Formation and evolution of black holes

From the exotic nature of black holes, it is natural to question if such bizarre objects could actually exist in nature or that they are merely pathological solutions to Einstein's equations. However in 1970, Hawking and Penrose proved the opposite; under generic conditions black holes are expected to form in any universe.Citation|last1=Hawking |first1=S.W. |last2=Penrose |first2=R. |title=The Singularities of Gravitational Collapse and Cosmology |journal=Proc.Roy.Soc.Lon |year=1970 |volume=314 |number=1519 |pages=529–548 |url=http://www.jstor.org/stable/2416467] The primary formation process for black holes is expected to be the gravitational collapse of heavy objects such as stars, but there are also more exotic processes that can lead to the production of black holes.

Gravitation collapse

Gravitational collapse occurs when an object's internal pressure is insufficient to resist the object's own gravity. For stars this usually occurs either because a star has too little "fuel" left to maintain its temperature, or because a star which would have been stable receives a lot of extra matter in a way which does not raise its core temperature. In either case the star's temperature is no longer high enough to prevent it from collapsing under its own weight (the ideal gas law explains the connection between pressure, temperature, and volume).

The collapse may be stopped by the degeneracy pressure of the star's constituents, condensing the matter in an exotic denser state. The result is one of the various types of compact star. Which type of compact star is formed depends on the mass of the remnant - the matter left over after changes triggered by the collapse (such as supernova or pulsations leading to a planetary nebula) have blown away the outer layers. Note that this can be substantially less than the original star - remnants exceeding 5 solar masses are produced by stars which were over 20 solar masses before the collapse.

If the mass of the remnant of exceeds ~3-4 solar masses (the Tolman-Oppenheimer-Volkoff limit)—either because the original star was very heavy or because the remnant collected additional mass through accretion of matter)—even the degeneracy pressure of neutrons is insufficient to stop the collapse. After this no known mechanism (except maybe the quark degeneracy pressure, see quark star) is powerful enough to stop the collapse and the object will inevitably collapse to a black hole.

This gravitational collapse of heavy stars is assumed to be responsible for the formation of most (if not all) stellar mass black holes.

Creation of primordial black holes in the big bang

Gravitational collapse requires great densities. In the current epoch of the universe these high densities are only found in stars, but in the early universe shortly after the big bang densities were much greater, possibly allowing for the creation of black holes. The high density alone is not enough to allow the formation of black holes since a uniform mass distribution will not allow the mass to bunch up. In order for primordial black holes to form in such a dense medium, there must be initial density perturbations which can then grow under their own gravity. Different models for the early universe vary widely in their predictions of the size of these perturbations. Various models predict the creation of black holes, ranging from a Planck mass to hundreds of thousands of solar masses. [citation|last=Carr |first=B.J. |contribution=Primordial Black Holes: Do They Exist and Are They Useful? |title=Proceedings of "Inflating Horizon of Particle Astrophysics and Cosmology" |publisher=Universal Academy Press Inc and Yamada Science Foundation |year=2005 |url=http://arxiv.org/abs/astro-ph/0511743] Primordial black holes could thus account for the creation of any type of black hole.

Production in high energy collisions

Gravitational collapse is not the only process that could create black holes. In principle, black holes could also be created in high energy collisions that create sufficient density. Since classically black holes can take any mass, one would expect micro black holes to be created in any such process no matter how low the energy. However, to date, no such events have ever been detected either directly or indirectly as a deficiency of the mass balance in particle accelerator experiments.Fact|date=June 2008 This suggests that there must be a lower limit for the mass of black holes.

Theoretically this bound is expect to lie around the Planck mass (~10¹⁹ GeV/c²), where quantum effects are expected to make the theory of general relativity break down completely.Fact|date=June 2008 This would put the creation of black holes firmly out of reach of any high energy process occurring on or near the Earth. Certain developments in quantum gravity however suggest that this bound could be much lower. Some braneworld scenarios for example put the Planck mass much lower, may be even as low as 1 TeV.Fact|date=June 2008 This would make it possible for micro black holes to be created in the high energy collisions occurring when cosmic rays hit the earth's atmosphere, or even maybe in the new Large Hadron Collider at CERN. These theories are however very speculative, and the creation of black holes in these processes is deemed unlikely by many specialists.

Growth

Once a black hole has formed, it can continue to grow by absorbing additional matter. Any black hole will continually absorb interstellar dust from its direct surroundings and omnipresent cosmic background radiation, but neither of these processes should significantly affect the mass of a stellar black hole. More significant contributions can occur when the black hole formed in a binary star system. After formation the black hole can then leech significant amounts of matter from its companion.

Much larger contributions can be obtained when a black hole merges with other stars or compact objects. The supermassive black holes suspected in the center of most galaxies are expected to have formed from the coagulation of many smaller objects. The process has also been proposed as the origin of some intermediate-mass black holes.

Evaporation

If Hawking's theory of black hole radiation is correct then black holes are expected to emit a thermal spectrum of radiation, and thereby lose mass, because according to the theory of relativity mass is just highly condensed energy (e = mc²). Black holes will thus shrink and evaporate over time. The temperature of this spectrum (Hawking temperature) is proportional to the surface gravity of the black hole, which in turn is inversely proportional to the mass. Large black holes thus emit less radiation than small black holes.

A stellar black hole of 5 solar masses has a Hawking temperature of about 12 nanoKelvin. This is far less than the 2.7 K produced by the Cosmic microwave background. Stellar mass (and larger) black holes thus receive more mass from the CMB than they emit through Hawking radiation and will thus grow instead of shrink. In order to have a Hawking temperature larger than 2.7 K (and thus be able to evaporate) a black hole needs to be lighter than the Moon (and thus have diameter of less than a tenth of a millimeter).

On the other hand if a black hole is very small the radiation effects are expected to become very strong. Even a black hole that is heavy compared to a human would evaporate in an instant. A black hole the weight of a car (~ 10^-24 m) would only take a nanosecond to evaporate, during which time it would briefly have a luminosity more than 200 times that of the sun. Lighter black holes are expected to evaporate even faster, for example a black hole of mass 1 TeV/c² would take less than 10^-88 seconds to evaporate completely. Of course, for such a small black hole quantum gravitation effects are expected to play an important role and could even —although current developments in quantum gravity do not indicate so— hypothetically make such a small black hole stable.