Gravastar
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In astrophysics, the Gravastar theory is a proposal by Emil Mottola and Pawel Mazur to replace the black hole. Instead of a star collapsing into a pinpoint of space with virtually infinite density, the gravastar theory proposes that as an object gravitationally collapses, space itself undergoes a phase transition preventing further collapse, being transformed into a spherical void surrounded by a form of super-dense matter.
The theory is a newcomer to the field which has generated little interest among astrophysicists. It was the topic of a conference proceeding, but has not been the topic of any scientific paper. The lack of interest comes from the fact that the concept requires one to accept a very speculative theory of quantum gravity yet provides no real benefit over black holes. Furthermore, there is no theoretical reason from quantum gravity that space should behave in the way that Mottola and Mazur assume.
The gravastar's name origin is simply: GRAvitational VAcuum STAR.
Mottola and Mazur have suggested that gravastars would be the solution for the black hole information paradox: the tremendous amounts of entropy that a black hole is said to have (a black hole apparently has a billion, billion times more entropy than the star it formed from) cannot yet be explained; there does not seem to be anywhere inside a black hole where such entropy would exist. The Gravastar is theorized to have very low amounts of entropy, thereby eliminating the need to answer the question.
The violent creation of a gravastar might be an alternate explanation for gamma ray bursts, adding yet one more speculative possibility to the dozens if not hundreds of ideas that have been proposed as the cause of GRB's.
However, the consensus among astrophysicists is that there are much less radical and speculative ways of resolving both issues.
A problem with the theory over the creation of a gravastar is whether or not a star would be capable of shedding enough entropy upon implosion.
Externally, a gravastar appears similar to a black hole: it is visible only by the high-energy emissions it creates while consuming matter. Astronomers observe the sky for X-rays emitted by infalling matter to detect black holes, and a gravastar would produce an identical signature.
Inside a gravastar, space-time would be "totally warped" by the extreme conditions there and the inner space would exert an outward force, like dark energy. Around this void would be a "bubble" of incredibly dense and durable matter. The phase of this matter is theorized to be similar to an extreme form of Bose-Einstein condensate in which all matter (protons, neutrons, electrons, etc.) goes into what is called a quantum state creating a "super-atom".
According to the theorizers Emil Mottola and Pawel Mazur the universe itself could very well be the inside of a giant gravastar.
Another alternative to the gravastar is the Dark-energy star. In April 2005, George Chapline, a physicist at the Lawrence Livermore National Laboratory in California, claimed that "It's a near certainty that black holes don't exist." Chapline argued that a massive star doesn't simply collapse to form a black hole; instead, its space-time fills with dark energy. The extremely powerful surface gravity of the proposed dark energy star would be like that of a black hole's, however, the dark energy may cause matter to eventually "bounce back."
External links
- Stable gravastars – an alternative to black holes? (http://www.fc.up.pt/pessoas/luis.beca/phisky/PhiSky%20Wiltshire.pdf)
- George Chapline article in Nature News (http://www.nature.com/news/2005/050328/full/050328-8.html)de:Gravastern