typical case, with the background source being stars in a remote galaxy, or, in another case, an even more distant
quasar A quasar is an extremely luminous active galactic nucleus (AGN). It is pronounced , and sometimes known as a quasi-stellar object, abbreviated QSO. This emission from a galaxy nucleus is powered by a supermassive black hole with a mass rangi ...
. 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 MACS J1149 Lensed Star 1, also known as Icarus,Other names include ''LS1'', ''MACS J1149 LS1'', ''MACS J1149 Lensed Star 1 (LS1)'' and ''MACS J1149+2223 Lensed Star 1'' is a blue supergiant star observed through a gravitational lens. It is th ...
(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 In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visib ...
, not just visible light, and also in non-electromagnetic radiation, like gravitational waves. Weak lensing effects are being studied for the
cosmic microwave background In Big Bang cosmology the cosmic microwave background (CMB, CMBR) is electromagnetic radiation that is a remnant from an early stage of the universe, also known as "relic radiation". The CMB is faint cosmic background radiation filling all spac ...
as well as
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 used ...
s. Strong lenses have been observed in
radio Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30  hertz (Hz) and 300  gigahertz (GHz). They are generated by an electronic device called a tr ...
and
x-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10  nanometers, corresponding to frequencies in the range 30&nb ...
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.


History

Henry Cavendish Henry Cavendish ( ; 10 October 1731 – 24 February 1810) was an English natural philosopher and scientist who was an important experimental and theoretical chemist and physicist. He is noted for his discovery of hydrogen, which he termed "infl ...
in 1784 (in an unpublished manuscript) and Johann Georg von Soldner in 1801 (published in 1804) had pointed out that Newtonian gravity predicts that starlight will bend around a massive object as had already been supposed by
Isaac Newton Sir Isaac Newton (25 December 1642 – 20 March 1726/27) was an English mathematician, physicist, astronomer, alchemist, Theology, theologian, and author (described in his time as a "natural philosophy, natural philosopher"), widely ...
in 1704 in his '' Queries'' No.1 in his book ''
Opticks ''Opticks: or, A Treatise of the Reflexions, Refractions, Inflexions and Colours of Light'' is a book by English natural philosopher Isaac Newton that was published in English in 1704 (a scholarly Latin translation appeared in 1706). (''Optic ...
''. (''Opticks'' was originally published in 1704). The same value as Soldner's was calculated by Einstein in 1911 based on the
equivalence principle In the theory of general relativity, the equivalence principle is the equivalence of gravitational and inertial mass, and Albert Einstein's observation that the gravitational "force" as experienced locally while standing on a massive body (su ...
alone. However, Einstein noted in 1915, in the process of completing general relativity, that his (and thus Soldner's) 1911-result is only half of the correct value. Einstein became the first to calculate the correct value for light bending. The first observation of light deflection was performed by noting the change in position of
star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...
s as they passed near the Sun on the
celestial sphere In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphe ...
. The observations were performed in 1919 by
Arthur Eddington Sir Arthur Stanley Eddington (28 December 1882 – 22 November 1944) was an English astronomer, physicist, and mathematician. He was also a philosopher of science and a populariser of science. The Eddington limit, the natural limit to the lumi ...
, Frank Watson Dyson, and their collaborators during the total
solar eclipse A solar eclipse occurs when the Moon passes between Earth and the Sun, thereby obscuring the view of the Sun from a small part of the Earth, totally or partially. Such an alignment occurs during an eclipse season, approximately every six mon ...
on
May 29 Events Pre-1600 * 363 – The Roman emperor Julian defeats the Sasanian army in the Battle of Ctesiphon, under the walls of the Sasanian capital, but is unable to take the city. * 1108 – Battle of Uclés: Almoravid troops under ...
. The solar eclipse allowed the stars near the Sun to be observed. Observations were made simultaneously in the cities of
Sobral, Ceará Sobral is a municipality in the state of Ceará, Brazil. Sobral is the fifth largest municipality of Ceará, after Fortaleza. Its economy is based on agriculture, services and some manufacturing industries. The city has two public universitie ...
, Brazil and in
