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White hole

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Ingeneral relativity,awhite holeis a hypothetical region ofspacetimeandsingularitythat cannot be entered from the outside, althoughenergy-matter,lightandinformationcan escape from it. In this sense, it is the reverse of ablack hole,from which energy-matter, light and information cannot escape. White holes appear in the theory ofeternal black holes.In addition to a black hole region in the future, such a solution of theEinstein field equationshas a white hole region in its past.[1]This region does not exist for black holes that have formed throughgravitational collapse,however, nor are there any observed physical processes through which a white hole could be formed.

Supermassive black holes(SMBHs) are theoretically predicted to be at the center of everygalaxy.Possibly, a galaxy cannot form without one.Stephen Hawking[2]and others have proposed that these supermassive black holesspawna supermassive white hole.[3]

Overview[edit]

Like black holes, white holes have properties such asmass,charge,andangular momentum.They attract matter like any other mass, but objects falling towards a white hole would never actually reach the white hole'sevent horizon(though in the case of themaximally extended Schwarzschild solution,discussed below, the white hole event horizon in the past becomes a black hole event horizon in the future, so any object falling towards it will eventually reach the black hole horizon). Imagine a gravitational field, without a surface. Acceleration due to gravity is the greatest on the surface of any body. But since black holes lack a surface, acceleration due to gravity increases exponentially, but never reaches a final value as there is no considered surface in a singularity.

Inquantum mechanics,the black hole emitsHawking radiationand so it can come tothermal equilibriumwith a gas of radiation (not compulsory). Because a thermal-equilibrium state is time-reversal-invariant,Stephen Hawkingargued that the time reversal of a black hole in thermal equilibrium results in a white hole in thermal equilibrium (each absorbing and emitting energy to equivalent degrees).[4][further explanation needed]Consequently, this may imply that black holes and white holes are reciprocal in structure, wherein the Hawking radiation from an ordinary black hole is identified with a white hole's emission of energy and matter. Hawking's semi-classical argument is reproduced in a quantum mechanicalAdS/CFTtreatment,[5]where a black hole inanti-de Sitter spaceis described by a thermal gas in agauge theory,whose time reversal is the same as itself.

Origin[edit]

A diagram of the structure of themaximally extended black holespacetime. The horizontal direction is space and the vertical direction is time.

The possibility of the existence of white holes was put forward by Soviet cosmologistIgor Novikovin 1964,[6]developed byNikolai Kardashev.[7]White holes are predicted as part of a solution to theEinstein field equationsknown as themaximally extendedversion of theSchwarzschild metric[clarification needed]describing an eternalblack holewith no charge and no rotation. Here, "maximally extended" implies thatspacetimeshould not have any "edges". For any possible trajectory of a free-falling particle (following ageodesic) in spacetime, it should be possible to continue this path arbitrarily far into the particle's future, unless the trajectory hits agravitational singularitylike the one at the center of the black hole's interior. In order to satisfy this requirement, it turns out that in addition to the black hole interior region that particles enter when they fall through theevent horizonfrom the outside, there must be a separate white hole interior region, which allows us to extrapolate the trajectories of particles that an outside observer sees rising upawayfrom the event horizon. For an observer outside usingSchwarzschild coordinates,infalling particles take an infinite time to reach the black hole horizon infinitely far in the future, while outgoing particles that pass the observer have been traveling outward for an infinite time since crossing the white hole horizon infinitely far in the past (however, the particles or other objects experience only a finiteproper timebetween crossing the horizon and passing the outside observer). The black hole/white hole appears "eternal" from the perspective of an outside observer, in the sense that particles traveling outward from the white hole interior region can pass the observer at any time, and particles traveling inward, which will eventually reach the black hole interior region can also pass the observer at any time.

Just as there are two separate interior regions of the maximally extended spacetime, there are also two separate exterior regions, sometimes called two different "universes", with the second universe allowing us to extrapolate some possible particle trajectories in the two interior regions. This means that the interior black-hole region can contain a mix of particles that fell in from either universe (and thus an observer who fell in from one universe might be able to see light that fell in from the other one), and likewise particles from the interior white-hole region can escape into either universe. All four regions can be seen in a spacetime diagram that usesKruskal–Szekeres coordinates(see figure).[8]

