A gravastar is an object hypothesized in astrophysics as an alternative to the black 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 incorporating quantum mechanics. The term gravastar is a portmanteau of the words Gravitational Vacuum Star.
The notion of gravastars builds on Einstein's theory of general relativity and imposes a universal "smallest size" that is known to exist according to well-accepted quantum theory. This size is known as the Planck length, and is derived using the speed of light, Planck's constant and the gravitational constant. Quantum theory says that any scale smaller than the Planck length is unobservable and meaningless to physics and physicists. This limit can be imposed on the wavelength of a beam of light so as to obtain a limit of blue shift that the light can undergo. This becomes important for the structure of a gravastar because general 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, the Planck 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. This can be created in a laboratory by supercooling 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 for X-rays 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, 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.
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.
Gravastar formation may provide an alternate explanation for sudden and intense gamma-ray bursts throughout space.
In comparison with black holes
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 the gravitational 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.
Dynamic stability of gravastars
In 2007 theoretical work since disproven by Hawking has shown 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 mathematically 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.
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