Huge-LQG From Wikipedia, the free encyclopedia Map of Huge-LQG Quasar 3C 273 Above: Map of the Huge-LQG noted by black circles, adjacent to the ClowesCampusan o LQG in red crosses. Map is by Roger Clowes of University of Central Lancashire . Bottom: Image of the bright quasar 3C 273. Each black circle and red cross on the map is a quasar similar to this one. The Huge Large Quasar Group, (Huge-LQG, also called U1.27) is a possible structu re or pseudo-structure of 73 quasars, referred to as a large quasar group, that measures about 4 billion light-years across. At its discovery, it was identified as the largest and the most massive known structure in the observable universe, [1][2][3] though it has been superseded by the Hercules-Corona Borealis Great Wa ll at 10 billion light-years. There are also issues about its structure (see Dis pute section below). Contents 1 Discovery 2 Characteristics 3 Cosmological principle 4 Dispute 5 See also 6 References 7 Further reading 8 External links Discovery[edit] Roger G. Clowes, together with colleagues from the University of Central Lancash ire in Preston, United Kingdom, has reported on January 11, 2013 a grouping of q uasars within the vicinity of the constellation Leo. They used data from the DR7 QSO catalogue of the comprehensive Sloan Digital Sky Survey, a major multi-imagi ng and spectroscopic redshift survey of the sky. They reported that the grouping was, as they announced, the largest known structure in the observable universe. The structure was initially discovered in November 2012 and took two months of verification before its announcement. News about the structure's announcement sp read worldwide, and has received great attention on the scientific community. Characteristics[edit] The Huge-LQG was estimated to be about 1.24 Gpc in length, by 640 Mpc and 370 Mp c on the other dimensions, and contains 73 quasars, respectively.[4] Quasars are very luminous active galactic nuclei, thought to be supermassive black holes fe eding on matter. Since they are only found in dense regions of the universe, qua sars can be used to find overdensities of matter within the universe. It has the approximate binding mass of 6.1×1018 (6.1 trillion (long scale) or 6.1 quintillio n (short scale)) M?. The Huge-LQG was initially named U1.27 due to its average r edshift of 1.27, placing its distance at about 9 billion light-years from Earth. [5] The Huge-LQG is 615Mpc from the ClowesCampusano LQG (U1.28), a group of 34 quasar s also discovered by Clowes in 1991. Cosmological principle[edit] Main article: Cosmological principle In Clowes' initial announcement of the structure, he has reported that the struc ture has contradicted the cosmological principle. The cosmological principle imp lies that at sufficiently large scales, the universe is approximately homogeneou s, meaning that the statistical fluctuations in quantities such as the matter de nsity between different regions of the universe are small. However, different de finitions exist for the homogeneity scale above which these fluctuations may be considered sufficiently small, and the appropriate definition depends on the con text in which it is used. Jaswant Yadav et al. have suggested a definition of th e homogeneity scale based on the fractal dimension of the universe; they conclud e that, according to this definition, an upper limit for the homogeneity scale i n the universe is 260/h Mpc.[6] Some studies that have attempted to measure the homogeneity scale according to this definition have found values in the range 701 30/h Mpc.[7][8][9] The Sloan Great Wall, discovered in 2003, has a length of 423Mpc,[10] which is m arginally larger than the homogeneity scale as defined above. The Huge-LQG is three times longer than, and twice as wide as the Yadav et al. u pper limit to the homogeneity scale, and has therefore been claimed to challenge our understanding of the universe on large scales.[3] However, due to the existence of long-range correlations, it is known that struc tures can be found in the distribution of galaxies in the universe that extend o ver scales larger than the homogeneity scale.[11] Dispute[edit] Seshadri Nadathur at the University of Bielefeld, has conducted an even more com prehensive study of the Huge-LQG. After a more detailed study, he announced that contrary to the claim by Clowes about a large clustering, has showed that in hi s new map, that there is no clear clustering of quasars within the vicinity of t he Huge-LQG. The map was actually a similar map created by Clowes above, but inc ludes all the quasars in that region. After performing a number of statistical s tudies on the quasar data, and finding extreme changes in the Huge-LQG membershi p and shape with small changes in the cluster finding parameters, he determined the probability that apparent clusters the size of the Huge-LQG would appear in a random assortment of quasars. He set up 10,000 regions identical in size to th at studied by Clowes, and filled them with randomly distributed quasars with the same position statistics as did the actual quasars in the sky.[9] The data is s upporting the study of the homogeneity scale by Yadav et al.,[6] and that there is therefore no challenge to the cosmological principle. The study also implies that the statistical algorithm used by Clowes to identify the Huge-LQG, when use d to correlate other quasars in the sky, produces more than a thousand clusterin gs identical to the Huge-LQG. While quasars can represent dense regions of the u niverse, one must note that all of the quasars in the sky are evenly distributed , that is, one quasar per few million light years, making their significance as a structure very unlikely. The identification of the Huge-LQG, together with the clusterings identified by Nadathur, is therefore referred to be false positive identifications, or errors in identifying structures, finally arriving to the co nclusion that the Huge-LQG is not a real structure at all. Several questions arouse from the structure's discovery. But it is not told how Clowes detected a clustering of quasars in the region, nor how he found any corr elation of quasars in the region. It is specified, that, not only the structure, but also other LQGs are not real structures at all. Nevertheless, Clowes et al. found independent support for the reality of the str ucture from its coincidence with Mg II absorbers (once-ionised magnesium gas, co mmonly used to probe distant galaxies). The Mg II gas suggests that the Huge-LQG is associated with an enhancement of the mass, rather than being a false positi ve. This point is not discussed by the critical paper.[9] Further support for the reality of the Huge-LQG comes from the work of Hutsemékers et al.