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A gravastar is an object hypothesized in astrophysics by Pawel O. Mazur and Emil Mottola as an alternative to the black hole theory. It has usual black hole metric outside of the horizon, but de Sitter metric inside. On the horizon there is a thin shell of matter. The term "gravastar" is a portmanteau of the words "gravitational vacuum star".[1]

Structure

In the original formulation by Mazur and Mottola,[2] gravastars contain a central region featuring a p = −ρ false vacuum or "dark energy", a thin shell of p = ρ perfect fluid, and a true vacuum p = ρ = 0 exterior. The dark energy-like behavior of the inner region prevents collapse to a singularity and the presence of the thin shell prevents the formation of an event horizon, avoiding the infinite blue shift. The inner region has thermodynamically no entropy and may be thought of as a gravitational Bose–Einstein condensate. Severe red-shifting of photons as they climb out of the gravity well would make the fluid shell also seem very cold, almost absolute zero.

In addition to the original thin shell formulation, gravastars with continuous pressure have been proposed. These objects must contain anisotropic stress.[3]

Externally, a gravastar appears similar to a black hole: It is visible by the high-energy radiation it emits while consuming matter, and by the Hawking radiation it creates. Astronomers search the sky for X-rays emitted by infalling matter to detect black holes. A gravastar would produce an identical signature. It is also possible, if the thin shell is transparent to radiation, that gravastars may be distinguished from ordinary black holes by different gravitational lensing properties as null geodesics may pass through.[4]

Mazur and Mottola suggest that the violent creation of a gravastar might be an explanation for the origin of our universe and many other universes, because all the matter from a collapsing star would implode "through" the central hole and explode into a new dimension and expand forever, which would be consistent with the current theories regarding the Big Bang.[5] This "new dimension" exerts an outward pressure on the Bose–Einstein condensate layer and prevents it from collapsing further.

Gravastars also could provide a mechanism for describing how dark energy accelerates the expansion of the universe. One possible hypothesis uses Hawking radiation as a means to exchange energy between the "parent" universe and the "child" universe, and so cause the rate of expansion to accelerate, but this area is under much speculation.

Gravastar formation may provide an alternate explanation for sudden and intense gamma-ray bursts throughout space.

LIGO's observations of gravitational waves from colliding objects have been found either to not be consistent with the gravastar concept,[6][7][8] or to be indistinguishable from ordinary black holes.[9][10]
In comparison with black holes

By taking quantum physics into account, the gravastar hypothesis attempts to resolve contradictions caused by conventional black hole theories.[11]
Event horizons

In a gravastar, the event horizon is not present. The layer of positive pressure fluid would lie just outside the 'event horizon', being prevented from complete collapse by the inner false vacuum.[1] Due to the absence of an event horizon the time coordinate of the exterior vacuum geometry is everywhere valid.
Dynamic stability of gravastars

In 2007, theoretical work indicated that under certain conditions gravastars as well as other alternative black hole models are not stable when they rotate.[12] Theoretical work has also shown that certain rotating gravastars are stable assuming certain angular velocities, shell thicknesses, and compactnesses. It is also possible that some gravastars which are mathematically unstable may be physically stable over cosmological timescales.[13] Theoretical support for the feasibility of gravastars does not exclude the existence of black holes as shown in other theoretical studies.[14]
See also

Acoustic metric
Acoustic Hawking radiation from sonic black holes
Black star (semiclassical gravity)
Dark-energy star
Generic object of dark energy

