What is the name of the theory that black holes emit radiation?

The theory that black holes emit radiation is called Hawking radiation. Proposed by Stephen Hawking in 1974, it showed that black holes are not completely black when quantum physics is considered.

Classical general relativity says that nothing, including light, can escape from inside a black hole's event horizon. Hawking applied quantum field theory to the curved spacetime around a black hole and found that an outside observer should detect faint thermal, or blackbody, radiation. The effect comes from quantum behavior near the horizon, not from ordinary light escaping from inside it.

The radiation carries energy away, so an isolated black hole should gradually lose mass. This predicted process is known as black-hole evaporation. The temperature is inversely related to mass: smaller black holes are hotter and radiate more strongly, while large astrophysical black holes are extraordinarily cold and faint.

The idea grew alongside Jacob Bekenstein's work connecting black-hole entropy with horizon area. Hawking radiation is therefore a major link between quantum theory, gravity, and thermodynamics. It remains a theoretical prediction; radiation from ordinary stellar-mass black holes has not been directly detected.