In this spacetime, it is possible to come up with coordinate systems such that if you pick ahypersurfaceof constant time (a set of points that all have the same time coordinate, such that every point on the surface has aspace-likeseparation, giving what is called a 'space-like surface') and draw an "embedding diagram" depicting the curvature of space at that time, the embedding diagram will look like a tube connecting the two exterior regions, known as an "Einstein-Rosen bridge" orSchwarzschild wormhole.[8]Depending on where the space-like hypersurface is chosen, the Einstein-Rosen bridge can either connect two black hole event horizons in each universe (with points in the interior of the bridge being part of the black hole region of the spacetime), or two white hole event horizons in each universe (with points in the interior of the bridge being part of the white hole region). It is impossible to use the bridge to cross from one universe to the other, however, because it is impossible to enter a white hole event horizon from the outside, and anyone entering a black hole horizon from either universe will inevitably hit the black hole singularity.

Note that the maximally extended Schwarzschild metric describes an idealized black hole/white hole that exists eternally from the perspective of external observers; a more realistic black hole that forms at some particular time from a collapsing star would require a different metric. When the infalling stellar matter is added to a diagram of a black hole's history, it removes the part of the diagram corresponding to the white hole interior region.[9]But because the equations of general relativity are time-reversible – they exhibitTime reversal symmetry– general relativity must also allow the time-reverse of this type of "realistic" black hole that forms from collapsing matter. The time-reversed case would be a white hole that has existed since the beginning of the universe, and that emits matter until it finally "explodes" and disappears.[10]Despite the fact that such objects are permitted theoretically, they are not taken as seriously as black holes by physicists, since there would be no processes that would naturally lead to their formation; they could exist only if they were built into the initial conditions of theBig Bang.[10]Additionally, it is predicted that such a white hole would be highly "unstable" in the sense that if any small amount of matter fell towards the horizon from the outside, this would prevent the white hole's explosion as seen by distant observers, with the matter emitted from the singularity never able to escape the white hole's gravitational radius.[11]

Properties[edit]

Depending on the type of black hole solution considered, there are several types of white holes. In the case of the Schwarzschild black hole mentioned above, a geodesic coming out of a white hole comes from the "gravitational singularity" it contains. In the case of a black hole possessing anelectric chargeψ ** Ώ ** ώ (Reissner-Nordström black hole) or anangular momentum,then the white hole happens to be the "exit door" of a black hole existing in another universe. Such a black hole - white hole configuration is called awormhole.In both cases, however, it is not possible to reach the region "in" the white hole, so the behavior of it - and, in particular, what may come out of it - is completely impossible to predict. In this sense, a white hole is a configuration according to which the evolution of the universe cannot be predicted, because it is not deterministic. A "bare singularity" is another example of a non-deterministic configuration, but does not have the status of a white hole, however, because there is no region inaccessible from a given region. In its basic conception, theBig Bangcan be seen as a naked singularity in outer space, but does not correspond to a white hole.[12]

Physical relevance[edit]

In its mode of formation, a black hole comes from a residue of a massivestarwhose core contracts until it turns into a black hole. Such a configuration is not static: we start from a massive and extended body which contracts to give a black hole. The black hole therefore does not exist for all eternity, and there is no corresponding white hole.

To be able to exist, a white hole must either arise from a physical process leading to its formation, or be present from the creation of theuniverse.None of these solutions appears satisfactory: there is no knownastrophysicalprocess that can lead to the formation of such a configuration, and imposing it from the creation of the universe amounts to assuming a very specific set of initial conditions which has no concrete motivation. Also the existence of white holes seems difficult to consider.

In view of the enormous quantities radiated byquasars,whose luminosity makes it possible to observe them from several billionlight-yearsaway, it had been assumed that they were the seat of exotic physical phenomena such as a white hole, or a phenomenon of continuous creation of matter (see the article on thesteady state theory). These ideas are now abandoned, the observed properties of quasars being very well explained by those of anaccretion diskin the center of which is asupermassive black hole.[12]

Big Bang/Supermassive White Hole[edit]

Main article:Big Bang
See also:Lee Smolin § Cosmological natural selection,andShockwave cosmology

A view of black holes first proposed in the late 1980s might be interpreted as shedding some light on the nature of classical white holes. Some researchers have proposed that when a black hole forms, a Big Bang may occur at the core/singularity,which would create a new universe thatexpands outside of theparent universe.[13][14][15]

