Gravastar
In astrophysics, a gravastar (a 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>Template:Citation</ref> It has the usual black hole metric outside of the horizon, but de Sitter metric inside. On the horizon there is a thin shell of exotic matter. This solution to the Einstein equations is stable and has no singularities.<ref name="notblackholes"> {{#invoke:citation/CS1|citation |CitationClass=web }} </ref> Further theoretical considerations of gravastars include the notion of a nestar (a second gravastar nested within the first one).<ref name="SA-20240220">Template:Cite news</ref><ref>Template:Cite journal</ref>
StructureEdit
In the original formulation by Mazur and Mottola,<ref>Template:Harvnb</ref> a gravastar is composed of three regions, differentiated by the relationship between pressure Template:Math and energy density Template:MathTemplate:Technical inline. The central region consists of false vacuum or "dark energy", and in this region Template:Nowrap. Surrounding it is a thin shell of perfect fluid where Template:Nowrap. On the exterior is true vacuum, where Template:Nowrap.
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 shiftTemplate:Technical inline. 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.<ref>Template:Cite journal</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.Template:Citation needed Astronomers search 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 photon like particles' pathsTemplate:Technical inline may pass through.<ref>Template:Cite journal</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>Template:Cite news {{#invoke:citation/CS1|citation |CitationClass=web }}</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.Template:Citation needed
Gravastar formation may provide an alternative explanation for sudden and intense gamma-ray bursts throughout space.Template:Citation needed
LIGO's observations of gravitational waves from colliding objects have been found either to not be consistent with the gravastar concept,<ref>Template:Cite journal</ref><ref>Template:Cite news</ref><ref>Template:Cite news</ref> or to be indistinguishable from ordinary black holes.<ref>Template:Cite news</ref><ref>Template:Cite journal</ref>
Comparison with black holesEdit
By taking quantum physics into account, the gravastar hypothesis attempts to resolve contradictions caused by conventional black hole theories.<ref> Template:Cite news </ref>
Event horizonsEdit
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 gravastarsEdit
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>Template:Cite journal</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>Template:Cite journal</ref> Theoretical support for the feasibility of gravastars does not exclude the existence of black holes as shown in other theoretical studies.<ref>Template:Cite journal</ref>
See alsoEdit
- Acoustic metric
- Acoustic Hawking radiation from sonic black holes
- Black star (semiclassical gravity)
- Dark-energy star
ReferencesEdit
Further readingEdit
- Template:Cite book
- Template:Cite news
- Template:Cite journal The original paper by Mazur and Mottola
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