[12] in September 2014. They measured the polarization of quasars in the Huge-LQG and found "a remarkable correlation" of the polarization vectors on sca les larger than 500 Mpc. See also[edit] Large-scale structure of the cosmos Galaxy filament Sloan Great Wall PiscesCetus Supercluster Complex CfA2 Great Wall HerculesCorona Borealis Great Wall References[edit] ^ Aron, Jacob. "Largest structure challenges Einstein's smooth cosmos". New Scie ntist. Retrieved 14 January 2013. ^ "Astronomers discover the largest structure in the universe". Royal astronomic al society. Retrieved 2013-01-13. ^ a b Clowes, Roger G.; Harris, Kathryn A.; Raghunathan, Srinivasan; Campusano, Luis E.; Söchting, Ilona K.; Graham, Matthew J. (2013-01-11). "A structure in the early Universe at z ~ 1.3 that exceeds the homogeneity scale of the R-W concorda nce cosmology". Monthly notices of the royal astronomical society 1211 (4): 6256 . arXiv:1211.6256. Bibcode:2013MNRAS.429.2910C. doi:10.1093/mnras/sts497. Retrie ved 14 January 2013. ^ "The Largest Structure in Universe Discovered Quasar Group 4 Billion Light-Yea rs Wide Challenges Current Cosmology". Retrieved 14 January 2013. ^ Prostak, Sergio (11 January 2013). "Universe's Largest Structure Discovered". scinews.com. Retrieved 15 January 2013. ^ a b Yadav, Jaswant; Bagla, J. S.; Khandai, Nishikanta (25 February 2010). "Fra ctal dimension as a measure of the scale of homogeneity". Monthly notices of the Royal Astronomical Society 405 (3): 20092015. arXiv:1001.0617. Bibcode:2010MNRAS .405.2009Y. doi:10.1111/j.1365-2966.2010.16612.x. Retrieved 15 January 2013. ^ Hogg, D.W. et al. (2005). "Cosmic Homogeneity Demonstrated with Luminous Red G alaxies". The Astrophysical Journal 624: 5458. arXiv:astro-ph/0411197. Bibcode:20 05ApJ...624...54H. doi:10.1086/429084. ^ Scrimgeour, Morag I. et al. (2012). "The WiggleZ Dark Energy Survey: the trans ition to large-scale cosmic homogeneity". Monthly Notices of the Royal Astronomi cal Society 425 (1): 116134. arXiv:1205.6812. Bibcode:2012MNRAS.425..116S. doi:10 .1111/j.1365-2966.2012.21402.x. ^ a b c Nadathur, Seshadri, (July 2013) "Seeing patterns in noise: gigaparsec-sc ale 'structures' that do not violate homogeneity". Monthly Notices of the Royal Astronomical Society in press. arXiv:1306.1700. Bibcode: 2013MNRAS.tmp.1690N doi :10.1093/mnras/stt1028 ^ Gott, J. Richard, III et al. (May 2005). "A Map of the Universe". The Astrophy sical Journal 624 (2): 463484. arXiv:astro-ph/0310571. Bibcode:2005ApJ...624..463 G. doi:10.1086/428890 ^ Gaite, Jose; Dominguez, Alvaro; Perez-Mercader, Juan (1999). "The fractal dist ribution of galaxies and the transition to homogeneity". The Astrophysical Journ al 522: 58. arXiv:astro-ph/9812132. Bibcode:1999ApJ...522L...5G. doi:10.1086/3122 04. ^ Hutsemekers, D.; Braibant, L.; Pelgrims, V.; Sluse, D. "Alignment of quasar po larizations with large-scale structures". Astronomy & Astrophysics, in press (as tro-ph/1409.6098). Further reading[edit] Clowes, Roger G.; Harris, Kathryn A.; Raghunathan, Srinivasan; Campusano, Luis E .; Soechting, Ilona K.; Graham, Matthew J. (2013). "A structure in the early uni verse at z ~ 1.3 that exceeds the homogeneity scale of the R-W concordance cosmo logy". Monthly Notices of the Royal Astronomical Society 429: 2910. arXiv:1211.6 256. Bibcode:2013MNRAS.429.2910C. doi:10.1093/mnras/sts497. External links[edit] http://www.star.uclan.ac.uk/~rgc/ Sixty Symbols: Biggest Thing in the Universe (Video) A large quasar group (LQG) is a collection of quasars (a form of supermassive bl ack hole active galactic nuclei) that form what are thought to constitute the la rgest astronomical structures in the known universe. LQGs are thought to be prec ursors to the sheets, walls and filaments of galaxies found in the relatively ne arby universe.[1] Contents [hide] 1 Prominent LQGs 2 List of LQGs 3 See also 4 Further reading 5 References Prominent LQGs[edit] On January 11, 2013, the discovery of the Huge-LQG was announced by the Universi ty of Central Lancashire, as the largest known structure in the universe by that time. It comprises 73 quasars and has a minimum diameter of 1.4 billion light-y ears, but over 4 billion light-years at its widest point.[2] According to resear cher and author, Roger Clowes, the existence of structures with the size of LQGs was believed theoretically impossible. Cosmological structures had been believe d to have a size limit of approximately 1.2 billion light-years.[3][4] List of LQGs[edit] An artist's impression of a single quasar powered by a black hole with a mass tw o billion times that of the Sun Large Quasar Groups LQG Date Mean Distance Dimension # of quasars Notes Webster LQG (LQG 1) 1982 z=0.37 100?Mpc 5 First LQG discovered. At the time of its discovery, it was the largest structure known.[1][5][6] CramptonCowleyHartwick LQG (LQG 2, CCH LQG, Komberg-Kravtsov-Lukash LQG 10) 1987 z=1.11 60?Mpc 28 Second LQG discovered [1][5][7] ClowesCampusano LQG (U1.28, CCLQG, LQG 3) 1991 z=1.28 longest dimension: 630?Mpc 34 Third LQG discovered [5][8] 1995 z=1.9 120?Mpc/h 10 Discovered by Graham, Clowes, Campusano. [1][7][9] 1995 z=0.19 60?Mpc/h 7 Discovered by Graham, Clowes, Campusano; this is a grouping of 7 Seyfert galaxies.[1][7][9] KombergKravtsovLukash LQG 1 1996 z=0.6 R=96?Mpc/h 12 Discover ed by Komberg, Kravtsov, Lukash.[1][7] KombergKravtsovLukash LQG 2 1996 z=0.6 R=111?Mpc/h 12 Discover ed by Komberg, Kravtsov, Lukash.[1][7] KombergKravtsovLukash LQG 3 1996 z=1.3 R=123?Mpc/h 14 Discover ed by Komberg, Kravtsov, Lukash.[1][7] KombergKravtsovLukash LQG 4 1996 z=1.9 R=104?Mpc/h 14 Discover ed by Komberg, Kravtsov, Lukash.[1][7] KombergKravtsovLukash LQG 5 1996 z=1.7 R=146?Mpc/h 13 Discover ed by Komberg, Kravtsov, Lukash.[1][7] KombergKravtsovLukash LQG 6 1996 z=1.5 R=94?Mpc/h 10 Discover ed by Komberg, Kravtsov, Lukash.[1][7] KombergKravtsovLukash LQG 7 1996 z=1.9 R=92?Mpc/h 10 Discover ed by Komberg, Kravtsov, Lukash.[1][7] KombergKravtsovLukash LQG 8 1996 z=2.1 R=104?Mpc/h 12 Discover ed by Komberg, Kravtsov, Lukash.[1][7] KombergKravtsovLukash LQG 9 1996 z=1.9 R=66?Mpc/h 18 Discover ed by Komberg, Kravtsov, Lukash.[1][7] KombergKravtsovLukash LQG 11 1996 z=0.7 R=157?Mpc/h 11 Discover ed by Komberg, Kravtsov, Lukash.[1][7] KombergKravtsovLukash LQG 12 1996 z=1.2 R=155?Mpc/h 14 Discover ed by Komberg, Kravtsov, Lukash.[1][7] Newman LQG (U1.54) 1998 z=1.54 150?Mpc/h 21 Discovered by P.R. Newman et al. This structure is parallel to the CCLQG, with its discovery, suggesting that th e cellular structure of sheets and voids already existed in this era, as found i n later void bubbles and walls of galaxies.,[1][8] TeschEngels LQG 2000 z=0.27 140?Mpc/h 7 The first X-ray selected LQG.[1] U1.11 2011 z=1.11 longest dimension: 780?Mpc 38 [5][8] Huge-LQG (U1.27) 2013 z=1.27 characteristic size: 500?Mpc longest dimension: 1240?Mpc 73 The largest structure known in the observable universe[5][10] until it w as eclipsed by the HerculesCorona Borealis Great Wall found one year later.