Gravastar
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In astrophysics, the Gravastar theory is a proposal by Pawel Mazur and Emil Mottola to replace the black hole. Instead of a star collapsing into a pinpoint of space with 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 origin of the word "gravastar" is simply: GRAvitational VAcuum STAR. In some references, the word "Condensate" is inserted after vacuum, resulting in "gravacstar".
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[edit] The basic idea
Inside a gravastar, spacetime would be "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".
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.
Mazur and Mottola have suggested that gravastars could explain very important problems in astrophysics. First, if a black hole cannot form in the real Universe, then there is no difficulty with the black hole information paradox, which is the problem that information seems to disappear. It seems that a gravastar should not suffer from the same problem.
Second, Mazur and Mottola suggest that the violent creation of a gravastar might be an alternate explanation for gamma ray bursts, adding yet one more possibility to the dozens if not hundreds of ideas that have been proposed as the cause of GRBs.
Mottola has even suggested that Universe itself could very well be the inside of a giant gravastar, as a possible explanation for the observed accelerating expansion of the Universe.
[edit] Problems with the theory
The theory is a relative newcomer to the field of astrophysics. As is typical for such ideas, it has not been fully understood, so there is no strong consensus either for acceptance or rejection. Still, few mainstream astrophysicists have paid much attention to the notion. Rather, most mainstream astrophysicists believe that there are far less radical and speculative ways of resolving problems with black holes.
The gravastar concept relies on highly speculative ideas from the poorly understood theory of quantum gravity, yet provides only limited benefits over the theory of black holes. Furthermore, there is no theoretical reason from quantum gravity that space should behave in the way that Mazur and Mottola assume. This means that there is no good model for the formation of a gravastar. A similar problem with the formation of black holes prevented mainstream scientists from accepting them until that problem was solved in the late 1960's.
A major criticism of this concept is that it simply makes no claim of substance. It suggests that something beyond the scope of present physics happens. While it is always important to push the boundaries of understanding in science, this theory is merely the suggestion that a new phenomenon occurs.
[edit] See also
- Dark energy star
- acoustic metric
- acoustic Hawking radiation
- analog model of gravity
- black hole
- dark star
- Magnetospheric eternally collapsing object
[edit] References
- The original paper by Mazur and Mottola (submitted to Physical Review Letters in 2001, but not yet published)
- George Chapline. "Black holes 'do not exist'", Nature News, 2005-03-28. (subscription only)
[edit] External links
- Visser, Matt; Wiltshire, David L.. Stable gravastars — an alternative to black holes? (PDF). Retrieved on 2004-10-02.
- Papers about gravastars on gr-qc
