Attempts to understand gravity at a microscopic level suggest that black holes are not entirely black. Applying subatomic physics to black holes, it was shown that it is possible for some matter and radiation to escape from a black hole. Such radiation from a black hole is known as Hawking radiation, named after astrophysicist Stephen Hawking. 

Black Hole Evaporation

    The laws of quantum physics allow a process known as pair creation to occur any where in space. A particle and its antiparticle (i.e. an electron and positron) can come into being spontaneously, provided the particles disappear by mutual annihilation within a short space of time. In effect, the laws of conservation of mass and energy can be broken as long as the imbalance is corrected before anyone notices it. 

    In most cases, pairs of particles appear and disappear so rapidly that energy is conserved on all macroscopic scales. However, should pair creation happen occur near a black hole, it is possible for one of the two particles to cross the event horizon before it meets and annihilates its partner. The other particle would then be free to leave, making the black hole appear as a source of matter or radiation to world outside. Ultimately, the energy needed to create the new particle comes from the black hole. As mass and energy are equivalent (related to each other by Einstein's equation E =MC2), the hole must decrease in mass as it radiates. Thus, black holes do not last forever but slowly evaporate. 

Temperature and Hawking radiation

    One result of this is that the spectrum of Hawking radiation is described by a Planck curve. The same curve that also characterizes emissions from any hot body. This implies that black holes emit black-body radiation. The temperature of the radiation turns out to be inversely related to the mass of the hole. Big black holes are very cold while small black holes are very hot. A hole the mass of the Sun would emit radiation at a temperature of 10-6K; the mass of a mountain, about 1012 kg, would have a temperature of 1012K. If  a black hole's temperature T and surface area A is known, its luminosity L can be calculated in the same way as for stars. L is proportional to AT4. 

    A black hole radiates energy (and hence mass) into space. As it radiates, its mass decreases and its temperature increases. In other words, the black hole's temperature is inversely proportional to its mass and the area decreases as a square of its mass. Hence, the black hole increases its luminosity as it evaporates. In turn, the increased luminosity leads to a faster mass loss. This runaway situation eventually ends violently in an explosion of gamma rays. 

    The lifetime of a hole depends on its original mass. For a 1 solar mass black hole, the explosion is predicted to occur after 1070 years. In contrast, a very small black hole with a mass of about 1012 kg should have roughly the lifetime of the current age of the universe. Although how such objects could be created is not known, it is plausible that conditions in the very earliest epochs of the universe might have been sufficient to compress bits of matter into miniature black holes. Such black holes would have a Schwarzschild radii of about 10-15m, comparable to the size of a subatomic particle. Should black holes of such small sizes exist, they would be exploding right now. Attempts to observe the resulting gamma rays have been unsuccessful so far.