[11][1 2][13] See also[edit] Large-scale structure of the cosmos Further reading[edit] R.G.Clowes; "Large Quasar Groups - A Short Review"; 'The New Era of Wide Field A stronomy', ASP Conference Series, Vol. 232.; 2001; Astronomical Society of the P acific; ISBN 1-58381-065-X ; Bibcode: 2001ASPC..232..108C References[edit] ^ Jump up to: a b c d e f g h i j k l m n o p q r R.G.Clowes; "Large Quasar Grou ps - A Short Review"; 'The New Era of Wide Field Astronomy', ASP Conference Seri es, Vol. 232.; 2001; Astronomical Society of the Pacific; ISBN 1-58381-065-X ; B ibcode: 2001ASPC..232..108C Jump up ^ Wall, Mike (2013-01-11). "Largest structure in universe discovered". F ox News. Jump up ^ Wall, Mike (2013-01-11). "Largest Structure In Universe, Large Quasar Group, Challenges Cosmological Principle". The Huffington Post. Jump up ^ Clowes, Roger; Kathryn A. Harris, Srinivasan Raghunathan, Luis E. Camp usano, Ilona K. Söchting, Matthew J. Graham.; Raghunathan, S.; Campusano, L. E.; S ochting, I. K.; Graham, M. J. (January 11, 2013). "A structure in the early Univ erse at z ~ 1.3 that exceeds the homogeneity scale of the R-W concordance cosmol ogy". Monthly Notices of the Royal Astronomical Society 429 (4): 2910. arXiv:121 1.6256. Bibcode:2013MNRAS.429.2910C. doi:10.1093/mnras/sts497. ^ Jump up to: a b c d e Clowes, Roger G.; Harris, Kathryn A.; Raghunathan, Srini vasan; Campusano, Luis E.; Soechting, Ilona K.; Graham, Matthew J.; "A structure in the early universe at z ~ 1.3 that exceeds the homogeneity scale of the R-W concordance cosmology"; arXiv:1211.6256 ; Bibcode: 2012arXiv1211.6256C ; doi:10. 1093/mnras/sts497 ; Monthly Notices of the Royal Astronomical Society, 11 Januar y 2013 Jump up ^ Webster, Adrian (May 1982). "The clustering of quasars from an objecti ve-prism survey". Monthly Notices of the Royal Astronomical Society 199: 683705. Bibcode:1982MNRAS.199..683W. doi:10.1093/mnras/199.3.683. ^ Jump up to: a b c d e f g h i j k l m n Boris V. Komberg, Andrey V. Kravtsov, Vladimir N. Lukash; "The search and investigation of the Large Groups of Quasars "; arXiv:astro-ph/9602090 ; Bibcode: 1996astro.ph..2090K ; ^ Jump up to: a b c Clowes, Roger; Luis E. Campusano, Matthew J. Graham and Ilon a K. S¨ochting (2001-09-01). "Two close Large Quasar Groups of size ~ 350 Mpc at z ~ 1.2". Mon. Not. R. Astron. Soc. arXiv:1108.6221. Bibcode:2012MNRAS.419..556C. doi:10.1111/j.1365-2966.2011.19719.x. ^ Jump up to: a b Graham, M. J.; Clowes, R. G.; Campusano, L. E.; "Finding Quasa r Superstructures"; Monthly Notices of the Royal Astronomical Society, Vol. 275, NO. 3/AUG1, P. 790, 1995 August; Bibcode: 1995MNRAS.275..790G Jump up ^ ScienceDaily, "Biggest Structure in Universe: Large Quasar Group Is 4 Billion Light Years Across", Royal Astronomical Society, 11 January 2013 (access ed 13 January 2013) Jump up ^ Horvath I., Hakkila J., and Bagoly Z. (2014). "Possible structure in t he GRB sky distribution at redshift two". Jump up ^ Horvath I., Hakkila J., and Bagoly Z.; Hakkila, J.; Bagoly, Z. (2013). "The largest structure of the Universe, defined by Gamma-Ray Bursts" 1311. p. 1 104. arXiv:1311.1104. Bibcode:2013arXiv1311.1104H. Jump up ^ Klotz, Irene (2013-11-19). "Universe's Larges Black hole From Wikipedia, the free encyclopedia For other uses, see Black hole (disambiguation). Schwarzschild black hole Simulation of gravitational lensing by a black hole, which distorts the image of a galaxy in the background General relativity Spacetime curvature schematic G_{\mu \nu} + \Lambda g_{\mu \nu}= {8\pi G\over c^4} T_{\mu \nu} Introduction History Mathematical formulation Resources Tests Fundamental concepts Equivalence principle Special relativity World line Riemannian geometry Phenomena Kepler problem Gravitational lensing Gravitational waves Frame-dragging Geodetic effect Event horizon Singularity Black hole Spacetime Spacetime diagrams Minkowski spacetime Wormhole Equations Formalisms Equations Linearized gravity Einstein field equations Friedmann Geodesics MathissonPapapetr ouDixon HamiltonJacobiEinstein Formalisms ADM BSSN Post-Newtonian Advanced theory KaluzaKlein theory Quantum gravity Solutions Schwarzschild ReissnerNordström Gödel Kerr KerrNewman Kasner LemaîtreTolman Taub-NUT Miln e RobertsonWalker pp-wave van Stockum dust Scientists Einstein Lorentz Hilbert Poincaré Schwarzschild de Sitter Reissner Nordström Weyl Ed dington Friedman Milne Zwicky Lemaître Gödel Wheeler Robertson Bardeen Walker Kerr C handrasekhar Ehlers Penrose Hawking Raychaudhuri Taylor Hulse van Stockum Taub N ewman Yau Thorne others v t e A black hole is a geometrically defined region of spacetime exhibiting such stro ng gravitational effects that nothingincluding particles and electromagnetic radi ation such as lightcan escape from inside it.[1] The theory of general relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole.[2][3] The boundary of the region from which no escape is possible is calle d the event horizon. Although crossing the event horizon has enormous effect on the fate of the object crossing it, it appears to have no locally detectable fea tures. In many ways a black hole acts like an ideal black body, as it reflects n o light.[4][5] Moreover, quantum field theory in curved spacetime predicts that event horizons emit Hawking radiation, with the same spectrum as a black body of a temperature inversely proportional to its mass. This temperature is on the or der of billionths of a kelvin for black holes of stellar mass, making it essenti ally impossible to observe. Objects whose gravitational fields are too strong for light to escape were first considered in the 18th century by John Michell and Pierre-Simon Laplace. The fi rst modern solution of general relativity that would characterize a black hole w as found by Karl Schwarzschild in 1916, although its interpretation as a region of space from which nothing can escape was first published by David Finkelstein in 1958. Long considered a mathematical curiosity, it was during the 1960s that theoretical work showed black holes were a generic prediction of general relativ ity. The discovery of neutron stars sparked interest in gravitationally collapse d compact objects as a possible astrophysical reality. Black holes of stellar mass are expected to form when very massive stars collaps e at the end of their life cycle. After a black hole has formed, it can continue to grow by absorbing mass from its surroundings. By absorbing other stars and m erging with other black holes, supermassive black holes of millions of solar mas ses (M?) may form. There is general consensus that supermassive black holes exis t in the centers of most galaxies. Despite its invisible interior, the presence of a black hole can be inferred thr ough its interaction with other matter and with electromagnetic radiation such a s visible light. Matter falling onto a black hole can form an accretion disk hea ted by friction, forming some of the brightest objects in the universe. If there are other stars orbiting a black hole, their orbit can be used to determine its mass and location. Such observations can be used to exclude possible alternativ es (such as neutron stars). In this way, astronomers have identified numerous st ellar black hole candidates in binary systems, and established that the radio so urce known as Sgr A*, at the core of our own Milky Way galaxy, contains a superm assive black hole of about 4.3 million M?.

