In the theory of general relativity, the Gibbons–Hawking effect is the statement that a temperature can be associated to each solution of the Einstein field equations that contains a causal horizon.
It is named after Gary Gibbons and Stephen Hawking.
For example, Schwarzschild spacetime contains an event horizon and so can be associated a temperature.
In the case of Schwarzschild spacetime this is the temperature
A second example is de Sitter space which contains an event horizon.