- Gravastar
A gravastar is an object hypothesized in
astrophysics as an alternative to theblack hole theory by Pawel 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 incorporatingquantum mechanics . The term "gravastar" is derived from the words "Gra"vitational "Va"cuum "Star".cite web
url=http://www.lanl.gov/news/releases/archive/02-035.shtml
title=Los Alamos researcher says 'black holes' aren't holes at all
publisher=www.lanl.gov
accessdate=2008-02-28
last=
first=]Structure
The theory of gravastars builds off of Einstein's
Theory of General Relativity and imposes a "smallest size" to the universe that is known to exist according to well acceptedquantum theory . This size is known as thePlanck length , and is derived using thespeed of light ,Planck's constant and thegravitational constant . Quantum Theory says that any scale smaller than thePlanck length is unobservable and meaningless. This limit can be imposed on the wavelength of a beam of light so as to obtain a limit ofblue shift that the light can undergo. This becomes important for the structure of a Gravastar becausegeneral 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, thePlanck 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 . [cite web
url=http://www.space.com/scienceastronomy/astronomy/gravastars_020423.html
title=SPACE.com -- Thick-Skinned Gravastars Vie to Replace Black Holes, in Theory
publisher=www.space.com
accessdate=2008-02-27
last=
first=] This can be created in a lab by super cooling 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 forX-ray s 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, as all of 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 thebig bang . 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 theexpansion of the universe . One possible theory 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 burst s throughout space.In comparison with black holes
By taking quantum physics into account, the gravastar hypothesis attempts to resolve contradictions caused by conventional
black hole theories. [cite web
url=http://archives.cnn.com/2002/TECH/space/01/21/black.holes/index.html?related
title=CNN.com - Is black hole theory full of hot air? - January 21, 2002
publisher=cnn.com
accessdate=2008-02-28
last=
first=]Event horizons
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 thegravitational 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.ee also
*
Dark energy star
*Acoustic metric
*Acoustic Hawking radiation
*Black hole
*Dark star
*Magnetospheric eternally collapsing object References
* [http://arxiv.org/abs/gr-qc/0109035 The original paper by Mazur and Mottola] (submitted to Physical Review Letters in 2001, but not yet published)
* cite news
url=http://www.nature.com/news/2005/050328/full/050328-8.html
author=George Chapline
publisher=Nature News
title=Black holes 'do not exist'
date=2005-03-28 ("subscription only")External links
* cite web
url=http://www.fc.up.pt/pessoas/luis.beca/phisky/PhiSky%20Wiltshire.pdf
title=Stable gravastars — an alternative to black holes?
author = Visser, Matt; Wiltshire, David L.
accessdate=2004-10-02
format=PDF
* [http://arxiv.org/find/grp_physics/1/abs:+gravastar/0/1/0/all/0/1 Papers about gravastars on gr-qc]
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