Predicted appearance of non-rotating black hole with toroidal ring of ionised ma tter, such as has been proposed[6] as a model for Sgr A*. The asymmetry is due t o the Doppler effect resulting from the enormous orbital speed needed for centri fugal balance of the very strong gravitational attraction of the hole. Contents 1 History 1.1 General relativity 1.2 Golden age 2 Properties and structure 2.1 Physical properties 2.2 Event horizon 2.3 Singularity 2.4 Photon sphere 2.5 Ergosphere 3 Formation and evolution 3.1 Gravitational collapse 3.1.1 Primordial black holes in the Big Bang 3.2 High-energy collisions 3.3 Growth 3.4 Evaporation 4 Observational evidence 4.1 Accretion of matter 4.2 X-ray binaries 4.2.1 Quiescence and advection-dominated accretion flow 4.2.2 Quasi-periodic oscillations 4.3 Galactic nuclei 4.4 Effects of strong gravity 4.5 Alternatives 5 Open questions 5.1 Entropy and thermodynamics 5.2 Information loss paradox 6 See also 7 Notes 8 References 9 Further reading 10 External links History Simulated view of a black hole in front of the Large Magellanic Cloud. Note the gravitational lensing effect, which produces two enlarged but highly distorted v iews of the Cloud. Across the top, the Milky Way disk appears distorted into an arc. The idea of a body so massive that even light could not escape was first put for ward by John Michell in a letter written to Henry Cavendish in 1783 of the Royal Society: If the semi-diameter of a sphere of the same density as the Sun were to exceed t hat of the Sun in the proportion of 500 to 1, a body falling from an infinite he ight towards it would have acquired at its surface greater velocity than that of light, and consequently supposing light to be attracted by the same force in pr oportion to its vis inertiae, with other bodies, all light emitted from such a b ody would be made to return towards it by its own proper gravity. John Michell[7] In 1796, mathematician Pierre-Simon Laplace promoted the same idea in the first and second editions of his book Exposition du système du Monde (it was removed fro m later editions).[8][9] Such "dark stars" were largely ignored in the nineteent h century, since it was not understood how a massless wave such as light could b e influenced by gravity.[10] General relativity In 1915, Albert Einstein developed his theory of general relativity, having earl ier shown that gravity does influence light's motion. Only a few months later, K arl Schwarzschild found a solution to the Einstein field equations, which descri bes the gravitational field of a point mass and a spherical mass.[11] A few mont hs after Schwarzschild, Johannes Droste, a student of Hendrik Lorentz, independe ntly gave the same solution for the point mass and wrote more extensively about its properties.[12][13] This solution had a peculiar behaviour at what is now ca lled the Schwarzschild radius, where it became singular, meaning that some of th e terms in the Einstein equations became infinite. The nature of this surface wa s not quite understood at the time. In 1924, Arthur Eddington showed that the si ngularity disappeared after a change of coordinates (see EddingtonFinkelstein coo rdinates), although it took until 1933 for Georges Lemaître to realize that this m eant the singularity at the Schwarzschild radius was an unphysical coordinate si ngularity.[14] In 1931, Subrahmanyan Chandrasekhar calculated, using special relativity, that a non-rotating body of electron-degenerate matter above a certain limiting mass ( now called the Chandrasekhar limit at 1.4 M?) has no stable solutions.[15] His a rguments were opposed by many of his contemporaries like Eddington and Lev Landa u, who argued that some yet unknown mechanism would stop the collapse.[16] They were partly correct: a white dwarf slightly more massive than the Chandrasekhar limit will collapse into a neutron star,[17] which is itself stable because of t he Pauli exclusion principle. But in 1939, Robert Oppenheimer and others predict ed that neutron stars above approximately 3 M? (the TolmanOppenheimerVolkoff limit ) would collapse into black holes for the reasons presented by Chandrasekhar, an d concluded that no law of physics was likely to intervene and stop at least som e stars from collapsing to black holes.[18] Oppenheimer and his co-authors interpreted the singularity at the boundary of th e Schwarzschild radius as indicating that this was the boundary of a bubble in w hich time stopped. This is a valid point of view for external observers, but not for infalling observers. Because of this property, the collapsed stars were cal led "frozen stars",[19] because an outside observer would see the surface of the star frozen in time at the instant where its collapse takes it inside the Schwa rzschild radius. Golden age See also: Golden age of general relativity In 1958, David Finkelstein identified the Schwarzschild surface as an event hori zon, "a perfect unidirectional membrane: causal influences can cross it in only one direction".[20] This did not strictly contradict Oppenheimer's results, but extended them to include the point of view of infalling observers. Finkelstein's solution extended the Schwarzschild solution for the future of observers fallin g into a black hole. A complete extension had already been found by Martin Krusk al, who was urged to publish it.[21] These results came at the beginning of the golden age of general relativity, whi ch was marked by general relativity and black holes becoming mainstream subjects of research. This process was helped by the discovery of pulsars in 1967,[22][2 3] which, by 1969, were shown to be rapidly rotating neutron stars.[24] Until th at time, neutron stars, like black holes, were regarded as just theoretical curi osities; but the discovery of pulsars showed their physical relevance and spurre d a further interest in all types of compact objects that might be formed by gra vitational collapse. In this period more general black hole solutions were found. In 1963, Roy Kerr f ound the exact solution for a rotating black hole. Two years later, Ezra Newman found the axisymmetric solution for a black hole that is both rotating and elect rically charged.[25] Through the work of Werner Israel,[26] Brandon Carter,[27][ 28] and David Robinson[29] the no-hair theorem emerged, stating that a stationar y black hole solution is completely described by the three parameters of the Ker rNewman metric; mass, angular momentum, and electric charge.[30] At first, it was suspected that the strange features of the black hole solutions were pathological artifacts from the symmetry conditions imposed, and that the singularities would not appear in generic situations. This view was held in part icular by Vladimir Belinsky, Isaak Khalatnikov, and Evgeny Lifshitz, who tried t o prove that no singularities appear in generic solutions. However, in the late 1960s Roger Penrose[31] and Stephen Hawking used global techniques to prove that singularities appear generically.[32] Work by James Bardeen, Jacob Bekenstein, Carter, and Hawking in the early 1970s led to the formulation of black hole thermodynamics.[33] These laws describe the behaviour of a black hole in close analogy to the laws of thermodynamics by rel ating mass to energy, area to entropy, and surface gravity to temperature. The a nalogy was completed when Hawking, in 1974, showed that quantum field theory pre dicts that black holes should radiate like a black body with a temperature propo rtional to the surface gravity of the black hole.[34] The first use of the term "black hole" in print was by journalist Ann Ewing in h er article "'Black Holes' in Space", dated 18 January 1964, which was a report o n a meeting of the American Association for the Advancement of Science.[35] John Wheeler used the term "black hole" at a lecture in 1967, leading some to credit him with coining the phrase. After Wheeler's use of the term, it was quickly ad opted in general use. Properties and structure The no-hair theorem states that, once it achieves a stable condition after forma tion, a black hole has only three independent physical properties: mass, charge, and angular momentum.[30] Any two black holes that share the same values for th ese properties, or parameters, are indistinguishable according to classical (i.e . non-quantum) mechanics. These properties are special because they are visible from outside a black hole. For example, a charged black hole repels other like charges just like any other charged object. Similarly, the total mass inside a sphere containing a black ho le can be found by using the gravitational analog of Gauss's law, the ADM mass, far away from the black hole.[36] Likewise, the angular momentum can be measured from far away using frame dragging by the gravitomagnetic field. When an object falls into a black hole, any information about the shape of the o bject or distribution of charge on it is evenly distributed along the horizon of the black hole, and is lost to outside observers. The behavior of the horizon i n this situation is a dissipative system that is closely analogous to that of a conductive stretchy membrane with friction and electrical resistancethe membrane paradigm.[37] This is different from other field theories like electromagnetism, which do not have any friction or resistivity at the microscopic level, because they are time-reversible. Because a black hole eventually achieves a stable sta te with only three parameters, there is no way to avoid losing information about the initial conditions: the gravitational and electric fields of a black hole g ive very little information about what went in. The information that is lost inc ludes every quantity that cannot be measured far away from the black hole horizo n, including approximately conserved quantum numbers such as the total baryon nu mber and lepton number. This behavior is so puzzling that it has been called the black hole information loss paradox.[38][39] Physical properties A simple illustration of a non-spinning black hole The simplest static black holes have mass but neither electric charge nor angula r momentum. These black holes are often referred to as Schwarzschild black holes after Karl Schwarzschild who discovered this solution in 1916.[11] According to Birkhoff's theorem, it is the only vacuum solution that is spherically symmetri c.[40] This means that there is no observable difference between the gravitation al field of such a black hole and that of any other spherical object of the same mass. The popular notion of a black hole "sucking in everything" in its surroun dings is therefore only correct near a black hole's horizon; far away, the exter nal gravitational field is identical to that of any other body of the same mass. [41] Solutions describing more general black holes also exist. Charged black holes ar e described by the ReissnerNordström metric, while the Kerr metric describes a rota ting black hole. The most general stationary black hole solution known is the Ke rrNewman metric, which describes a black hole with both charge and angular moment um.