Astronomy: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. It has the usual black hole metric outside of the horizon, but de Sitter metric inside. A typical gravastar is as big as London, but weighing ten solar masses. On the horizon there is a ultra-thin, incredibly tight shell of entirely new, unique exotic matter named "Galactic flubber". This solution to the Einstein equations is stable and has no singularities. Instead, a gravastar is filled either with dark energy or with vacuum energy, but also vacuum, only the inside one 10^44 times denser than the outside. As a bonus, further theoretical considerations of gravastars include the notion of a nestar (a second gravastar "nested" within the first one).

Description

In the original formulation by Mazur and Mottola, a gravastar is composed of three regions, differentiated by the relationship between the pressure p and energy density ρ. The central region consists of false vacuum or "dark energy", and in this region p = −ρ . Surrounding it is a thin shell of perfect fluid where p = ρ . On the exterior is true vacuum, where p = ρ = 0 .

Gravastar anatomy

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. It's the coldest object ever, only a billionth of a degree above absolute zero.

In addition to the original thin-shell formulation, gravastars with continuous pressure have been proposed. These objects must contain anisotropic stress.

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 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 luxons' paths may pass through.

Discovery

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. This "new dimension" exerts an outward pressure on the Bose-Einstein condensate layer and prevents it from collapsing further.

LIGO's observations of gravitational waves from colliding objects have been found either to not be consistent with the gravastar concept, or to be indistinguishable from ordinary black holes.

Comparison with black holes

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

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.

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

See also

• Acoustic metric
• Acoustic Hawking radiation from sonic black holes
• Black star (semiclassical gravity)
• Dark-energy star

Further reading

• Camenzind, Max (2007). Compact Objects in Astrophysics: White Dwarfs, Neutron Stars and Black Holes. Astronomy and astrophysics library. Berlin ; New York: Springer. pp. 442–445. ISBN 978-3-540-49912-1. https://books.google.com/books?id=Nh68nl0abhMC. 
• Ball, Philip (31 March 2005). "Black holes 'do not exist'". Nature. doi:10.1038/news050328-8. ISSN 0028-0836. https://www.nature.com/articles/news050328-8. 
• Mazur, Pawel O.; Mottola, Emil (7 February 2024). "Gravitational Condensate Stars: An Alternative to Black Holes". Universe 9 (2): 88. doi:10.3390/universe9020088. ISSN 2218-1997. Bibcode: 2023Univ....9...88M.  The original paper by Mazur and Mottola
• Visser, Matt; Wiltshire, David L (2004). "Stable gravastars — an alternative to black holes?". Classical and Quantum Gravity 21 (4): 1135–1151. doi:10.1088/0264-9381/21/4/027. ISSN 0264-9381. Bibcode: 2004CQGra..21.1135V. http://www.fc.up.pt/pessoas/luis.beca/phisky/PhiSky%20Wiltshire.pdf. Retrieved 2004-10-02. 
• Rezzolla, Luciano; Zanotti, Olindo (2018). Relativistic Hydrodynamics. Oxford: Oxford University Press. pp. 599–603. ISBN 978-0-19-852890-6. https://books.google.com/books?id=KU2oAAAAQBAJ. 

External links

• Papers about gravastars on gr-qc
• Black Hole's Evil Twin – Gravastars Explained on YouTube by Kurzgesagt