View source for Gravastar

{{distinguish|Gravatar|Gravitar }}
[[File:Gravastar.png|thumb|A diagram comparing the structure of a classical black hole with a gravastar.]]<!-- Note: "Gravistar" redirects here -->
{{short description|Hypothesized alternative to a black hole}}

In [[astrophysics]], a '''gravastar'''  (a [[portmanteau|blend word]] of "gravitational [[vacuum]] star") is an object hypothesized in a 2001 paper by [[Pawel O. Mazur]] and [[Emil Mottola]] as an alternative to the [[black hole]] theory.<ref>{{Citation |last1=Mazur |first1=Pawel O. |title=Gravitational Condensate Stars: An Alternative to Black Holes |date=2023-02-15 |last2=Mottola |first2=Emil|journal=Universe |volume=9 |issue=2 |page=88 |doi=10.3390/universe9020088 |doi-access=free |arxiv=gr-qc/0109035 |bibcode=2023Univ....9...88M }}</ref> It has the usual black hole [[Metric tensor (general relativity)|metric]] outside of the [[Horizon (general relativity)|horizon]], but [[de Sitter metric]] inside. On the horizon there is a thin shell of [[exotic matter]]. This [[Solutions of the Einstein field equations|solution]] to the [[Einstein field equations|Einstein equations]] is stable and has no [[Gravitational singularity|singularities]].<ref name="notblackholes">
{{cite web
|url=http://www.lanl.gov/news/releases/archive/02-035.shtml
|archive-url=https://web.archive.org/web/20061213095149/http://www.lanl.gov/news/releases/archive/02-035.shtml
|archive-date=13 December 2006
|title=Los Alamos researcher says 'black holes' aren't holes at all
|publisher=[[Los Alamos National Laboratory]]
|access-date=10 April 2014
}}
</ref> Further theoretical considerations of gravastars include the notion of a '''nestar''' (a second gravastar ''nested'' within the first one).<ref name="SA-20240220">{{cite news |last=McRae |first=Mike |title=Bubble-Like 'Stars Within Stars' Could Explain Black Hole Weirdness |url=https://www.sciencealert.com/bubble-like-stars-within-stars-could-explain-black-hole-weirdness |date=20 February 2024 |work=[[ScienceAlert]] |url-status=live |archiveurl=https://archive.today/20240220150012/https://www.sciencealert.com/bubble-like-stars-within-stars-could-explain-black-hole-weirdness |archivedate=20 February 2024 |accessdate=20 February 2024 }}</ref><ref>{{Cite journal |last1=Jampolski |first1=Daniel |last2=Rezzolla |first2=Luciano |date=15 February 2024 |title=Nested solutions of gravitational condensate stars |journal=[[Classical and Quantum Gravity]] |volume=41 |issue=6 |page=065014 |arxiv=2310.13946 |bibcode=2024CQGra..41f5014J |doi=10.1088/1361-6382/ad2317 |s2cid=264426808}}</ref>

== Structure ==
In the original formulation by Mazur and Mottola,<ref>{{harvnb|Mazur|Mottola|2024|p=88}}</ref> a gravastar is composed of three regions, differentiated by the relationship between pressure {{math| ''p''}} and energy density {{math| ''ρ''}}{{Technical inline|date=March 2024}}. The central region consists of [[false vacuum]] or "dark energy", and in this region {{nowrap|{{math| ''p'' {{=}} −''ρ''}} }}.   Surrounding it is a thin shell of [[perfect fluid]] where {{nowrap|{{math| ''p'' {{=}} ''ρ''}} }}.  On the exterior is true vacuum, where {{nowrap|{{math| ''p'' {{=}} ''ρ'' {{=}} 0}} }}.

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]]{{Technical inline|date=January 2025}}. The inner region has thermodynamically no [[entropy (thermodynamics)|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.<ref>{{Cite journal |last1=Cattoen |first1=Celine |last2=Faber |first2=Tristan |last3=Visser |first3=Matt |date=21 October 2005 |title=Gravastars must have anisotropic pressures |journal=[[Classical and Quantum Gravity]] |volume=22 |issue=20 |pages=4189–4202 |arxiv=gr-qc/0505137 |bibcode=2005CQGra..22.4189C |doi=10.1088/0264-9381/22/20/002 |issn=0264-9381 |s2cid=10023130}}</ref>

