Gravastar

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A gravastar is an object hypothesized in astrophysics as an alternative to the black hole theory by Pawel O. Mazur and Emil Mottola. It results from assuming real, physical limitations on the formation of black holes. These limits, such as discrete length and time quanta (chronon), were not known to exist when black holes were originally theorized, so the concept of a gravastar is an attempt to "modernize" the theory by incorporating quantum mechanics. The term "gravastar" is a portmanteau of the words "Gravitational Vacuum Star".[1]

Structure[edit]

The notion of gravastars builds on Einstein's theory of general relativity and imposes a universal "smallest size" that is known to exist according to well-accepted quantum theory. This size is known as the Planck length, and is derived using the speed of light, Planck's constant and the gravitational constant. Quantum theory says that any scale smaller than the Planck length is unobservable and meaningless to physics and physicists. This limit can be imposed on the wavelength of a beam of light so as to obtain a limit of blue shift that the light can undergo. This becomes important for the structure of a gravastar because general relativity says that a gravitational well blue-shifts incoming light, so around the extremely large mass of a gravastar there is a region of "immeasurability" to the outside universe as the wavelength of the light approaches, and then passes, the Planck length. This region is called a "gravitational vacuum", because it is a void in the fabric of space and time.

Mazur and Mottola hypothesize that just outside this region there will be a very dense form of matter, Bose–Einstein condensate. This can be created in a laboratory by supercooling atoms to expand their wavelengths, enabling the atoms to superimpose their wave-functions to create one very dense form of atom. To outside observers, the outer core of a gravastar would appear to be Bose–Einstein condensate. The severe red-shifting of space-time as photons climb out of the gravity well would make the core seem very cold, almost absolute zero.

Externally, a gravastar appears similar to a black hole: it is visible only 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.

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.[2] 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,[3][4][5] or to leave the question unanswered.[6][7]

In comparison with black holes[edit]

By taking quantum physics into account, the gravastar hypothesis attempts to resolve contradictions caused by conventional black hole theories.[8]

Event horizons[edit]

In a gravastar, the event horizon is not a well-defined surface. Each wavelength of light has its own 'event horizon', inside which an observer in flat space-time would never measure that wavelength because of the gravitational red shift. The thick layer of Bose–Einstein condensate would lie just outside the 'event horizon', being prevented from complete collapse by the inner void, exerting a balance pressure outwards on the condensate.[1]

Dynamic stability of gravastars[edit]

In 2007, theoretical work since disproven by Hawking[citation needed] indicated that under certain conditions gravastars as well as other alternative black hole models are not stable when they rotate.[9] Theoretical work has also shown that certain rotating gravastars are mathematically 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.[10] Theoretical support for the feasibility of gravastars does not exclude the existence of black holes as shown in other theoretical studies.[11]

See also[edit]

References[edit]

  1. ^ a b "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. ^ "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. 
  3. ^ Chirenti, Cecilia; Rezzolla, Luciano (2016-10-11). "Did GW150914 produce a rotating gravastar?". Physical Review D. 94 (8): 084016. 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. 
  4. ^ "Did LIGO detect black holes or gravastars?". ScienceDaily. October 19, 2016. Retrieved 2017-11-04. 
  5. ^ "LIGO's black hole detection survives the gravastar test - ExtremeTech". ExtremeTech. 2016-10-26. Retrieved 2017-11-04. 
  6. ^ "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 
  7. ^ 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). doi:10.1103/PhysRevLett.116.171101. ISSN 0031-9007. 
  8. ^ Stenger, Richard (22 January 2002). "Is black hole theory full of hot air?". CNN.com. Retrieved 10 April 2014. 
  9. ^ Vitor Cardoso; Paolo Pani; Mariano Cadoni; Marco Cavaglia (2007). "Ergoregion instability of ultra-compact astrophysical objects". Physical Review D. 77 (12). arXiv:0709.0532Freely accessible [gr-qc]. Bibcode:2008PhRvD..77l4044C. doi:10.1103/PhysRevD.77.124044. 
  10. ^ Chirenti, Cecilia; Rezzolla, Luciano (October 2008). "Ergoregion instability in rotating gravastars" (PDF). Physical Review D. 78 (8). arXiv:0808.4080Freely accessible. Bibcode:2008PhRvD..78h4011C. doi:10.1103/PhysRevD.78.084011. Retrieved 10 April 2014. 
  11. ^ Rocha; Miguelote; Chan; da Silva; Santos; Anzhong Wang (2008). "Bounded excursion stable gravastars and black holes". arXiv:0803.4200Freely accessible [gr-qc]. 

Further reading[edit]

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