São Tomé and Príncipe São Tomé and Príncipe (; pt, São Tomé e Príncipe (); English: " Saint Thomas and Prince"), officially the Democratic Republic of São Tomé and Príncipe ( pt, República Democrática de São Tomé e Príncipe), is a Portuguese-speaking ...
on the west coast of Africa. The observations demonstrated that the light from
star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...
s passing close to the
Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
was slightly bent, so that stars appeared slightly out of position. The result was considered spectacular news and made the front page of most major newspapers. It made Einstein and his theory of general relativity world-famous. When asked by his assistant what his reaction would have been if general relativity had not been confirmed by Eddington and Dyson in 1919, Einstein said "Then I would feel sorry for the dear Lord. The theory is correct anyway." In 1912, Einstein had speculated that an observer could see multiple images of a single light source, if the light were deflected around a mass. This effect would make the mass act as a kind of gravitational lens. However, as he only considered the effect of deflection around a single star, he seemed to conclude that the phenomenon was unlikely to be observed for the foreseeable future since the necessary alignments between stars and observer would be highly improbable. Several other physicists speculated about gravitational lensing as well, but all reached the same conclusion that it would be nearly impossible to observe. Although Einstein made unpublished calculations on the subject, the first discussion of the gravitational lens in print was by Khvolson, in a short article discussing the "halo effect" of gravitation when the source, lens, and observer are in near-perfect alignment, now referred to as the
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 ...
. In 1936, after some urging by Rudi W. Mandl, Einstein reluctantly published the short article "Lens-Like Action of a Star By the Deviation of Light In the Gravitational Field" in the journal ''Science''. In 1937,
Fritz Zwicky Fritz Zwicky (; ; February 14, 1898 – February 8, 1974) was a Swiss astronomer. He worked most of his life at the California Institute of Technology in the United States of America, where he made many important contributions in theoretical an ...
first considered the case where the newly discovered
galaxies 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 ...
(which were called 'nebulae' at the time) could act as both source and lens, and that, because of the mass and sizes involved, the effect was much more likely to be observed. In 1963 Yu. G. Klimov, S. Liebes, and
Sjur Refsdal Sjur Refsdal (30 December 1935 – 29 January 2009) was a Norwegian astrophysicist, born in Oslo. He is best known for his pioneer work on gravitational lensing, including the Chang-Refsdal lens. Biography In 1970 he earned a doctorate at the ...
recognized independently that quasars are an ideal light source for the gravitational lens effect. It was not until 1979 that the first gravitational lens would be discovered. It became known as the " Twin QSO" since it initially looked like two identical quasistellar objects. (It is officially named SBS 0957+561.) This gravitational lens was discovered by Dennis Walsh, 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 ...
using the
Kitt Peak National Observatory The Kitt Peak National Observatory (KPNO) is a United States astronomical observatory located on Kitt Peak of the Quinlan Mountains in the Arizona-Sonoran Desert on the Tohono Oʼodham Nation, west-southwest of Tucson, Arizona. With more than ...
2.1 meter
telescope A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to obse ...
. In the 1980s, astronomers realized that the combination of CCD imagers and computers would allow the brightness of millions of stars to be measured each night. In a dense field, such as the galactic center or the Magellanic clouds, many microlensing events per year could potentially be found. This led to efforts such as
Optical Gravitational Lensing Experiment The Optical Gravitational Lensing Experiment (OGLE) is a Polish astronomical project based at the University of Warsaw that runs a long-term variability sky survey (1992–present). The main goals are the detection and classification of variable ...
, or OGLE, that have characterized hundreds of such events, including those of
OGLE-2016-BLG-1190Lb OGLE-2016-BLG-1190Lb is an extremely massive exoplanet, with a mass about 13.4 times that of Jupiter (), or is, possibly, a low mass brown dwarf, orbiting the G-dwarf star OGLE-2016-BLG-1190L, located about 22,000 light years from Earth, in the ...
and
OGLE-2016-BLG-1195Lb OGLE-2016-BLG-1195Lb is an extrasolar planet located about 22,000 light-years from Earth, in the galactic bulge, orbiting the 0.57 star OGLE-2016-BLG-1195L, discovered in 2017. The planet was detected using gravitational microlensing techniques ...
.


