Gravastar
Encyclopedia
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 derived from the words Gravitational Vacuum Star.
. 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-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 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 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 bursts throughout space.
theories.
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.
Astrophysics
Astrophysics is the branch of astronomy that deals with the physics of the universe, including the physical properties of celestial objects, as well as their interactions and behavior...
as an alternative to the black hole
Black hole
A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...
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
Planck time
In physics, the Planck time, , is the unit of time in the system of natural units known as Planck units. It is the time required for light to travel, in a vacuum, a distance of 1 Planck length...
(chronon
Chronon
A chronon is a proposed quantum of time, that is, a discrete and indivisible "unit" of time as part of a theory that proposes that time is not continuous...
), 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
Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...
. The term gravastar is derived from the words Gravitational Vacuum Star.
Structure
The theory of gravastars builds on Einstein's Theory of General Relativity and imposes a "smallest size" to the universe that is known to exist according to well accepted quantum theoryQuantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...
. This size is known as the Planck length, and is derived using the speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
, Planck's constant and the gravitational constant
Gravitational constant
The gravitational constant, denoted G, is an empirical physical constant involved in the calculation of the gravitational attraction between objects with mass. It appears in Newton's law of universal gravitation and in Einstein's theory of general relativity. It is also known as the universal...
. 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
Blue shift
A blueshift is any decrease in wavelength ; the opposite effect is referred to as redshift. In visible light, this shifts the colour from the red end of the spectrum to the blue end...
that the light can undergo. This becomes important for the structure of a Gravastar because general relativity
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...
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
Supercooling
Supercooling, also known as undercooling, is the process of lowering the temperature of a liquid or a gas below its freezing point without it becoming a solid....
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
Hawking radiation
Hawking radiation is a thermal radiation with a black body spectrum predicted to be emitted by black holes due to quantum effects. It is named after the physicist Stephen Hawking, who provided a theoretical argument for its existence in 1974, and sometimes also after the physicist Jacob Bekenstein...
it creates. Astronomers observe the sky for X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
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
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...
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 the big bang
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...
. 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
Dark energy
In physical cosmology, astronomy and celestial mechanics, dark energy is a hypothetical form of energy that permeates all of space and tends to accelerate the expansion of the universe. Dark energy is the most accepted theory to explain recent observations that the universe appears to be expanding...
accelerates the expansion 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 bursts throughout space.
In comparison with black holes
By taking quantum physics into account, the gravastar hypothesis attempts to resolve contradictions caused by conventional black holeBlack hole
A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...
theories.
Event horizons
In a gravastar, the event horizonEvent horizon
In general relativity, an event horizon is a boundary in spacetime beyond which events cannot affect an outside observer. In layman's terms it is defined as "the point of no return" i.e. the point at which the gravitational pull becomes so great as to make escape impossible. The most common case...
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
Recent theoretical work 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.External links
- Papers about gravastars on gr-qc The original paper by Mazur and Mottola