[42] While the mass of a black hole can take any positive value, the charge and angul ar momentum are constrained by the mass. In Planck units, the total electric cha rge Q and the total angular momentum J are expected to satisfy Q^2+\left ( \tfrac{J}{M} \right )^2\le M^2\, for a black hole of mass M. Black holes saturating this inequality are called ex tremal. Solutions of Einstein's equations that violate this inequality exist, bu t they do not possess an event horizon. These solutions have so-called naked sin gularities that can be observed from the outside, and hence are deemed unphysica l. The cosmic censorship hypothesis rules out the formation of such singularitie s, when they are created through the gravitational collapse of realistic matter. [2] This is supported by numerical simulations.[43] Due to the relatively large strength of the electromagnetic force, black holes f orming from the collapse of stars are expected to retain the nearly neutral char ge of the star. Rotation, however, is expected to be a common feature of compact objects. The black-hole candidate binary X-ray source GRS 1915+105[44] appears to have an angular momentum near the maximum allowed value. Black hole classifications Class Mass Size Supermassive black hole ~1051010 MSun ~0.001400 AU Intermediate-mass black hole ~103 MSun ~103 km REarth Stellar black hole ~10 MSun ~30 km Micro black hole up to ~MMoon up to ~0.1 mm Black holes are commonly classified according to their mass, independent of angu lar momentum J or electric charge Q. The size of a black hole, as determined by the radius of the event horizon, or Schwarzschild radius, is roughly proportiona l to the mass M through r_\mathrm{sh} =\frac{2GM}{c^2} \approx 2.95\, \frac{M}{M_\mathrm{Sun}}~\mathrm{k m,} where rsh is the Schwarzschild radius and MSun is the mass of the Sun.[45] This relation is exact only for black holes with zero charge and angular momentum; fo r more general black holes it can differ up to a factor of 2. Event horizon Main article: Event horizon BH-no-escape-1.svg Far away from the black hole, a particle can move in any direction, as illustrat ed by the set of arrows. It is only restricted by the speed of light. BH-no-escape-2.svg Closer to the black hole, spacetime starts to deform. There are more paths going towards the black hole than paths moving away.[Note 1] BH-no-escape-3.svg Inside of the event horizon, all paths bring the particle closer to the center o f the black hole. It is no longer possible for the particle to escape. The defining feature of a black hole is the appearance of an event horizona bound ary in spacetime through which matter and light can only pass inward towards the mass of the black hole. Nothing, not even light, can escape from inside the eve nt horizon. The event horizon is referred to as such because if an event occurs within the boundary, information from that event cannot reach an outside observe r, making it impossible to determine if such an event occurred.[47] As predicted by general relativity, the presence of a mass deforms spacetime in such a way that the paths taken by particles bend towards the mass.[48] At the e vent horizon of a black hole, this deformation becomes so strong that there are no paths that lead away from the black hole. To a distant observer, clocks near a black hole appear to tick more slowly than those further away from the black hole.[49] Due to this effect, known as gravita tional time dilation, an object falling into a black hole appears to slow down a s it approaches the event horizon, taking an infinite time to reach it.[50] At t he same time, all processes on this object slow down, for a fixed outside observ er, causing emitted light to appear redder and dimmer, an effect known as gravit ational redshift.[51] Eventually, the falling object becomes so dim that it can no longer be seen. On the other hand, an indestructible observer falling into a black hole does not notice any of these effects as he crosses the event horizon. According to his o wn clock, which appears to him to tick normally, he crosses the event horizon af ter a finite time without noting any singular behaviour. In particular, he is un able to determine exactly when he crosses it, as it is impossible to determine t he location of the event horizon from local observations.[52] The shape of the event horizon of a black hole is always approximately spherical .[Note 2][55] For non-rotating (static) black holes the geometry is precisely sp herical, while for rotating black holes the sphere is somewhat oblate. Singularity Main article: Gravitational singularity At the center of a black hole as described by general relativity lies a gravitat ional singularity, a region where the spacetime curvature becomes infinite.[56] For a non-rotating black hole, this region takes the shape of a single point and for a rotating black hole, it is smeared out to form a ring singularity lying i n the plane of rotation.[57] In both cases, the singular region has zero volume. It can also be shown that the singular region contains all the mass of the blac k hole solution.[58] The singular region can thus be thought of as having infini te density. Observers falling into a Schwarzschild black hole (i.e., non-rotating and not ch arged) cannot avoid being carried into the singularity, once they cross the even t horizon. They can prolong the experience by accelerating away to slow their de scent, but only up to a point; after attaining a certain ideal velocity, it is b est to free fall the rest of the way.[59] When they reach the singularity, they are crushed to infinite density and their mass is added to the total of the blac k hole. Before that happens, they will have been torn apart by the growing tidal forces in a process sometimes referred to as spaghettification or the "noodle e ffect".[60] In the case of a charged (ReissnerNordström) or rotating (Kerr) black hole, it is p ossible to avoid the singularity. Extending these solutions as far as possible r eveals the hypothetical possibility of exiting the black hole into a different s pacetime with the black hole acting as a wormhole.[61] The possibility of travel ing to another universe is however only theoretical, since any perturbation will destroy this possibility.[62] It also appears to be possible to follow closed t imelike curves (going back to one's own past) around the Kerr singularity, which lead to problems with causality like the grandfather paradox.[63] It is expecte d that none of these peculiar effects would survive in a proper quantum treatmen t of rotating and charged black holes.[64] The appearance of singularities in general relativity is commonly perceived as s ignaling the breakdown of the theory.[65] This breakdown, however, is expected; it occurs in a situation where quantum effects should describe these actions, du e to the extremely high density and therefore particle interactions. To date, it has not been possible to combine quantum and gravitational effects into a singl e theory, although there exist attempts to formulate such a theory of quantum gr avity. It is generally expected that such a theory will not feature any singular ities.[66][67] Photon sphere Main article: Photon sphere The photon sphere is a spherical boundary of zero thickness such that photons mo ving along tangents to the sphere will be trapped in a circular orbit. For non-r otating black holes, the photon sphere has a radius 1.5 times the Schwarzschild radius. The orbits are dynamically unstable, hence any small perturbation (such as a particle of infalling matter) will grow over time, either setting it on an outward trajectory escaping the black hole or on an inward spiral eventually cro ssing the event horizon.[68] While light can still escape from inside the photon sphere, any light that cross es the photon sphere on an inbound trajectory will be captured by the black hole . Hence any light reaching an outside observer from inside the photon sphere mus t have been emitted by objects inside the photon sphere but still outside of the event horizon.[68] Other compact objects, such as neutron stars, can also have photon spheres.[69] This follows from the fact that the gravitational field of an object does not de pend on its actual size, hence any object that is smaller than 1.5 times the Sch warzschild radius corresponding to its mass will indeed have a photon sphere. Ergosphere Main article: Ergosphere The ergosphere is an oblate spheroid region outside of the event horizon, where objects cannot remain stationary. Rotating black holes are surrounded by a region of spacetime in which it is impo ssible to stand still, called the ergosphere. This is the result of a process kn own as frame-dragging; general relativity predicts that any rotating mass will t end to slightly "drag" along the spacetime immediately surrounding it. Any objec t near the rotating mass will tend to start moving in the direction of rotation. For a rotating black hole, this effect becomes so strong near the event horizon that an object would have to move faster than the speed of light in the opposit e direction to just stand still.[70] The ergosphere of a black hole is bounded by the (outer) event horizon on the in side and an oblate spheroid, which coincides with the event horizon at the poles and is noticeably wider around the equator. The outer boundary is sometimes cal led the ergosurface. Objects and radiation can escape normally from the ergosphere. Through the Penro se process, objects can emerge from the ergosphere with more energy than they en tered. This energy is taken from the rotational energy of the black hole causing it to slow down.[71] Formation and evolution Considering the exotic nature of black holes, it may be natural[clarification ne eded] to question if such bizarre objects could exist in nature or to suggest th at they are merely pathological solutions to Einstein's equations. Einstein hims elf wrongly thought that black holes would not form, because he held that the an gular momentum of collapsing particles would stabilize their motion at some radi us.[72] This led the general relativity community to dismiss all results to the contrary for many years. However, a minority of relativists continued to contend that black holes were physical objects,[73] and by the end of the 1960s, they h ad persuaded the majority of researchers in the field that there is no obstacle to forming an event horizon. Once an event horizon forms, Penrose proved that a singularity will form somewhe re inside it.