Explanation in terms of spacetime curvature

In general relativity, light follows the curvature of spacetime, hence when light passes around a massive object, it is bent. This means that the light from an object on the other side will be bent towards an observer's eye, just like an ordinary lens. In general relativity the speed of light depends on the gravitational potential (i.e. the metric) and this bending can be viewed as a consequence of the light traveling along a gradient in light speed. Light rays are the boundary between the future, the spacelike, and the past regions. The gravitational attraction can be viewed as the motion of undisturbed objects in a background curved ''
geometry Geometry (; ) is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space such as the distance, shape, size, and relative position of figures. A mathematician who works in the field of geometry is c ...
'' or alternatively as the response of objects to a ''force'' in a flat geometry. The angle of deflection is: :\theta = \frac toward the mass ''M'' at a distance ''r'' from the affected radiation, where ''G'' is the universal constant of gravitation and ''c'' is the speed of light in a vacuum. Since the
Schwarzschild radius The Schwarzschild radius or the gravitational radius is a physical parameter in the Schwarzschild solution to Einstein's field equations that corresponds to the radius defining the event horizon of a Schwarzschild black hole. It is a characteri ...
r_\text is defined as r_\text = / and
escape velocity In celestial mechanics, escape velocity or escape speed is the minimum speed needed for a free, non- propelled object to escape from the gravitational influence of a primary body, thus reaching an infinite distance from it. It is typically ...
v_\text is defined as v_\text = \sqrt = \beta_\text c, this can also be expressed in simple form as :\theta = 2 \frac = 2 \left(\frac\right)^2 = 2\beta_\text^2


Search for gravitational lenses

Most of the gravitational lenses in the past have been discovered accidentally. A search for gravitational lenses in the northern hemisphere (Cosmic Lens All Sky Survey, CLASS), done in radio frequencies using the Very Large Array (VLA) in New Mexico, led to the discovery of 22 new lensing systems, a major milestone. This has opened a whole new avenue for research ranging from finding very distant objects to finding values for cosmological parameters so we can understand the universe better. A similar search in the southern hemisphere would be a very good step towards complementing the northern hemisphere search as well as obtaining other objectives for study. If such a search is done using well-calibrated and well-parameterized instrument and data, a result similar to the northern survey can be expected. The use of the Australia Telescope 20 GHz (AT20G) Survey data collected using the Australia Telescope Compact Array (ATCA) stands to be such a collection of data. As the data were collected using the same instrument maintaining a very stringent quality of data we should expect to obtain good results from the search. The AT20G survey is a blind survey at 20 GHz frequency in the radio domain of the electromagnetic spectrum. Due to the high frequency used, the chances of finding gravitational lenses increases as the relative number of compact core objects (e.g. quasars) are higher (Sadler et al. 2006). This is important as the lensing is easier to detect and identify in simple objects compared to objects with complexity in them. This search involves the use of interferometric methods to identify candidates and follow them up at higher resolution to identify them. Full detail of the project is currently under works for publication. Microlensing techniques have been used to search for planets outside our solar system. A statistical analysis of specific cases of observed microlensing over the time period of 2002 to 2007 found that most stars in the
Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...
galaxy hosted at least one orbiting planet within 0.5 to 10 AUs. In 2009, weak gravitational lensing was used to extend the mass-X-ray-luminosity relation to older and smaller structures than was previously possible to improve measurements of distant galaxies. the most distant gravitational lens galaxy, J1000+0221, had been found using
NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeedin ...
’s
Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most vers ...
. While it remains the most distant quad-image lensing galaxy known, an even more distant two-image lensing galaxy was subsequently discovered by an international team of astronomers using a combination of
Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most vers ...
and
Keck telescope The W. M. Keck Observatory is an astronomical observatory with two telescopes at an elevation of 4,145 meters (13,600 ft) near the summit of Mauna Kea in the U.S. state of Hawaii. Both telescopes have aperture primary mirrors, and when co ...
imaging and spectroscopy. The discovery and analysis of the IRC 0218 lens was published in the ''
Astrophysical Journal Letters ''The Astrophysical Journal'', often abbreviated ''ApJ'' (pronounced "ap jay") in references and speech, is a peer-reviewed scientific journal of astrophysics and astronomy, established in 1895 by American astronomers George Ellery Hale and Ja ...
'' on June 23, 2014. Research published Sep 30, 2013 in the online edition of ''
Physical Review Letters ''Physical Review Letters'' (''PRL''), established in 1958, is a peer-reviewed, scientific journal that is published 52 times per year by the American Physical Society. As also confirmed by various measurement standards, which include the ''Journa ...
'', led by
McGill University McGill University (french: link=no, Université McGill) is an English-language public research university located in Montreal, Quebec, Canada. Founded in 1821 by royal charter granted by King George IV,Frost, Stanley Brice. ''McGill Univer ...
in
Montreal Montreal ( ; officially Montréal, ) is the second-most populous city in Canada and most populous city in the Canadian province of Quebec. Founded in 1642 as '' Ville-Marie'', or "City of Mary", it is named after Mount Royal, the triple- ...
,
Québec Quebec ( ; )According to the Canadian government, ''Québec'' (with the acute accent) is the official name in Canadian French and ''Quebec'' (without the accent) is the province's official name in Canadian English is one of the thirte ...
, Canada, has discovered the
B-modes In Big Bang cosmology the cosmic microwave background (CMB, CMBR) is electromagnetic radiation that is a remnant from an early stage of the universe, also known as "relic radiation". The CMB is faint cosmic background radiation filling all spac ...
, that are formed due to gravitational lensing effect, using
National Science Foundation The National Science Foundation (NSF) is an independent agency of the United States government that supports fundamental research and education in all the non-medical fields of science and engineering. Its medical counterpart is the National ...
's
South Pole Telescope The South Pole Telescope (SPT) is a diameter telescope located at the Amundsen–Scott South Pole Station, Antarctica. The telescope is designed for observations in the microwave, millimeter-wave, and submillimeter-wave regions of the electroma ...
and with help from the Herschel space observatory. This discovery would open the possibilities of testing the theories of how our universe originated.


