Infographic describing the best current theory on the evolution of PBHs. Credit: NASA Goddard Spaceflight Center Black holes live forever, at least according to general relativity. Once material crosses a black hole's event horizon, it is trapped forever, until the last day of cosmic time. But we know that isn't true. General relativity is a classical model. It doesn't take into account the fuzzy, indeterminate nature of the quantum. We don't have a complete and consistent theory of quantum gravity, but we do have some understanding of quantum black holes. One major feature of quantum black holes is that matter and energy can escape them through the process of Hawking radiation. Very roughly, quantum particles can tunnel through the event horizon now and then, which means quantum black holes slowly lose mass. The smaller the mass of a black hole, the greater the rate of Hawking radiation. So, as a black hole ages, it loses mass, which makes it radiate more, and so on until it completely evaporates.…