[31] Shortly afterwards, Hawking showed that many cosmological solu tions describing the Big Bang have singularities without scalar fields or other exotic matter (see PenroseHawking singularity theorems). The Kerr solution, the n o-hair theorem and the laws of black hole thermodynamics showed that the physica l properties of black holes were simple and comprehensible, making them respecta ble subjects for research.[74] The primary formation process for black holes is expected to be the gravitational collapse of heavy objects such as stars, but th ere are also more exotic processes that can lead to the production of black hole s. Gravitational collapse Main article: Gravitational collapse Gravitational collapse occurs when an object's internal pressure is insufficient to resist the object's own gravity. For stars this usually occurs either becaus e a star has too little "fuel" left to maintain its temperature through stellar nucleosynthesis, or because a star that would have been stable receives extra ma tter in a way that does not raise its core temperature. In either case the star' s temperature is no longer high enough to prevent it from collapsing under its o wn weight.[75] The collapse may be stopped by the degeneracy pressure of the sta r's constituents, condensing the matter in an exotic denser state. The result is one of the various types of compact star. The type of compact star formed depen ds on the mass of the remnantthe matter left over after the outer layers have bee n blown away, such from a supernova explosion or by pulsations leading to a plan etary nebula. Note that this mass can be substantially less than the original st arremnants exceeding 5 M? are produced by stars that were over 20 M? before the c ollapse.[75] If the mass of the remnant exceeds about 34 M? (the TolmanOppenheimerVolkoff limit[ 18])either because the original star was very heavy or because the remnant collec ted additional mass through accretion of mattereven the degeneracy pressure of ne utrons is insufficient to stop the collapse. No known mechanism (except possibly quark degeneracy pressure, see quark star) is powerful enough to stop the implo sion and the object will inevitably collapse to form a black hole.[75] The gravitational collapse of heavy stars is assumed to be responsible for the f ormation of stellar mass black holes. Star formation in the early universe may h ave resulted in very massive stars, which upon their collapse would have produce d black holes of up to 103 M?. These black holes could be the seeds of the super massive black holes found in the centers of most galaxies.[76] While most of the energy released during gravitational collapse is emitted very quickly, an outside observer does not actually see the end of this process. Even though the collapse takes a finite amount of time from the reference frame of i nfalling matter, a distant observer sees the infalling material slow and halt ju st above the event horizon, due to gravitational time dilation. Light from the c ollapsing material takes longer and longer to reach the observer, with the light emitted just before the event horizon forms delayed an infinite amount of time. Thus the external observer never sees the formation of the event horizon; inste ad, the collapsing material seems to become dimmer and increasingly red-shifted, eventually fading away.[77] Primordial black holes in the Big Bang Gravitational collapse requires great density. In the current epoch of the unive rse these high densities are only found in stars, but in the early universe shor tly after the big bang densities were much greater, possibly allowing for the cr eation of black holes. The high density alone is not enough to allow the formati on of black holes since a uniform mass distribution will not allow the mass to b unch up. In order for primordial black holes to form in such a dense medium, the re must be initial density perturbations that can then grow under their own grav ity. Different models for the early universe vary widely in their predictions of the size of these perturbations. Various models predict the creation of black h oles, ranging from a Planck mass to hundreds of thousands of solar masses.[78] P rimordial black holes could thus account for the creation of any type of black h ole. High-energy collisions A simulated event in the CMS detector, a collision in which a micro black hole m ay be created. Gravitational collapse is not the only process that could create black holes. In principle, black holes could be formed in high-energy collisions that achieve s ufficient density. As of 2002, no such events have been detected, either directl y or indirectly as a deficiency of the mass balance in particle accelerator expe riments.[79] This suggests that there must be a lower limit for the mass of blac k holes. Theoretically, this boundary is expected to lie around the Planck mass (mP = vhc/G 1.2×1019 GeV/c2 2.2×10-8 kg), where quantum effects are expected to inva lidate the predictions of general relativity.[80] This would put the creation of black holes firmly out of reach of any high-energy process occurring on or near the Earth. However, certain developments in quantum gravity suggest that the Pl anck mass could be much lower: some braneworld scenarios for example put the bou ndary as low as 1 TeV/c2.[81] This would make it conceivable for micro black hol es to be created in the high-energy collisions occurring when cosmic rays hit th e Earth's atmosphere, or possibly in the Large Hadron Collider at CERN. Yet thes e theories are very speculative, and the creation of black holes in these proces ses is deemed unlikely by many specialists.[82] Even if micro black holes should be formed in these collisions, it is expected that they would evaporate in abou t 10-25 seconds, posing no threat to the Earth.[83] Growth Once a black hole has formed, it can continue to grow by absorbing additional ma tter. Any black hole will continually absorb gas and interstellar dust from its direct surroundings and omnipresent cosmic background radiation. This is the pri mary process through which supermassive black holes seem to have grown.[76] A si milar process has been suggested for the formation of intermediate-mass black ho les in globular clusters.[84] Another possibility is for a black hole to merge with other objects such as star s or even other black holes. Although not necessary for growth, this is thought to have been important, especially for the early development of supermassive bla ck holes, which could have formed from the coagulation of many smaller objects.[ 76] The process has also been proposed as the origin of some intermediate-mass b lack holes.[85][86] Evaporation Main article: Hawking radiation In 1974, Hawking predicted that black holes are not entirely black but emit smal l amounts of thermal radiation;[34] this effect has become known as Hawking radi ation. By applying quantum field theory to a static black hole background, he de termined that a black hole should emit particles in a perfect black body spectru m. Since Hawking's publication, many others have verified the result through var ious approaches.[87] If Hawking's theory of black hole radiation is correct, the n black holes are expected to shrink and evaporate over time because they lose m ass by the emission of photons and other particles.[34] The temperature of this thermal spectrum (Hawking temperature) is proportional to the surface gravity of the black hole, which, for a Schwarzschild black hole, is inversely proportiona l to the mass. Hence, large black holes emit less radiation than small black hol es.[88] A stellar black hole of 1 M? has a Hawking temperature of about 100 nanokelvins. This is far less than the 2.7 K temperature of the cosmic microwave background radiation. Stellar-mass or larger black holes receive more mass from the cosmic microwave background than they emit through Hawking radiation and thus will grow instead of shrink.[citation needed] To have a Hawking temperature larger than 2 .7 K (and be able to evaporate), a black hole needs to have less mass than the M oon. Such a black hole would have a diameter of less than a tenth of a millimete r.[89] If a black hole is very small the radiation effects are expected to become very strong. Even a black hole that is heavy compared to a human would evaporate in a n instant. A black hole with the mass of a car would have a diameter of about 10 -24 m and take a nanosecond to evaporate, during which time it would briefly hav e a luminosity more than 200 times that of the Sun. Lower-mass black holes are e xpected to evaporate even faster; for example, a black hole of mass 1 TeV/c2 wou ld take less than 10-88 seconds to evaporate completely. For such a small black hole, quantum gravitation effects are expected to play an important role and cou ld evenalthough current developments in quantum gravity do not indicate so[90]hypo thetically make such a small black hole stable.[91] Observational evidence Gas cloud ripped apart by black hole at the centre of the Milky Way.[92] By their very nature, black holes do not directly emit any signals other than th e hypothetical Hawking radiation; since the Hawking radiation for an astrophysic al black hole is predicted to be very weak, this makes it impossible to directly detect astrophysical black holes from the Earth. A possible exception to the Ha wking radiation being weak is the last stage of the evaporation of light (primor dial) black holes; searches for such flashes in the past have proven unsuccessfu l and provide stringent limits on the possibility of existence of light primordi al black holes.[93] NASA's Fermi Gamma-ray Space Telescope launched in 2008 will continue the search for these flashes.[94] Astrophysicists searching for black holes thus have to rely on indirect observat ions. A black hole's existence can sometimes be inferred by observing its gravit ational interactions with its surroundings. A project run by MIT's Haystack Obse rvatory is attempting to observe the event horizon of a black hole directly. Ini tial results are encouraging.[95] Accretion of matter See also: Accretion disc Black hole with corona, X-ray source (artist's concept).[96] Due to conservation of angular momentum, gas falling into the gravitational well created by a massive object will typically form a disc-like structure around th e object. Artist's impressions such as the accompanying representation of a blac k hole with corona commonly depict the black hole as if it were a flat-space mat erial body hiding the part of the disc just behind it, but detailed mathematical modelling[97] shows that the image of the disc would actually be distorted by l ight bending in such a way that the upper side of the disc is entirely visible, while there is even a partially visible secondary image of the underside.