Solar gravitational lens

Albert Einstein predicted in 1936 that rays of light from the same direction that skirt the edges of the
Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
would converge to a focal point approximately 542 AUs from the Sun. Thus, a probe positioned at this distance (or greater) from the Sun could use the Sun as a gravitational lens for magnifying distant objects on the opposite side of the Sun. A probe's location could shift around as needed to select different targets relative to the Sun. This distance is far beyond the progress and equipment capabilities of space probes such as ''
Voyager 1 ''Voyager 1'' is a space probe launched by NASA on September 5, 1977, as part of the Voyager program to study the outer Solar System and interstellar space beyond the Sun's heliosphere. Launched 16 days after its twin '' Voyager 2'', ''V ...
'', and beyond the known planets and dwarf planets, though over thousands of years
90377 Sedna Sedna ( minor-planet designation 90377 Sedna) is a dwarf planet in the outer reaches of the Solar System that is in the innermost part of its orbit; it is 84 astronomical units (AU), or 1.26×1010 km, from the Sun, almost three times fart ...
will move farther away on its highly elliptical orbit. The high gain for potentially detecting signals through this lens, such as microwaves at the 21-cm
hydrogen line The hydrogen line, 21 centimeter line, or H I line is the electromagnetic radiation spectral line that is created by a change in the energy state of neutral hydrogen atoms. This electromagnetic radiation has a precise frequency of , w ...
, led to the suggestion by
Frank Drake Frank Donald Drake (May 28, 1930 – September 2, 2022) was an American astrophysicist and astrobiologist. He began his career as a radio astronomer, studying the planets of the Solar System and later pulsars. Drake expanded his interests ...
in the early days of
SETI The search for extraterrestrial intelligence (SETI) is a collective term for scientific searches for intelligent extraterrestrial life, for example, monitoring electromagnetic radiation for signs of transmissions from civilizations on other pl ...
that a probe could be sent to this distance. A multipurpose probe SETISAIL and later FOCAL was proposed to the ESA in 1993, but is expected to be a difficult task.Geoffrey A. Landis
"Mission to the Gravitational Focus of the Sun: A Critical Analysis,"
ArXiv, paper 1604.06351, Cornell University, 21 Apr 2016 (downloaded 30 April 2016)
If a probe does pass 542 AU, magnification capabilities of the lens will continue to act at farther distances, as the rays that come to a focus at larger distances pass further away from the distortions of the Sun's corona. A critique of the concept was given by Landis, who discussed issues including interference of the solar corona, the high magnification of the target, which will make the design of the mission focal plane difficult, and an analysis of the inherent
spherical aberration In optics, spherical aberration (SA) is a type of aberration found in optical systems that have elements with spherical surfaces. Lenses and curved mirrors are prime examples, because this shape is easier to manufacture. Light rays that strik ...
of the lens. In 2020, NASA physicist Slava Turyshev presented his idea of Direct Multipixel Imaging and Spectroscopy of an Exoplanet with a
Solar Gravitational Lens A solar gravitational lens (or solar gravity lens) (SGL) is a theoretical method of using the Sun as a large lens with a physical effect called gravitational lensing. It is considered the best method to directly image habitable exoplanets. The so ...
Mission. The lens could reconstruct the exoplanet image with ~25 km-scale surface resolution, enough to see surface features and signs of habitability.