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.{{citation needed|date=December 2017}} Astronomers search the sky for [[X-ray]]s 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 photon like particles' paths{{Technical inline|date=January 2025}} may pass through.<ref>{{Cite journal |last1=Sakai |first1=Nobuyuki |last2=Saida |first2=Hiromi |last3=Tamaki |first3=Takashi |date=17 November 2014 |title=Gravastar shadows |journal=[[Physical Review D]] |volume=90 |issue=10 |page=104013 |arxiv=1408.6929 |bibcode=2014PhRvD..90j4013S |doi=10.1103/PhysRevD.90.104013 |issn=1550-7998 |s2cid=119102542}}</ref>

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]].<ref>{{cite news |first=Marcus |last=Chown |date=7 June 2006 |title=Is space-time a superfluid? |magazine=[[New Scientist]] |language=en-US |url=https://www.newscientist.com/article/mg19025551-000-is-space-time-actually-a-superfluid/ |access-date=2017-11-04 |url-access=subscription |url-status=live |archive-url=https://web.archive.org/web/20160412232613/https://www.newscientist.com/article/mg19025551-000-is-space-time-actually-a-superfluid/ |archive-date=2016-04-12 |quote="It’s the big bang," says Mazur. "Effectively, we are inside a gravastar."}} {{cite web |title=alternative URL |website=bibliotecapleyades.net |url=https://www.bibliotecapleyades.net/ciencia/time_travel/esp_ciencia_timetravel12.htm}}</ref> 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.{{citation needed|date=December 2017}}

Gravastar formation may provide an alternative explanation for sudden and intense [[gamma-ray burst]]s throughout space.{{citation needed|date=December 2017}}

LIGO's observations of gravitational waves from colliding objects have been found either to not be consistent with the gravastar concept,<ref>{{Cite journal |last1=Chirenti |first1=Cecilia |last2=Rezzolla |first2=Luciano |date=11 October 2016 |title=Did GW150914 produce a rotating gravastar? |journal=[[Physical Review D]] |volume=94 |issue=8 |page=084016 |arxiv=1602.08759 |bibcode=2016PhRvD..94h4016C |doi=10.1103/PhysRevD.94.084016 |issn=2470-0010 |s2cid=16097346 |quote=We conclude it is not possible to model the measured ringdown of GW150914 as due to a rotating gravastar.}}</ref><ref>{{Cite news |title=Did LIGO detect black holes or gravastars? |date=October 19, 2016 |website=[[ScienceDaily]] |url=https://www.sciencedaily.com/releases/2016/10/161019082757.htm |access-date=2017-11-04 |language=en}}</ref><ref>{{cite news |title=LIGO's black hole detection survives the gravastar test |date=2016-10-26 |website=Extreme Tech |url=https://www.extremetech.com/extreme/237917-ligos-black-hole-detection-survives-the-gravatstar-test |access-date=2017-11-04 |language=en-US}}</ref> or to be indistinguishable from ordinary black holes.<ref>{{cite news |title=Was gravitational wave signal from a gravastar, not black holes? |date=2016-05-04 |magazine=New Scientist |url=https://www.newscientist.com/article/mg23030724-100-was-gravitational-wave-signal-from-a-gravastar-not-black-holes/ |access-date=2017-11-04 |language=en-US |quote=Our signal is consistent with both the formation of a black hole and a horizonless object – we just can’t tell.}}</ref><ref>{{Cite journal |last1=Cardoso |first1=Vitor |last2=Franzin |first2=Edgardo |last3=Pani |first3=Paolo |date=27 April 2016 |title=Is the Gravitational-Wave Ringdown a Probe of the Event Horizon? |journal=[[Physical Review Letters]] |volume=116 |issue=17 |page=171101 |arxiv=1602.07309 |bibcode=2016PhRvL.116q1101C |doi=10.1103/PhysRevLett.116.171101 |issn=0031-9007 |pmid=27176511 |s2cid=206273829}}</ref>

==Comparison with black holes==
By taking quantum physics into account, the gravastar hypothesis attempts to resolve contradictions caused by conventional [[black hole]] theories.<ref>
{{cite news
|url=http://edition.cnn.com/2002/TECH/space/01/22/gravastars/index.html
|title= Is black hole theory full of hot air? 
|date= 22 January 2002
|publisher=[[CNN.com]]
|access-date=10 April 2014
|last=Stenger
|first=Richard
}}
</ref>

