Primordial black hole
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.
It has been proposed that dark matter is made up of primordial black holes. One theory proposes that they are in the mass range of 1017 g to 1026 g, based on the expectation that at this low mass they would behave as expected of other particle candidates for dark matter. They would be within the typical mass range of asteroids. Another theory is that dark matter consists of larger black holes, about 30 times the mass of the sun (about 6 × 1034 g).
According to the Big Bang Model, pressure and temperature were extremely high following the Big Bang. Under these conditions, simple fluctuations in the density of matter may have resulted in local regions dense enough to create black holes. Although other regions of high density would be quickly dispersed by the expansion of the universe, a large enough primordial black hole would be stable, persisting to the present.
Observational limits and detection strategies
A variety of observations have been interpreted to place limits on how many primordial black holes might exist at various sizes, including observations of gravitational lensing, the cosmic microwave background, etc. The size range around 1025 kg seems least constrained.
One way to detect primordial black holes, or to constrain their mass and abundance, is by their Hawking radiation. 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 theorized 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 a hydrogen bomb yielding millions of megatons of explosive force. A regular black hole (of about 3 solar masses) cannot lose all of its mass within the current age 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.
Two gravitational wave events detected by LIGO fit within the window of masses 20M⊙≲Mbh≲100M⊙ where primordial black holes (PBHs) may constitute the dark matter, while not being ruled out by other constraints. This raised the question of whether these LIGO events were caused by primordial black holes which constitute the dark matter content of our Universe. However the claim that the LIGO events are caused by primordial black hole dark matter is not widely accepted by the physics community, and is likely in contradiction with other observations.
The evaporation of primordial black holes has been suggested as one possible explanation for gamma-ray bursts. This explanation is, however, considered unlikely.[clarification needed] 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.
General relativity 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.
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While their existence has not been established with certainty, primordial black holes have in the past been suggested as a possible solution to the dark matter mystery. Because there’s so little evidence of them, though, the primordial black hole-dark matter hypothesis has not gained a large following among scientists. The LIGO findings, however, raise the prospect anew, especially as the objects detected in that experiment conform to the mass predicted for dark matter. Predictions made by scientists in the past held that conditions at the birth of the universe would produce lots of these primordial black holes distributed roughly evenly in the universe, clustering in halos around galaxies. All this would make them good candidates for dark matter.
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