Black holes are not quite black. Given time, they should evaporate.
Nothing escapes a black hole, or so physics said until 1974. Then Stephen Hawking combined quantum theory with gravity and found that black holes should glow faintly and slowly shrink. For any real black hole, the process is so slow and dim that no one has ever observed it.
Classically, a black hole is a region where gravity is so strong that nothing, not even light, can escape past its boundary, the event horizon. Early theorists treated black holes as simple objects with no entropy, no internal disorder. In 1972 Jacob Bekenstein argued they must have an entropy proportional to the area of their horizon. Hawking initially disagreed, but after meeting the Soviet physicist Yakov Zeldovich in Moscow in 1973, who had argued that spinning black holes should emit particles, he set out to calculate what quantum theory really predicted.
His 1974 result was startling: every black hole should emit radiation as if it were a warm object, with a temperature that depends on its mass. The effect arises from quantum fluctuations of empty space near the horizon, which can produce pairs of particles, one of which escapes while its partner falls in. The escaping radiation carries away energy, so the black hole loses mass. Because this also implies black holes have a temperature, Bekenstein's entropy was vindicated, and the effect is sometimes called Bekenstein–Hawking radiation.
The numbers are extreme. The bigger the black hole, the colder it is. One with the mass of Earth would have a temperature of about a trillionth of a kelvin. Stellar black holes are colder than the 2.7 kelvin cosmic microwave background that fills space, so none of them can currently evaporate at all. A black hole with the mass of the Sun would take more than 10^67 years to evaporate, unimaginably longer than the universe's current age of about 14 billion years. Only tiny black holes, if they exist, would evaporate quickly, ending in a final burst of radiation that has never been seen.
Hawking radiation remains unobserved in space. Physicists have instead built laboratory analogues, such as 'sonic black holes' in which sound cannot escape a flowing fluid, and reported similar radiation there. The theory also raised the black hole information paradox: if a black hole evaporates into random radiation, what happens to the information that fell in? That question is still debated.
Source: Wikipedia — Hawking radiation · Text summarised from Wikipedia (CC BY-SA 4.0)
