Gibbons–Hawking effect , , ,

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 William Hawking.

The term "causal horizon" does not necessarily refer to event horizons only, but could also stand for the horizon of the visible universe, for instance.

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 ${\displaystyle T}$ of a black hole of mass ${\displaystyle M}$, satisfying ${\displaystyle T\propto M^{-1}}$ (see also Hawking radiation).

A second example is de Sitter space which contains an event horizon. In this case the temperature ${\displaystyle T}$ is proportional to the Hubble parameter ${\displaystyle H}$, i.e. ${\displaystyle T\propto H}$..

In general relativity, an event horizon is a boundary in spacetime beyond which events cannot affect an outside observer. In layman's terms, it is defined as the shell of "points of no return", i.e., the points at which the gravitational pull becomes so great as to make escape impossible, even for light. An event horizon is most commonly associated with black holes. Light emitted from inside the event horizon can never reach the outside observer. Likewise, any object approaching the horizon from the observer's side appears to slow down and never quite pass through the horizon, with its image becoming more and more redshifted as time elapses. This means that the wavelength is getting longer as the object moves away from the observer. The traveling object, however, experiences no strange effects and does, in fact, pass through the horizon in a finite amount of proper time.

More specific types of horizon include the related but distinct absolute and apparent horizons found around a black hole. Still other distinct notions include the Cauchy and Killing horizon; the photon spheres and ergospheres of the Kerr solution; particle andcosmological horizons relevant to cosmology; and isolated and dynamical horizons important in current black hole research.