TheEinstein–Cartan–Sciama–Kibbletheory of gravity extends general relativity by removing a constraint of the symmetry of the affine connection and regarding its antisymmetric part, thetorsion tensor,as a dynamical variable. Torsion naturally accounts for the quantum-mechanical, intrinsic angular momentum (spin) of matter. According to general relativity, the gravitational collapse of a sufficiently compact mass forms a singular black hole. In the Einstein–Cartan theory, however, the minimal coupling between torsion andDirac spinorsgenerates a repulsive spin–spin interaction that is significant infermionic matterat extremely high densities. Such an interaction prevents the formation of a gravitational singularity. Instead, the collapsing matter on the other side of the event horizon reaches an enormous but finite density and rebounds, forming a regular Einstein–Rosen bridge.[16]The other side of the bridge becomes a new, growing baby universe. For observers in the baby universe, the parent universe appears as the only white hole. Accordingly, theobservable universeis the Einstein–Rosen interior of a black hole existing as one of possibly many inside a larger universe. The Big Bang was a nonsingularBig Bounceat which the observable universe had a finite, minimum scale factor.[17]

Shockwave cosmology,proposed by Joel Smoller and Blake Temple in 2003, has the “big bang” as an explosion inside a black hole, producing the expanding volume of space and matter that includes the observable universe.[18]This black hole eventually becomes a white hole as the matter density reduces with the expansion. A related theory gives an alternative to dark energy.[19]

A 2012 paper argues that theBig Bangitself is a white hole.[20]It further suggests that the emergence of a white hole, which was named a "Small Bang", is spontaneous—all the matter is ejected at a single pulse. Thus, unlike black holes, white holes cannot be continuously observed; rather, their effects can be detected only around the event itself. The paper even proposed identifying a new group ofgamma-ray burstswith white holes.

Various hypotheses[edit]

Unlike black holes for which there is a well-studied physical process,gravitational collapse(which gives rise to black holes when a star somewhat more massive than the sun exhausts its nuclear "fuel" ), there is no clear analogous process that leads reliably to the production of white holes. Although some hypotheses have been put forward:

  • White holes as a kind of "exit" from black holes, both types of singularities would probably be connected by awormhole(note that, like white holes, wormholes have not yet been found); whenquasarswere discovered it was assumed that these were the sought-after white holes but this assumption has now been discarded.[21]
  • Another widespread idea is that white holes would be very unstable, would last a very short time and even after forming could collapse and become black holes.
  • Israeli astronomers Alon Retter and Shlomo Heller suggest that theGRB 060614anomalous gamma-ray burst that occurred in 2006 was a "white hole".[22][23]
  • In 2014, the idea of the Big Bang being produced by a supermassive white hole explosion was explored in the framework of a five-dimensional vacuum by Madriz Aguilar, Moreno and Bellini.[24]
  • Finally, it has been postulated that white holes could be the temporal inverse of ablack hole.[25][26]

At present, very few scientists believe in the existence of white holes and it is considered only a mathematical exercise with no real-world counterpart.[27]

In popular culture[edit]

  • Awhite holeappears in theRed Dwarfepisodeof the same name,wherein the protagonists must find a way to deal with its temporal effects.
  • Awhite holeserves as a major source of conflict in theYu-Gi-Oh! GXanime, as the radiance it exudes is both sentient and evil, known as the Light of Destruction.
  • Awhite holeserves as a very important location in thevideo gameOuter Wilds.In this game, falling into the black hole in the center of the planetBrittle Hollowleads to thiswhite hole.
  • Awhite holeappears in the animated television seriesVoltron: Legendary Defender.

See also[edit]

References[edit]