===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.<ref name="notblackholes"/> 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.<ref>{{Cite journal |last1=Cardoso |first1=Vitor |last2=Pani |first2=Paolo |last3=Cadoni |first3=Mariano |last4=Cavaglià |first4=Marco |date=26 June 2008 |title=Ergoregion instability of ultracompact astrophysical objects |journal=[[Physical Review D]] |language=en |volume=77 |issue=12 |page=124044 |arxiv=0709.0532 |bibcode=2008PhRvD..77l4044C |doi=10.1103/PhysRevD.77.124044 |issn=1550-7998 |s2cid=119119838}}</ref> 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.<ref>{{Cite journal |last1=Chirenti |first1=Cecilia B. M. H. |last2=Rezzolla |first2=Luciano |date=8 October 2008 |title=Ergoregion instability in rotating gravastars |url=http://pubman.mpdl.mpg.de/pubman/item/escidoc:52853:2/component/escidoc:52854/PRD78-084011.pdf |url-status=dead |journal=[[Physical Review D]] |volume=78 |issue=8 |page=084011 |arxiv=0808.4080 |bibcode=2008PhRvD..78h4011C |doi=10.1103/PhysRevD.78.084011 |hdl=11858/00-001M-0000-0013-136E-D |issn=1550-7998 |s2cid=34564980 |archive-url=https://web.archive.org/web/20160304040157/http://pubman.mpdl.mpg.de/pubman/item/escidoc:52853:2/component/escidoc:52854/PRD78-084011.pdf |archive-date=4 March 2016 |access-date=10 April 2014}}</ref> Theoretical support for the feasibility of gravastars does not exclude the existence of black holes as shown in other theoretical studies.<ref>{{Cite journal |last1=Rocha |first1=P |last2=Miguelote |first2=A Y |last3=Chan |first3=R |last4=da Silva |first4=M F |last5=Santos |first5=N O |last6=Wang |first6=Anzhong |date=23 June 2008 |title=Bounded excursion stable gravastars and black holes |journal=[[Journal of Cosmology and Astroparticle Physics]] |volume=2008 |issue=6 |page=25 |arxiv=0803.4200 |bibcode=2008JCAP...06..025R |doi=10.1088/1475-7516/2008/06/025 |issn=1475-7516 |s2cid=118669175}}</ref> 
<!-- TODO: talk about entropy -->

==See also==
* [[Acoustic metric]]
* [[Hawking radiation#Experimental|Acoustic Hawking radiation]] from [[sonic black hole]]s
* [[Black star (semiclassical gravity)]]
* [[Dark-energy star]]

==References==
{{reflist|25em}}

==Further reading==
{{Refbegin}}
* {{Cite book |last=Camenzind |first=Max |title=Compact Objects in Astrophysics: White Dwarfs, Neutron Stars and Black Holes |date=2007 |publisher=Springer |isbn=978-3-540-49912-1 |series=Astronomy and astrophysics library |location=Berlin ; New York |pages=442&ndash;445}}
* {{cite news
 | url=http://www.nature.com/news/2005/050328/full/050328-8.html
 | author=George Chapline
 | publisher=Nature News
 | title=Black holes 'do not exist'
 | date=2005-03-28
}}
* {{Cite journal |last1=Mazur |first1=Pawel O. |last2=Mottola |first2=Emil |date=7 February 2024 |title=Gravitational Condensate Stars: An Alternative to Black Holes |journal=[[Universe (journal)|Universe]] |volume=9 |issue=2 |page=88 |arxiv=gr-qc/0109035 |bibcode=2023Univ....9...88M |doi=10.3390/universe9020088 |issn=2218-1997 |doi-access=free}} The original paper by Mazur and Mottola
* {{cite web
 | url=http://www.fc.up.pt/pessoas/luis.beca/phisky/PhiSky%20Wiltshire.pdf
 | title=Stable gravastars — an alternative to black holes?
 |author1=Visser, Matt |author2=Wiltshire, David L. | access-date=2004-10-02
 }}
* {{Cite book |last1=Rezzolla |first1=Luciano |title=Relativistic Hydrodynamics |last2=Zanotti |first2=Olindo |date=2018 |publisher=[[Oxford University Press]] |isbn=978-0-19-852890-6 |location=Oxford |pages=599&ndash;603}}
{{Refend}}

==External links==
* [https://arxiv.org/find/grp_physics/1/abs:+gravastar/0/1/0/all/0/1 Papers about gravastars on gr-qc]
* {{YouTube|BmUZ2wp1lM8|Black Hole's Evil Twin – Gravastars Explained}} by [[Kurzgesagt]]

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