Measuring weak lensing

Kaiser, Squires and Broadhurst (1995), Luppino & Kaiser (1997) and Hoekstra et al. (1998) prescribed a method to invert the effects of the point spread function (PSF) smearing and shearing, recovering a shear estimator uncontaminated by the systematic distortion of the PSF. This method (KSB+) is the most widely used method in weak lensing shear measurements. Galaxies have random rotations and inclinations. As a result, the shear effects in weak lensing need to be determined by statistically preferred orientations. The primary source of error in lensing measurement is due to the convolution of the PSF with the lensed image. The KSB method measures the ellipticity of a galaxy image. The shear is proportional to the ellipticity. The objects in lensed images are parameterized according to their weighted quadrupole moments. For a perfect ellipse, the weighted quadrupole moments are related to the weighted ellipticity. KSB calculate how a weighted ellipticity measure is related to the shear and use the same formalism to remove the effects of the PSF. KSB's primary advantages are its mathematical ease and relatively simple implementation. However, KSB is based on a key assumption that the PSF is circular with an anisotropic distortion. This is a reasonable assumption for cosmic shear surveys, but the next generation of surveys (e.g. LSST) may need much better accuracy than KSB can provide.


Gallery

Image:The Sunburst Arc PSZ1 G311.65-18.48.jpg, Sunburst Arc galaxy. Image:Hubble’s view on distant quasar J043947.08+163415.7.tif, Gravitationally lensed Quasar. Image:On the hunt for newborn stars.tif, In SDSS J0952+3434, the lower arc-shaped galaxy has the characteristic shape of a galaxy that has been gravitationally lensed. Image:Warped and distorted SDSS J1050+0017.jpg, Warped and distorted around SDSS J1050+0017. Image:Stretched out image of distant galaxy.jpg, Galaxy SPT0615-JD existed when the Universe was just 500 million years old. Image:Cosmic snake pregnant with stars.tif, Gravitational lenses found in the DESI Legacy Survey data Image:Gravitational_lenses_found_in_the_DESI_Legacy_Survey_data.jpg, Gravitational lenses found in the DESI Legacy Survey data Image:Hubble captures gallery of ultra-bright galaxies.jpg, The lensing phenomenon allows for features as small as about 100 light-years or less. Image:Detailed look at a gravitationally lensed supernova.jpg, Detailed look at a gravitationally lensed
type Ia supernova A Type Ia supernova (read: "type one-A") is a type of supernova that occurs in binary systems (two stars orbiting one another) in which one of the stars is a white dwarf. The other star can be anything from a giant star to an even smaller white ...
iPTF16geu. Image:HST-Smiling-GalaxyClusterSDSS-J1038+4849-20150210.jpg, "Smiley" image of
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 ...
(SDSS J1038+4849) & gravitational lensing (an
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 ...
) ( HST). Image:Gravitationell-lins-4.jpg,
Abell 1689 Abell 1689 is a galaxy cluster in the constellation Virgo over 2.3 billion light-years away. Details Abell 1689 is one of the biggest and most massive galaxy clusters known and acts as a gravitational lens, distorting the images of galaxies that ...
- actual gravitational lensing effects (
Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most vers ...
). Image:COSMOS 3D dark matter map.jpg,
Dark matter Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not ...
distribution - weak gravitational lensing (
Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most vers ...
). Image:The most distant gravitational lens yet discovered.jpg, Gravitational lens discovered at
redshift In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and simultaneous increase in fr ...
z = 1.53. Image:PIA23641-GravitationalLensing-20200108.jpg, Gravitational Lensing Graphic (January 8, 2020) File:Hubble Showcases Hamilton’s Object.jpg, Hubble Showcases Hamilton's Object File:Gravitational lensing by a black hole.jpg, Illustration of graviational lensing Gravitationally-lensed distant star-forming galaxy.jpg, Gravitationally-lensed distant star-forming galaxies.