Predicted view from outside the horizon of a Schwarzschild black hole lit by a t hin accretion disc Within such a disc, friction will cause angular momentum to be transported outwa rd, allowing matter to fall further inward, releasing potential energy and incre asing the temperature of the gas.[98]

Blurring of X-rays near Black hole (NuSTAR; 12 August 2014).[96] In the case of compact objects such as white dwarfs, neutron stars, and black ho les, the gas in the inner regions becomes so hot that it will emit vast amounts of radiation (mainly X-rays), which may be detected by telescopes. This process of accretion is one of the most efficient energy-producing processes known; up t o 40% of the rest mass of the accreted material can be emitted in radiation.[98] (In nuclear fusion only about 0.7% of the rest mass will be emitted as energy.) In many cases, accretion discs are accompanied by relativistic jets emitted alo ng the poles, which carry away much of the energy. The mechanism for the creatio n of these jets is currently not well understood. As such many of the universe's more energetic phenomena have been attributed to the accretion of matter on black holes. In particular, active galactic nuclei an d quasars are believed to be the accretion discs of supermassive black holes.[99 ] Similarly, X-ray binaries are generally accepted to be binary star systems in which one of the two stars is a compact object accreting matter from its compani on.[99] It has also been suggested that some ultraluminous X-ray sources may be the accretion disks of intermediate-mass black holes.[100] X-ray binaries See also: X-ray binary File:A star is consumed by a black hole.ogv A computer simulation of a star being consumed by a black hole. The blue dot ind icates the location of the black hole. X-ray binaries are binary star systems that are luminous in the X-ray part of th e spectrum. These X-ray emissions are generally thought to be caused by one of t he component stars being a compact object accreting matter from the other (regul ar) star. The presence of an ordinary star in such a system provides a unique op portunity for studying the central object and determining if it might be a black hole. File:RXTE Detects Heartbeat Of Smallest Black Hole Candidate.ogv This animation compares the X-ray 'heartbeats' of GRS 1915 and IGR J17091, two b lack holes that ingest gas from companion stars. If such a system emits signals that can be directly traced back to the compact o bject, it cannot be a black hole. The absence of such a signal does, however, no t exclude the possibility that the compact object is a neutron star. By studying the companion star it is often possible to obtain the orbital parameters of the system and obtain an estimate for the mass of the compact object. If this is mu ch larger than the TolmanOppenheimerVolkoff limit (that is, the maximum mass a neu tron star can have before collapsing) then the object cannot be a neutron star a nd is generally expected to be a black hole.[99] The first strong candidate for a black hole, Cygnus X-1, was discovered in this way by Charles Thomas Bolton,[101] Louise Webster and Paul Murdin[102] in 1972.[ 103][104] Some doubt, however, remained due to the uncertainties resultant from the companion star being much heavier than the candidate black hole.[99] Current ly, better candidates for black holes are found in a class of X-ray binaries cal led soft X-ray transients.[99] In this class of system the companion star is rel atively low mass allowing for more accurate estimates in the black hole mass. Mo reover, these systems are only active in X-ray for several months once every 1050 years. During the period of low X-ray emission (called quiescence), the accreti on disc is extremely faint allowing for detailed observation of the companion st ar during this period. One of the best such candidates is V404 Cyg. Quiescence and advection-dominated accretion flow The faintness of the accretion disc during quiescence is suspected to be caused by the flow entering a mode called an advection-dominated accretion flow (ADAF). In this mode, almost all the energy generated by friction in the disc is swept along with the flow instead of radiated away. If this model is correct, then it forms strong qualitative evidence for the presence of an event horizon.[105] Bec ause, if the object at the center of the disc had a solid surface, it would emit large amounts of radiation as the highly energetic gas hits the surface, an eff ect that is observed for neutron stars in a similar state.[98] Quasi-periodic oscillations Main article: Quasi-periodic oscillations The X-ray emission from accretion disks sometimes flickers at certain frequencie s. These signals are called quasi-periodic oscillations and are thought to be ca used by material moving along the inner edge of the accretion disk (the innermos t stable circular orbit). As such their frequency is linked to the mass of the c ompact object. They can thus be used as an alternative way to determine the mass of potential black holes.[106] Galactic nuclei See also: Active galactic nucleus Magnetic waves, called Alfvén S-waves, flow from the base of black hole jets. Astronomers use the term "active galaxy" to describe galaxies with unusual chara cteristics, such as unusual spectral line emission and very strong radio emissio n. Theoretical and observational studies have shown that the activity in these a ctive galactic nuclei (AGN) may be explained by the presence of supermassive bla ck holes, which can be millions of time more massive than stellar ones. The mode ls of these AGN consist of a central black hole that may be millions or billions of times more massive than the Sun; a disk of gas and dust called an accretion disk; and two jets that are perpendicular to the accretion disk.[107][108]

Detection of unusually bright X-Ray flare from Sagittarius A*, a black hole in t he center of the Milky Way galaxy on 5 January 2015.[109] Although supermassive black holes are expected to be found in most AGN, only som e galaxies' nuclei have been more carefully studied in attempts to both identify and measure the actual masses of the central supermassive black hole candidates . Some of the most notable galaxies with supermassive black hole candidates incl ude the Andromeda Galaxy, M32, M87, NGC 3115, NGC 3377, NGC 4258, NGC 4889, NGC 1277, OJ 287, APM 08279+5255 and the Sombrero Galaxy.[110] It is now widely accepted that the center of nearly every galaxy, not just activ e ones, contains a supermassive black hole.[111] The close observational correla tion between the mass of this hole and the velocity dispersion of the host galax y's bulge, known as the M-sigma relation, strongly suggests a connection between the formation of the black hole and the galaxy itself.[112]

Simulation of gas cloud after close approach to the black hole at the centre of the Milky Way.[113] Currently, the best evidence for a supermassive black hole comes from studying t he proper motion of stars near the center of our own Milky Way.[114] Since 1995 astronomers have tracked the motion of 90 stars in a region called Sagittarius A *. By fitting their motion to Keplerian orbits they were able to infer in 1998 t hat 2.6 million M? must be contained in a volume with a radius of 0.02 light-yea rs.[115] Since then one of the starscalled S2has completed a full orbit. From the orbital data they were able to place better constraints on the mass and size of the object causing the orbital motion of stars in the Sagittarius A* region, fin ding that there is a spherical mass of 4.3 million M? contained within a radius of less than 0.002 lightyears.[114] While this is more than 3000 times the Schwa rzschild radius corresponding to that mass, it is at least consistent with the c entral object being a supermassive black hole, and no "realistic cluster [of sta rs] is physically tenable".[115] Effects of strong gravity Another way that the black hole nature of an object may be tested in the future is through observation of effects caused by strong gravity in their vicinity. On e such effect is gravitational lensing: The deformation of spacetime around a ma ssive object causes light rays to be deflected much like light passing through a n optic lens. Observations have been made of weak gravitational lensing, in whic h light rays are deflected by only a few arcseconds. However, it has never been directly observed for a black hole.[116] One possibility for observing gravitati onal lensing by a black hole would be to observe stars in orbit around the black hole. There are several candidates for such an observation in orbit around Sagi ttarius A*.