References

. This solution of Einstein equations is stable and has no singularities. "Los Alamos researcher says 'black holes' aren't holes at all". Los Alamos National Laboratory. Archived from the original on 13 December 2006. Retrieved 10 April 2014.
Mazur, Pawel O.; Mottola, Emil (27 February 2002). "arXiv:gr-qc/0109035v5 27 Feb 2002Gravitational Condensate Stars: An Alternative to Black Holes" (PDF). ArXiv: 1–4.arXiv:gr-qc/0109035. Retrieved 12 November 2020.
Cattoen, Celine; Faber, Tristan; Visser, Matt (2005-09-25). "Gravastars must have anisotropic pressures". Classical and Quantum Gravity. 22 (20): 4189–4202.arXiv:gr-qc/0505137. Bibcode:2005CQGra..22.4189C. doi:10.1088/0264-9381/22/20/002. S2CID 10023130.
Sakai, Nobuyuki; Saida, Hiromi; Tamaki, Takashi (2014-11-17). "Gravastar shadows". Physical Review D. 90 (10): 104013.arXiv:1408.6929. Bibcode:2014PhRvD..90j4013S. doi:10.1103/physrevd.90.104013. S2CID 119102542.
Chown, Marcus (7 June 2006). "Is space-time actually a superfluid?". New Scientist. Archived from the original on 2016-04-12. Retrieved 2017-11-04. "It’s the big bang," says Mazur. "Effectively, we are inside a gravastar." "alternate URL". bibliotecapleyades.net.
Chirenti, Cecilia; Rezzolla, Luciano (2016-10-11). "Did GW150914 produce a rotating gravastar?". Physical Review D. 94 (8): 084016.arXiv:1602.08759. Bibcode:2016PhRvD..94h4016C. doi:10.1103/PhysRevD.94.084016. S2CID 16097346. "We conclude it is not possible to model the measured ringdown of GW150914 as due to a rotating gravastar."
"Did LIGO detect black holes or gravastars?". ScienceDaily. October 19, 2016. Retrieved 2017-11-04.
"LIGO's black hole detection survives the gravastar test". Extreme Tech. 2016-10-26. Retrieved 2017-11-04.
"Was gravitational wave signal from a gravastar, not black holes?". New Scientist. 2016-05-04. Retrieved 2017-11-04. "Our signal is consistent with both the formation of a black hole and a horizonless object – we just can’t tell."
Cardoso, Vitor; Franzin, Edgardo; Pani, Paolo (2016-04-27). "Is the gravitational-wave ringdown a probe of the event horizon?". Physical Review Letters. 116 (17): 171101.arXiv:1602.07309. Bibcode:2016PhRvL.116q1101C. doi:10.1103/PhysRevLett.116.171101. ISSN 0031-9007. PMID 27176511. S2CID 206273829.
Stenger, Richard (22 January 2002). "Is black hole theory full of hot air?". CNN.com. Retrieved 10 April 2014.
Vitor Cardoso; Paolo Pani; Mariano Cadoni; Marco Cavaglia (2008). "Ergoregion instability of ultra-compact astrophysical objects". Physical Review D. 77 (12): 124044.arXiv:0709.0532. Bibcode:2008PhRvD..77l4044C. doi:10.1103/PhysRevD.77.124044. S2CID 119119838.
Chirenti, Cecilia; Rezzolla, Luciano (October 2008). "Ergoregion instability in rotating gravastars" (PDF). Physical Review D. 78 (8): 084011.arXiv:0808.4080. Bibcode:2008PhRvD..78h4011C. doi:10.1103/PhysRevD.78.084011. S2CID 34564980. Archived from the original (PDF) on 4 March 2016. Retrieved 10 April 2014.

Rocha; Miguelote; Chan; da Silva; Santos; Anzhong Wang (2008). "Bounded excursion stable gravastars and black holes". Journal of Cosmology and Astroparticle Physics. 2008 (6): 025.arXiv:0803.4200. Bibcode:2008JCAP...06..025R. doi:10.1088/1475-7516/2008/06/025. S2CID 118669175.

Further reading

Camenzind, Max (2007). Compact objects in astrophysics white dwarfs, neutron stars and black holes. Berlin: Springer. pp. 442–445. ISBN 9783540499121.
George Chapline (2005-03-28). "Black holes 'do not exist'". Nature News.
Mazur; Emil Mottola (2001). "Gravitational Condensate Stars: An Alternative to Black Holes".arXiv:gr-qc/0109035. The original paper by Mazur and Mottola
Visser, Matt; Wiltshire, David L. "Stable gravastars — an alternative to black holes?" (PDF). Retrieved 2004-10-02.
Zanotti, Luciano Rezzolla, Olindo (2013). Relativistic hydrodynamics (1. publ. ed.). Oxford: Oxford University Press. pp. 599–603. ISBN 9780198528906.

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