See also

*
Terrestrial atmospheric lens A Terrestrial Atmospheric Lens (TAL) is a theoretical method of using the Earth as a large lens with a physical effect called atmospheric refraction. The sun's image appears about a half degree above its real position during sunset due to Earth's ...
* Galactic lens * Gravitational lensing formalism *
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 '' crit ...
** Einstein cross **
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 ...
*
Weak gravitational lensing While the presence of any mass bends the path of light passing near it, this effect rarely produces the giant arcs and multiple images associated with strong gravitational lensing. Most lines of sight in the universe are thoroughly in the weak le ...
*
Gravitational microlensing Gravitational microlensing is an astronomical phenomenon due to the gravitational lens effect. It can be used to detect objects that range from the mass of a planet to the mass of a star, regardless of the light they emit. Typically, astronomers ...
*
SN Refsdal SN Refsdal is the first detected multiply-lensed supernova, visible within the field of the galaxy cluster MACS J1149+2223. It was given its nickname in honor of the Norwegian astrophysicist Sjur Refsdal, who, in 1964, first proposed using time ...
*
Euclid (spacecraft) Euclid is a visible to near-infrared space telescope currently under development by the European Space Agency (ESA) and the Euclid Consortium. The objective of the Euclid mission is to better understand dark energy and dark matter by accurately ...
, to measure gravitational lensing


Historical papers and references

* * *


References

;Notes ;Bibliography * "
Accidental Astrophysicists
''". Science News, June 13, 2008. * "

'". A Computer Program to visualize Gravitational Lenses, Francisco Frutos-Alfaro * "
G-LenS
'". A Point Mass Gravitational Lens Simulation, Mark Boughen. * Newbury, Pete, "

'". Institute of Applied Mathematics, The University of British Columbia. * Cohen, N., "Gravity's Lens: Views of the New Cosmology", Wiley and Sons, 1988. * "

'". Harvard.edu. * Bridges, Andrew, "

'".
Associated Press The Associated Press (AP) is an American non-profit news agency headquartered in New York City. Founded in 1846, it operates as a cooperative, unincorporated association. It produces news reports that are distributed to its members, U.S. new ...
. February 15, 2004. (Farthest galaxy found by gravitational lensing, using Abell 2218 and Hubble Space Telescope.)
Analyzing Corporations ... and the Cosmos
An unusual career path in gravitational lensing. * "
HST images of strong gravitational lenses
'". Harvard-Smithsonian Center for Astrophysics. * "
A planetary microlensing event
'" and "
A Jovian-mass Planet in Microlensing Event OGLE-2005-BLG-071
'", the first extra-solar planet detections using microlensing.
Gravitational lensing on arxiv.org



AT20G survey

''A diffraction limit on the gravitational lens effect'' (Bontz, R. J. and Haugan, M. P. "Astrophysics and Space Science" vol. 78, no. 1, p. 199-210. August 1981)
;Further reading * * * . *
''Tools for the evaluation of the possibilities of using parallax measurements of gravitationally lensed sources'' (Stein Vidar Hagfors Haugan. June 2008)


External links


Video: Evalyn Gates – ''Einstein's Telescope: The Search for Dark Matter and Dark Energy in the Universe''
, presentation in Portland, Oregon, on April 19, 2009, from the author's recent book tour.
Audio: Fraser Cain and Dr. Pamela Gay – Astronomy Cast: Gravitational Lensing, May 2007
{{DEFAULTSORT:Gravitational Lens Astrophysics Effects of gravitation Large-scale structure of the cosmos Concepts in astronomy Articles containing video clips