[116] Another option would be the direct observation of gravitational waves produced b y an object falling into a black hole, for example a compact object falling into a supermassive black hole through an extreme mass ratio inspiral. Matching the observed waveform to the predictions of general relativity would allow precision measurements of the mass and angular momentum of the central object, while at t he same time testing general relativity.[117] These types of events are a primar y target for the proposed Laser Interferometer Space Antenna. Alternatives See also: Exotic star The evidence for stellar black holes strongly relies on the existence of an uppe r limit for the mass of a neutron star. The size of this limit heavily depends o n the assumptions made about the properties of dense matter. New exotic phases o f matter could push up this bound.[99] A phase of free quarks at high density mi ght allow the existence of dense quark stars,[118] and some supersymmetric model s predict the existence of Q stars.[119] Some extensions of the standard model p osit the existence of preons as fundamental building blocks of quarks and lepton s, which could hypothetically form preon stars.[120] These hypothetical models c ould potentially explain a number of observations of stellar black hole candidat es. However, it can be shown from general arguments in general relativity that a ny such object will have a maximum mass.[99] Since the average density of a black hole inside its Schwarzschild radius is inv ersely proportional to the square of its mass, supermassive black holes are much less dense than stellar black holes (the average density of a 108 M? black hole is comparable to that of water).[99] Consequently, the physics of matter formin g a supermassive black hole is much better understood and the possible alternati ve explanations for supermassive black hole observations are much more mundane. For example, a supermassive black hole could be modelled by a large cluster of v ery dark objects. However, such alternatives are typically not stable enough to explain the supermassive black hole candidates.[99] The evidence for stellar and supermassive black holes implies that in order for black holes not to form, general relativity must fail as a theory of gravity, pe rhaps due to the onset of quantum mechanical corrections. A much anticipated fea ture of a theory of quantum gravity is that it will not feature singularities or event horizons (and thus no black holes).[121] In 2002,[122] much attention has been drawn by the fuzzball model in string theory. Based on calculations in spe cific situations in string theory, the proposal suggests that generically the in dividual states of a black hole solution do not have an event horizon or singula rity, but that for a classical/semi-classical observer the statistical average o f such states does appear just like an ordinary black hole in general relativity .[123] Open questions Entropy and thermodynamics Further information: Black hole thermodynamics S=1/4 k c3h-1G-1 A. The formula for the BekensteinHawking entropy (S) of a black hole, which depends on the area of the black hole (A). The constants are the speed of light (c), the Boltzmann constant (k), Newton's constant (G), and the reduced Planck constant (h). In 1971, Hawking showed under general conditions[Note 3] that the total area of the event horizons of any collection of classical black holes can never decrease , even if they collide and merge.[124] This result, now known as the second law of black hole mechanics, is remarkably similar to the second law of thermodynami cs, which states that the total entropy of a system can never decrease. As with classical objects at absolute zero temperature, it was assumed that black holes had zero entropy. If this were the case, the second law of thermodynamics would be violated by entropy-laden matter entering a black hole, resulting in a decrea se of the total entropy of the universe. Therefore, Bekenstein proposed that a b lack hole should have an entropy, and that it should be proportional to its hori zon area.[125] The link with the laws of thermodynamics was further strengthened by Hawking's d iscovery that quantum field theory predicts that a black hole radiates blackbody radiation at a constant temperature. This seemingly causes a violation of the s econd law of black hole mechanics, since the radiation will carry away energy fr om the black hole causing it to shrink. The radiation, however also carries away entropy, and it can be proven under general assumptions that the sum of the ent ropy of the matter surrounding a black hole and one quarter of the area of the h orizon as measured in Planck units is in fact always increasing. This allows the formulation of the first law of black hole mechanics as an analogue of the firs t law of thermodynamics, with the mass acting as energy, the surface gravity as temperature and the area as entropy.[125] One puzzling feature is that the entropy of a black hole scales with its area ra ther than with its volume, since entropy is normally an extensive quantity that scales linearly with the volume of the system. This odd property led Gerard 't H ooft and Leonard Susskind to propose the holographic principle, which suggests t hat anything that happens in a volume of spacetime can be described by data on t he boundary of that volume.[126] Although general relativity can be used to perform a semi-classical calculation of black hole entropy, this situation is theoretically unsatisfying. In statisti cal mechanics, entropy is understood as counting the number of microscopic confi gurations of a system that have the same macroscopic qualities (such as mass, ch arge, pressure, etc.). Without a satisfactory theory of quantum gravity, one can not perform such a computation for black holes. Some progress has been made in v arious approaches to quantum gravity. In 1995, Andrew Strominger and Cumrun Vafa showed that counting the microstates of a specific supersymmetric black hole in string theory reproduced the BekensteinHawking entropy.[127] Since then, similar results have been reported for different black holes both in string theory and in other approaches to quantum gravity like loop quantum gravity.[128] Information loss paradox Main article: Black hole information paradox List of unsolved problems in physics Is physical information lost in black holes? Because a black hole has only a few internal parameters, most of the information about the matter that went into forming the black hole is lost. Regardless of t he type of matter which goes into a black hole, it appears that only information concerning the total mass, charge, and angular momentum are conserved. As long as black holes were thought to persist forever this information loss is not that problematic, as the information can be thought of as existing inside the black hole, inaccessible from the outside. However, black holes slowly evaporate by em itting Hawking radiation. This radiation does not appear to carry any additional information about the matter that formed the black hole, meaning that this info rmation appears to be gone forever.[129] The question whether information is truly lost in black holes (the black hole in formation paradox) has divided the theoretical physics community (see ThorneHawki ngPreskill bet). In quantum mechanics, loss of information corresponds to the vio lation of vital property called unitarity, which has to do with the conservation of probability. It has been argued that loss of unitarity would also imply viol ation of conservation of energy.[130] Over recent years evidence has been buildi ng that indeed information and unitarity are preserved in a full quantum gravita tional treatment of the problem.[131] See also Portal icon Cosmology portal Portal icon Star portal Portal icon Astronomy portal List of nearest black holes Black brane Black hole complementarity Black hole starship Black holes in fiction Black string BTZ black hole Dumb hole General relativity Kugelblitz (astrophysics) List of black holes Susskind-Hawking battle Timeline of black hole physics White hole Wormhole Notes ^ The set of possible paths, or more accurately the future light cone containing all possible world lines (in this diagram represented by the yellow/blue grid), is tilted in this way in EddingtonFinkelstein coordinates (the diagram is a "car toon" version of an EddingtonFinkelstein coordinate diagram), but in other coordi nates the light cones are not tilted in this way, for example in Schwarzschild c oordinates they simply narrow without tilting as one approaches the event horizo n, and in KruskalSzekeres coordinates the light cones don't change shape or orien tation at all.[46] ^ This is true only for 4-dimensional spacetimes. 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