Does a black hole emit radiation?
According to the general theory of relativity, a black hole is a region of space from which nothing, not even light, can escape. It is the result of the deformation of space-time caused by a very compact mass. Around a black hole there is an undetectable surface which marks the point of no return, called an event horizon. It is called "black" because it absorbs all the light that hits it, reflecting nothing, just like a perfect black body in thermodynamics. Under the theory of quantum mechanics, black holes possess a temperature and emit Hawking radiation.
Hawking radiation (sometimes also called Bekenstein-Hawking radiation) is a theoretical prediction from British physicist Stephen Hawking, which explains thermal properties relating to black hole.Normally, a black hole is considered to draw all matter and energy in the surrounding region into it, as a result of the intense gravitational fields. However, in 1972 the Israeli physicist Jacob Bekenstein suggested that black holes should have a well-defined entropy, and initiated the development of black hole thermodynamics, including the emission of energy.
In 1974, British physicist Stephen Hawking worked out the exact theoretical model for how a black hole could emit black body radiation.In a simplified version of the explanation, Hawking predicted that energy fluctuations from the vacuum causes the generation of particle-antiparticle pairs near the event horizon of the black hole. One of the particles falls into the black hole while the other escapes, before they have an opportunity to annihilate each other. The net result is that, to someone viewing the black hole, it would appear that a particle had been emitted.
Since the particle that is emitted has positive energy, the particle that gets absorbed by the black hole has a negative energy relative to the outside universe. This results in the black hole losing energy, and thus mass (because E = mc2).
Smaller primordial black holes can actually emit more energy than they absorb, which results in them losing net mass. Larger black holes, such as those that are one solar mass, absorb more cosmic radiation than they emit through Hawking radiation.
Hawking radiation was one of the first theoretical predictions which provided insight into how gravity can relate to other forms of energy, which is a necessary part of any theory of quantum gravity.
Though Hawking radiation is generally accepted by the scientific community, there is still some controversy associated with it. There are some concerns that it ultimately results in information being lost, which makes physicists uncomfortable. Alternately, those who don't actually believe that black holes themselves exist are similarly reluctant to accept that they absorb particles.
The prediction that black holes radiate due to quantum effects is often considered one of the most secure in quantum field theory in curved space-time. Yet this prediction rests on two dubious assumptions: that ordinary physics may be applied to vacuum fluctuations at energy scales increasing exponentially without bound; and that quantum-gravitational effects may be neglected. Various suggestions have been put forward to address these issues: that they might be explained away by lessons from sonic black hole models; that the prediction is indeed successfully reproduced by quantum gravity; that the success of the link provided by the prediction between black holes and thermodynamics justifies the prediction.
Thus, a definitive theoretical treatment will require an understanding of quantum gravity in at least some regimes. Until then, no compelling theoretical case for or against radiation by black holes is likely to be made.
The possibility that non-radiating `mini' black holes exist should be taken seriously; such holes could be part of the dark matter in the Universe. Attempts to place observational limits on the number of `mini' black holes (independent of the assumption that they radiate) would be most welcome.
2.About.com.physics
3.Does black holes radiate? Adam D Helfer 2003 Rep. Prog. Phys. 66 943
