In astrophysics, an event horizon is a boundary beyond which events cannot affect an observer. The term was coined by Wolfgang Rindler.In 1784, John Michell proposed that in the vicinity of compact massive objects, gravity can be strong enough that even light cannot escape. At that time, the Newtonian theory of gravitation and the so-called corpuscular theory of light were dominant. In these theories, if the escape velocity of an object exceeds the speed of light, then light originating inside or from it can escape temporarily but will return. In 1958, David Finkelstein used General Relativity to introduce a stricter definition of a local black hole event horizon as a boundary beyond which events of any kind cannot affect an outside observer. This led to information and firewall paradoxes, which encouraged the re-examination of the concept of local event horizons and the notion of black holes. Several theories were subsequently developed, some with, and some without, event horizons. Stephen Hawking, who was one of the leading developers of theories to describe black holes, suggested that an apparent horizon should be used instead of an event horizon, saying "gravitational collapse produces apparent horizons but no event horizons". He eventually concluded that "the absence of event horizons means that there are no black holes – in the sense of regimes from which light can't escape to infinity."Any object that approaches the horizon from the observer's side appears to slow down and never quite crosses the horizon. Due to gravitational redshift, its image reddens over time as the object moves away from the observer.In an expanding universe the speed of expansion reaches and even exceeds the speed of light, which prevents signals from travelling to some regions. A cosmic event horizon is a real event horizon because it affects all kinds of signals, including gravitational waves which travel at the speed of light.
More specific types of horizon include the related but distinct absolute and apparent horizons found around a black hole. Other distinct types include the Cauchy and Killing horizons; the photon spheres and ergospheres of the Kerr solution; particle and cosmological horizons relevant to cosmology; and isolated and dynamical horizons important in current black hole research.