Gravastar

A gravastar is an object hypothesized in astrophysics as an alternative to the black hole theory by Pawel O. Mazur and Emil Mottola. 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, 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.[2]

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 observe 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.[3]

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.[4] 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,[5][6][7] or to leave the question unanswered.[8][9]

In comparison with black holes

By taking quantum physics into account, the gravastar hypothesis attempts to resolve contradictions caused by conventional black hole theories.[10]

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.[11] 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.[12] Theoretical support for the feasibility of gravastars does not exclude the existence of black holes as shown in other theoretical studies.[13]

See also

References

  1. ^ a b . 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.
  2. ^ Cattoen, Celine; Faber, Tristan; Visser, Matt (2005-09-25). "Gravastars must have anisotropic pressures". Classical and Quantum Gravity. 22 (20). arXiv:gr-qc/0505137. Bibcode:2005CQGra..22.4189C. doi:10.1088/0264-9381/22/20/002.
  3. ^ Sakai, Nobuyuki; Saida, Hiromi; Tamaki, Takashi (2014-11-17). "Gravastar shadows". Phys. Rev. D. 90. arXiv:1408.6929. Bibcode:2014PhRvD..90j4013S. doi:10.1103/physrevd.90.104013.
  4. ^ "Is space-time actually a superfluid?". New Scientist. Archived from the original on 2006-06-09. Retrieved 2017-11-04. It’s the big bang," says Mazur. "Effectively, we are inside a gravastar.
  5. ^ 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. we conclude it is not possible to model the measured ringdown of GW150914 as due to a rotating gravastar.
  6. ^ "Did LIGO detect black holes or gravastars?". ScienceDaily. October 19, 2016. Retrieved 2017-11-04.
  7. ^ "LIGO's black hole detection survives the gravastar test - ExtremeTech". ExtremeTech. 2016-10-26. Retrieved 2017-11-04.
  8. ^ "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
  9. ^ 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). arXiv:1602.07309. Bibcode:2016PhRvL.116q1101C. doi:10.1103/PhysRevLett.116.171101. ISSN 0031-9007.
  10. ^ Stenger, Richard (22 January 2002). "Is black hole theory full of hot air?". CNN.com. Retrieved 10 April 2014.
  11. ^ Vitor Cardoso; Paolo Pani; Mariano Cadoni; Marco Cavaglia (2007). "Ergoregion instability of ultra-compact astrophysical objects". Physical Review D. 77 (12). arXiv:0709.0532. Bibcode:2008PhRvD..77l4044C. doi:10.1103/PhysRevD.77.124044.
  12. ^ Chirenti, Cecilia; Rezzolla, Luciano (October 2008). "Ergoregion instability in rotating gravastars" (PDF). Physical Review D. 78 (8). arXiv:0808.4080. Bibcode:2008PhRvD..78h4011C. doi:10.1103/PhysRevD.78.084011. Retrieved 10 April 2014.
  13. ^ Rocha; Miguelote; Chan; da Silva; Santos; Anzhong Wang (2008). "Bounded excursion stable gravastars and black holes". arXiv:0803.4200 [gr-qc].

Further reading

External links

Black star (semiclassical gravity)

A black star is a gravitational object composed of matter. It is a theoretical alternative to the black hole concept from general relativity. The theoretical construct was created through the use of semiclassical gravity theory. A similar

structure should also exist for the Einstein–Maxwell–Dirac equations system, which is the (super)classical limit of quantum electrodynamics, and for the Einstein–Yang–Mills–Dirac system, which is the (super)classical limit of the standard model.

A black star doesn't need to have an event horizon, and may or may not be a transitional phase between a collapsing star and a singularity. A black star is created when matter compresses at a rate significantly less than the freefall velocity of a hypothetical particle falling to the center of its star, because quantum processes create vacuum polarization, which creates a form of degeneracy pressure, preventing spacetime (and the particles held within it) from occupying the same space at the same time. This vacuum energy is theoretically unlimited, and if built up quickly enough, will stop gravitational collapse from creating a singularity. This may entail an ever-decreasing rate of collapse, leading to an infinite collapse time, or asymptotically approaching a radius bigger than zero.

A black star with a radius slightly greater than the predicted event horizon for an equivalent-mass black hole will appear very dark, because almost all light produced will be drawn back to the star, and any escaping light will be severely gravitationally redshifted. It will appear almost exactly like a black hole. It will feature Hawking radiation, as virtual particle pairs created in its vicinity may still be split, with one particle escaping and the other being trapped. Additionally, it will create thermal Planckian radiation that will closely resemble the expected Hawking radiation of an equivalent black hole.

The predicted interior of a black star will be composed of this strange state of spacetime, with each length in depth heading inward appearing the same as a black star of equivalent mass and radius with the overlayment stripped off. Temperatures increase with depth towards the centre.

Bright giant

The luminosity class II in the Yerkes spectral classification is given to bright giants. These are stars which straddle the boundary between ordinary giants and supergiants, based on the appearance of their spectra.

CN star

A CN star is a star with strong cyanogen bands in its spectrum. Cyanogen is a simple molecule of one carbon atom and one nitrogen atom, with absorption bands around 388.9 and 421.6 nm. This group of stars was first noticed by Nancy G. Roman who called them 4150 stars.

Dark-energy star

A dark-energy star is a hypothetical compact astrophysical object, which a minority of physicists think might constitute an alternative explanation for observations of astronomical black hole candidates.

