Primordial black hole
Encyclopedia
A primordial black hole is a hypothetical type of black hole
that is formed not by the gravitational collapse
of a large star but by the extreme density of matter present during the universe's early expansion.
According to the Big Bang Model
, during the first few moments after the Big Bang, pressure and temperature were extremely great. Under these conditions, simple fluctuations in the density of matter may have resulted in local regions dense enough to create black holes. Although most regions of high density would be quickly dispersed by the expansion of the universe
, a primordial black hole would be stable, persisting to the present.
It has been proposed that primordial black holes could be a candidate for dark matter
. Specifically those forming in the mass range of 1014 kg to 1023 kg. This is due to the possibility that at this low mass they would behave as expected of other particle candidates for dark matter. Being within the typical mass range of asteroids, this excludes those black holes too small to persist until our era and those too large to explain gravitational lensing observations.
. Stephen Hawking
theorized in 1974 that large numbers of such smaller primordial black holes might exist in the Milky Way
in our galaxy
's Halo
region. All black holes are believed by many theorists to emit Hawking radiation at a rate inversely proportional to their mass. Since this emission further decreases their mass, black holes with very small mass would experience runaway evaporation, creating a massive burst of radiation at the final phase, equivalent to millions of megatons hydrogen bomb exploding. A regular black hole (of about 3 solar mass
es) cannot lose all of its mass within the lifetime of the universe (they would take about 1069 years to do so, even without any matter falling in). However, since primordial black holes are not formed by stellar core collapse, they may be of any size. A black hole with a mass of about 1011 kg would have a lifetime about equal to the age of the universe. If such low-mass black holes were created in sufficient number in the Big Bang, we should be able to observe some of those that are relatively nearby in our own Milky Way
galaxy
exploding today. NASA
's Fermi Gamma-ray Space Telescope satellite, launched in June 2008, is designed in part to search for such evaporating primordial black holes. However, if theoretical Hawking radiation does not actually exist, such primordial black holes would be extremely difficult, if not impossible, to detect in space due to their small size and lack of large gravitational influence. It has been suggested that a small black hole passing through the Earth would produce a detectable acoustic signal. Because of its tiny diameter, large mass compared to a nucleon
, and relatively high speed, such primordial black holes would simply transit earth virtually unimpeded with only a few impacts on nucleons, exiting the planet with no ill effects.
Another way to detect primordial black holes could be by watching for ripples on the surfaces of stars. If the black hole passed through a star, its density would cause observable vibrations.
s. This explanation is, however, considered unlikely. Other problems for which primordial black holes have been suggested as a solution include the dark matter
problem, the cosmological domain wall
problem and the cosmological monopole
problem. Since a primordial black hole does not necessarily have to be small (they can have any size), primordial black holes may also have contributed to the later formation of galaxies
.
Even if they do not solve these problems, the low number of primordial black holes (as of 2010, only two intermediate mass black holes were confirmed) aids cosmologists
by putting constraints on the spectrum of density fluctuations in the early universe.
predicts the smallest primordial black holes would have evaporated by now, but if there were a fourth spatial dimension — as predicted by string theory
— it would affect how gravity acts on small scales and "slow down the evaporation quite substantially." This could mean there are several thousand black holes in our galaxy. To test this theory scientists will use the Fermi Gamma-ray Space Telescope which was put in orbit by NASA
on June 11, 2008. If they observe specific small interference patterns within gamma-ray bursts, it could be the first indirect evidence for primordial black holes and string theory.
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...
that is formed not by the gravitational collapse
Gravitational collapse
Gravitational collapse is the inward fall of a body due to the influence of its own gravity. In any stable body, this gravitational force is counterbalanced by the internal pressure of the body, in the opposite direction to the force of gravity...
of a large star but by the extreme density of matter present during the universe's early expansion.
According to the Big Bang Model
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...
, during the first few moments after the Big Bang, pressure and temperature were extremely great. Under these conditions, simple fluctuations in the density of matter may have resulted in local regions dense enough to create black holes. Although most regions of high density would be quickly dispersed by the expansion of the universe
Metric expansion of space
The metric expansion of space is the increase of distance between distant parts of the universe with time. It is an intrinsic expansion—that is, it is defined by the relative separation of parts of the universe and not by motion "outward" into preexisting space...
, a primordial black hole would be stable, persisting to the present.
It has been proposed that primordial black holes could be a candidate for dark matter
Dark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...
. Specifically those forming in the mass range of 1014 kg to 1023 kg. This is due to the possibility that at this low mass they would behave as expected of other particle candidates for dark matter. Being within the typical mass range of asteroids, this excludes those black holes too small to persist until our era and those too large to explain gravitational lensing observations.
Possible detection
One way to detect primordial black holes is by their Hawking radiationHawking 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...
