Hawking radiation

Hawking radiation is electromagnetic radiation (EMR) emitted from a black hole through a particular process worked out by Steven Hawking. This was surprising because black holes "allow nothing to escape", including EMR. The process is the consequence of quantum theory overcoming what general relativity would otherwise dictate. There are various ways to conceive the mechanism, a common one being that quantum fluctuations just outside the event horizon produce a pair of virtual particles, which happen to be such that one escapes and the other falls through event horizon. With this permanent separation, the particles cannot recombine (annihilation would have been the typical result) and the particle that escaped effectively becomes "no longer virtual". Another way to conceive the mechanism involves quantum tunneling outward through the event horizon. The resulting EMR spectral energy distribution (SED) is worked out to be black-body radiation as if black holes have temperatures proportional to their mass. A third way to conceive it is as a consequence of the Unruh effect, which is a quantum effect consisting of the appearance of a temperature in something accelerating: the gravitational field drawing toward the black hole is equivalent to acceleration in that direction, and from outside the black hole, resulting Unruh radiation coming from it, the black-body radiation due this apparent temperature, which is the temperature specified by black hole thermodynamics.

A consequence of Hawking radiation is that unless mass continues falling into a black hole at a sufficient rate, the black hole grows smaller and given enough time, eventually evaporates into nothing. Its temperature rises and the contraction accelerates over time, so even though an SMBH or even a stellar-mass black hole would last billions of times the age of the universe, a black hole of a billion kg (the mass of a large oil tanker fully loaded) would dissipate in roughly a year and any smaller black hole would be gone in less time.