The concept was proposed by physicist George Chapline. The theory states that infalling matter is converted into vacuum energy or dark energy, as the matter falls through the event horizon. The space within the event horizon would end up with a large value for the cosmological constant and have negative pressure to exert against gravity. There would be no information-destroying singularity.

Hybrid (British band)

Hybrid are a British electronic music group comprising Mike and Charlotte Truman. The group was formed in 1995 by Mike Truman and Chris Healings, with Lee Mullin performing drums. At the time, they were primarily known as a breakbeat collective, although they overlapped considerably with progressive house and trance.

Their 1999 single, "Finished Symphony" was their first charting release, and their debut studio album, Wide Angle, was released that year to critical acclaim. Hybrid are considered pioneers of the electronic genre, and are known for their cinematic approach to their production, specifically with the use of orchestral recordings. After Mullin left the group, their second studio album, Morning Sci-Fi (2003), was made with Adam Taylor and featured collaborations with Peter Hook and Kirsty Hawkshaw. In 2006, Truman and Healings released their acclaimed third studio album I Choose Noise.

Charlotte Truman (née James) joined as a vocalist shortly afterwards in 2007. Her first recording with Hybrid was "The Formula of Fear" in 2008, the first single from their fourth studio album, Disappear Here (2010).

After a hiatus, founding member Chris Healings left the group in 2015, and their long-awaited fifth studio album, Light of the Fearless, was released in 2018.

Over their career, they have also produced over one hundred remixes for over forty artists including U2, Moby, Rob Dougan, R.E.M., The Future Sound of London. The group was formerly based in Swansea, Wales, but have relocated to Worcestershire, England.

Hypothetical star

A hypothetical star is a star, or type of star, that is speculated to exist but has yet to be definitively observed. Hypothetical types of stars have been conjectured to exist, have existed or will exist in the future universe.

Infrared dark cloud

An infrared dark cloud (IRDC) is a cold, dense region of a giant molecular cloud. They can be seen in silhouette against the bright diffuse mid-infrared emission from the galactic plane.

Iron star

In astronomy, an iron star is a hypothetical type of compact star that could occur in the universe in the extremely far future, after perhaps 101500 years.

The premise behind iron stars states that cold fusion occurring via quantum tunnelling would cause the light nuclei in ordinary matter to fuse into iron-56 nuclei. Fission and alpha-particle emission would then make heavy nuclei decay into iron, converting stellar-mass objects to cold spheres of iron. The formation of these stars is only a possibility if protons do not decay. Though the surface of a neutron star may be iron, according to some predictions, it is distinct from an iron star.

Unrelatedly, the term is also used for blue supergiants which have a forest of forbidden FeII lines in their spectra. They are potentially quiescent hot luminous blue variables. Eta Carinae has been described as a prototypical example.

Lambda Boötis star

A Lambda Boötis star is a type of peculiar star which has an unusually low abundance of iron peak elements in its surface layers. One possible explanation for this is that it is the result of accretion of metal-poor gas from a circumstellar disc, a second possibility is the accretion of material from a hot Jupiter suffering from mass loss. The prototype is Lambda Boötis.

Lead star

A lead star is a low-metallicity star with an overabundance of lead and bismuth as compared to other products of the S-process.

Luciano Rezzolla

Luciano Rezzolla (born 1967) is an Italian professor of relativistic astrophysics and

numerical relativity at the Goethe University Frankfurt. His main field of study is the physics and astrophysics of compact objects such as black holes and neutron stars.

Photometric-standard star

Photometric-standard stars are a series of stars that have had their light output in various passbands of photometric system measured very carefully. Other objects can be observed using CCD cameras or photoelectric photometers connected to a telescope, and the flux, or amount of light received, can be compared to a photometric-standard star to determine the exact brightness, or stellar magnitude, of the object.A current set of photometric-standard stars for UBVRI photometry was published by Arlo U. Landolt in 1992 in the Astronomical Journal.

Photosphere

The photosphere is a star's outer shell from which light is radiated. The term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to it being a spherical surface that is perceived to emit light. It extends into a star's surface until the plasma becomes opaque, equivalent to an optical depth of approximately 2/3, or equivalently, a depth from which 50% of light will escape without being scattered.

In other words, a photosphere is the deepest region of a luminous object, usually a star, that is transparent to photons of certain wavelengths.

Q star

A Q-Star, also known as a grey hole, is a hypothetical type of a compact, heavy neutron star with an exotic state of matter. The Q stands for a conserved particle number. A Q-Star may be mistaken for a stellar black hole.

Starfield (astronomy)

A starfield refers to a set of stars visible in an arbitrarily-sized field of view, usually in the context of some region of interest within the celestial sphere. For example: the starfield surrounding the stars Betelgeuse and Rigel could be defined as encompassing some or all of the Orion constellation.

Stellar atmosphere

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Stellar mass

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Supernova impostor

Supernova impostors are stellar explosions that appear at first to be a supernova but do not destroy their progenitor stars. As such, they are a class of extra-powerful novae. They are also known as Type V supernovae, Eta Carinae analogs, and giant eruptions of luminous blue variables (LBV).

Yellow giant

A yellow giant is a luminous giant star of low or intermediate mass (roughly 0.5–11 solar masses (M)) in a late phase of its stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature as low as 5,200-7500 K. The appearance of the yellow giant is from white to yellow, including the spectral types F and G. About 10.6 percent of all giant stars are yellow giants.

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