. Stephen Hawking
Stephen Hawking
Stephen William Hawking, CH, CBE, FRS, FRSA is an English theoretical physicist and cosmologist, whose scientific books and public appearances have made him an academic celebrity...
theorized in 1974 that large numbers of such smaller primordial black holes might exist in the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
in our galaxy
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
's Halo
Galactic halo
The term galactic halo is used to denote an extended, roughly spherical component of a galaxy, which extends beyond the main, visible component. It can refer to any of several distinct components which share these properties:* the galactic spheroid...
region. All black holes are believed by many theorists to emit Hawking radiation at a rate inversely proportional to their mass. Since this emission further decreases their mass, black holes with very small mass would experience runaway evaporation, creating a massive burst of radiation at the final phase, equivalent to millions of megatons hydrogen bomb exploding. A regular black hole (of about 3 solar mass
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
es) cannot lose all of its mass within the lifetime of the universe (they would take about 1069 years to do so, even without any matter falling in). However, since primordial black holes are not formed by stellar core collapse, they may be of any size. A black hole with a mass of about 1011 kg would have a lifetime about equal to the age of the universe. If such low-mass black holes were created in sufficient number in the Big Bang, we should be able to observe some of those that are relatively nearby in our own Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
galaxy
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
exploding today. NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
's Fermi Gamma-ray Space Telescope satellite, launched in June 2008, is designed in part to search for such evaporating primordial black holes. However, if theoretical Hawking radiation does not actually exist, such primordial black holes would be extremely difficult, if not impossible, to detect in space due to their small size and lack of large gravitational influence. It has been suggested that a small black hole passing through the Earth would produce a detectable acoustic signal. Because of its tiny diameter, large mass compared to a nucleon
Nucleon
In physics, a nucleon is a collective name for two particles: the neutron and the proton. These are the two constituents of the atomic nucleus. Until the 1960s, the nucleons were thought to be elementary particles...
, and relatively high speed, such primordial black holes would simply transit earth virtually unimpeded with only a few impacts on nucleons, exiting the planet with no ill effects.
Another way to detect primordial black holes could be by watching for ripples on the surfaces of stars. If the black hole passed through a star, its density would cause observable vibrations.
Implications
The evaporation of primordial black holes has been suggested as one possible explanation for gamma ray burstGamma ray burst
Gamma-ray bursts are flashes of gamma rays associated with extremely energetic explosions that have been observed in distant galaxies. They are the most luminous electromagnetic events known to occur in the universe. Bursts can last from ten milliseconds to several minutes, although a typical...
s. This explanation is, however, considered unlikely. Other problems for which primordial black holes have been suggested as a solution include the dark matter
Dark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...
problem, the cosmological domain wall
Domain wall
A domain wall is a term used in physics which can have one of two distinct but similar meanings in magnetism, optics, or string theory. These phenomena can all be generically described as topological solitons which occur whenever a discrete symmetry is spontaneously broken.-Magnetism:In magnetism,...
problem and the cosmological monopole
Magnetic monopole
A magnetic monopole is a hypothetical particle in particle physics that is a magnet with only one magnetic pole . In more technical terms, a magnetic monopole would have a net "magnetic charge". Modern interest in the concept stems from particle theories, notably the grand unified and superstring...
problem. Since a primordial black hole does not necessarily have to be small (they can have any size), primordial black holes may also have contributed to the later formation of galaxies
Galaxy formation and evolution
The study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universe from a homogeneous beginning, the formation of the first galaxies, the way galaxies change over time, and the processes that have generated the variety of structures observed in nearby...
.
Even if they do not solve these problems, the low number of primordial black holes (as of 2010, only two intermediate mass black holes were confirmed) aids cosmologists
Physical cosmology
Physical cosmology, as a branch of astronomy, is the study of the largest-scale structures and dynamics of the universe and is concerned with fundamental questions about its formation and evolution. For most of human history, it was a branch of metaphysics and religion...
by putting constraints on the spectrum of density fluctuations in the early universe.
String theory
General relativityGeneral 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...
predicts the smallest primordial black holes would have evaporated by now, but if there were a fourth spatial dimension — as predicted by string theory
String theory
String theory is an active research framework in particle physics that attempts to reconcile quantum mechanics and general relativity. It is a contender for a theory of everything , a manner of describing the known fundamental forces and matter in a mathematically complete system...
— it would affect how gravity acts on small scales and "slow down the evaporation quite substantially." This could mean there are several thousand black holes in our galaxy. To test this theory scientists will use the Fermi Gamma-ray Space Telescope which was put in orbit by NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
on June 11, 2008. If they observe specific small interference patterns within gamma-ray bursts, it could be the first indirect evidence for primordial black holes and string theory.