  1. ^Carroll, Sean M. (2004).Spacetime and Geometry(5.7 ed.).Addison-Wesley.ISBN0-8053-8732-3.
  2. ^Hawking and Penrose,The Nature of Space and Time(Princeton, 1996)
  3. ^"Is the Big Bang a black hole?".math.ucr.edu.
  4. ^Hawking, S. W. (1976). "Black Holes and Thermodynamics".Physical Review D.13(2): 191–197.Bibcode:1976PhRvD..13..191H.doi:10.1103/PhysRevD.13.191.
  5. ^Klebanov, Igor R. (19 May 2006). "TASI lectures: Introduction to the AdS/CFT correspondence".Strings, Branes and Gravity.pp. 615–650.arXiv:hep-th/0009139.Bibcode:2001sbg..conf..615K.doi:10.1142/9789812799630_0007.ISBN978-981-02-4774-4.S2CID14783311.{{cite book}}:|journal=ignored (help)
  6. ^Старобинский, А. А.(1988). "БЁЛАЯ ДЫРА" [White hole]. InПРОХОРОВ, А.М.(ed.).ФИЗИЧЕСКАЯ ЭНЦИКЛОПЕДИЯ(in Russian). Vol. 1. Москва: Советская энциклопедия. p. 184.
  7. ^Вселенная, жизнь, разум(in Russian). Наука. 1976. p. 310.
  8. ^abAndrew Hamilton."White Holes and Wormholes".Archived fromthe originalon 27 September 2011.Retrieved12 October2011.
  9. ^Andrew Hamilton."Collapse to a black hole".Retrieved12 October2011.
  10. ^abWheeler, J. Craig (2007).Cosmic Catastrophes: Exploding Stars, Black Holes, and Mapping the Universe.Cambridge University Press.pp.197–198.ISBN978-0-521-85714-7.
  11. ^Frolov, Valeri P.;Igor D. Novikov(1998).Black Hole Physics: Basic Concepts and New Developments.Springer. pp.580–581.ISBN978-0-7923-5145-0.
  12. ^ab"Trou blanc: définition et explications".Techno-Science.net.
  13. ^E. Fahri & A. H. Guth (1987)."An Obstacle to Creating a Universe in the Laboratory"(PDF).Physics Letters B.183(2): 149–155.Bibcode:1987PhLB..183..149F.doi:10.1016/0370-2693(87)90429-1.
  14. ^Nikodem J. Popławski (2010)."Radial motion into an Einstein–Rosen bridge".Physics Letters B.687(2–3): 110–113.arXiv:0902.1994.Bibcode:2010PhLB..687..110P.doi:10.1016/j.physletb.2010.03.029.S2CID5947253.
  15. ^"Every Black Hole Contains Another Universe?".National Geographic News.12 April 2010. Archived fromthe originalon 27 August 2019.
  16. ^N. J. Popławski (2010). "Cosmology with torsion: An alternative to cosmic inflation".Physics Letters B.694(3): 181–185.arXiv:1007.0587.Bibcode:2010PhLB..694..181P.doi:10.1016/j.physletb.2010.09.056.
  17. ^N. Popławski(2012). "Nonsingular, big-bounce cosmology from spinor-torsion coupling".Physical Review D.85(10): 107502.arXiv:1111.4595.Bibcode:2012PhRvD..85j7502P.doi:10.1103/PhysRevD.85.107502.S2CID118434253.
  18. ^"Did cosmos begin as a black hole?".NBC News.17 September 2003.Retrieved23 March2024.
  19. ^Clara Moskowitz (17 August 2009)."'Big Wave' Theory Offers Alternative to Dark Energy ".Space.Retrieved23 March2024.
  20. ^A. Retter & S. Heller (2012). "The revival of white holes as Small Bangs".New Astronomy.17(2): 73–75.arXiv:1105.2776.Bibcode:2012NewA...17...73R.doi:10.1016/j.newast.2011.07.003.S2CID118505127.
  21. ^Sitio oficial de la Nasa en donde se explica la cuestión: los cuásares fueron supuestos como agujeros blancos pero la hipótesis quedó descartada
  22. ^Alon Retter; Shlomo Heller (17 July 2011). "The Revival of White Holes as Small Bangs".New Astronomy.17(2) (New Astronomy ed.): 73–75.arXiv:1105.2776.Bibcode:2012NewA...17...73R.doi:10.1016/j.newast.2011.07.003.S2CID118505127.
  23. ^Леонид Попов (27 May 2011)."Израильтяне нашли белую дыру".Archived fromthe originalon 4 August 2012.Retrieved3 May2012.
  24. ^J. E. Madriz Aguilar, C. Moreno, M. Bellini. "The primordial explosion of a false white hole from a 5D vacuum".Physics Letters.B728, 244 (2014).[1].
  25. ^Descubren nuevas evidencias de la transición al blanco de los agujeros negros, Universidad Complutense de Madrid.
  26. ^Carlos Barceló, Raúl Carballo Rubio y Luis J. Garay. “Exponential fading to white of black holes in quantum gravity”.Classical and Quantum Gravity.Volume 34. Number 10.2017.DOI: 10.1088/1361-6382/aa6962.
  27. ^"¿Hemos detectado ya agujeros blancos y no los hemos reconocido?".abc(in Spanish). 17 December 2018.Retrieved12 March2020.

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