Growth
Once a black hole has formed, it cancontinue to grow by absorbing additional matter. Any black hole willcontinually absorb gas and interstellar dust from its surroundings.This growth process is one possible way through which somesupermassive black holes may have been formed, although the formationof supermassive black holes is still an open field of research. Asimilar process has been suggested for the formation ofintermediate-mass black holes found in globular clusters. Black holescan also merge with other objects such as stars or even other blackholes. This is thought to have been important, especially in theearly growth of supermassive black holes, which could have formedfrom the aggregation of many smaller objects. The process has alsobeen proposed as the origin of some intermediate-mass black holes.
Evaporation
In 1974, Hawking predicted that blackholes are not entirely black but emit small amounts of thermalradiation at a temperature ℏc3/(8πGMkB); this effect has becomeknown as Hawking radiation. By applying quantum field theory to astatic black hole background, he determined that a black hole shouldemit particles that display a perfect black body spectrum. SinceHawking's publication, many others have verified the result throughvarious approaches. If Hawking's theory of black hole radiation iscorrect, then black holes are expected to shrink and evaporate overtime as they lose mass by the emission of photons and otherparticles. The temperature of this thermal spectrum (Hawkingtemperature) is proportional to the surface gravity of the blackhole, which, for a Schwarzschild black hole, is inverselyproportional to the mass. Hence, large black holes emit lessradiation than small black holes.
A stellar black hole of 1 M☉ has aHawking temperature of 62 nanokelvins. This is far less than the 2.7K temperature of the cosmic microwave background radiation.Stellar-mass or larger black holes receive more mass from the cosmicmicrowave background than they emit through Hawking radiation andthus will grow instead of shrinking. To have a Hawking temperaturelarger than 2.7 K (and be able to evaporate), a black hole would needa mass less than the Moon. Such a black hole would have a diameter ofless than a tenth of a millimeter.
If a black hole is very small, theradiation effects are expected to become very strong. A black holewith the mass of a car would have a diameter of about 10−24 m andtake a nanosecond to evaporate, during which time it would brieflyhave a luminosity of more than 200 times that of the Sun. Lower-massblack holes are expected to evaporate even faster; for example, ablack hole of mass 1 TeV/c2 would take less than 10−88 seconds toevaporate completely. For such a small black hole, quantum gravityeffects are expected to play an important role and couldhypothetically make such a small black hole stable, although currentdevelopments in quantum gravity do not indicate this is the case.
The Hawking radiation for anastrophysical black hole is predicted to be very weak and would thusbe exceedingly difficult to detect from Earth. A possible exception,however, is the burst of gamma rays emitted in the last stage of theevaporation of primordial black holes. Searches for such flashes haveproven unsuccessful and provide stringent limits on the possibilityof existence of low mass primordial black holes. NASA's FermiGamma-ray Space Telescope launched in 2008 will continue the searchfor these flashes.
If black holes evaporate via Hawkingradiation, a solar mass black hole will evaporate (beginning once thetemperature of the cosmic microwave background drops below that ofthe black hole) over a period of 1064 years. A supermassive blackhole with a mass of 1011 M☉ will evaporate in around 2×10100years. Some monster black holes in the universe are predicted tocontinue to grow up to perhaps 1014 M☉ during the collapse ofsuper-clusters of galaxies. Even these would evaporate over atimescale of up to 10106 years.
