An event horizon, a term introduced in astrophysics by Wolfgang Rindler in the 1950s, is a boundary in space beyond which events cannot impact an observer.
John Michell suggested in 1784 that the gravitational pull near large compact objects can be intense enough to prevent even light from escaping. By this logic, if the escape velocity associated with a gravitational force exceeds the speed of light, light originating within its reach can momentarily escape but will eventually return.
In 1958, David Finkelstein used general relativity to propose a stricter definition of a local black hole event horizon as a boundary from which no events can affect an outside observer, leading to debate over the concept of event horizons and black holes.
Stephen Hawking, one of the leading developers of theories about black holes, later suggested using an "apparent horizon" concept, stating that "Gravitational collapse produces apparent horizons but no event horizons." He concluded that the absence of event horizons means there are no actual black holes where light can't escape infinitely.
An interesting point to note is that as an object gets closer to the horizon from the observer's side, it appears to slow down and never truly crosses the horizon. Owing to gravitational redshift, the object's image gradually turns red as it moves away from the observer.
In the context of an expanding universe, the expansion speed can reach or exceed the speed of light, which obstructs signals from reaching certain regions. This creates a cosmic event horizon that affects all types of signals, including gravitational waves.
Some specific types of horizons include absolute and apparent horizons found around a black hole, Cauchy and Killing horizons, photon spheres, and ergospheres of the Kerr solution. There are also particle and cosmological horizons relevant to cosmology, and isolated and dynamical horizons, which hold importance for ongoing black hole research.
In cosmology, the event horizon of the observable universe represents the largest possible distance from which light emitted now can ever reach the observer in the future. This differs from the particle horizon, which is the largest distance light emitted in the past could reach an observer at a certain time. The particle horizon's evolution relies on the nature of the universe's expansion; specific characteristics could imply that certain parts of the universe will forever be unobservable, these areas beyond the boundary are termed as event horizons.
In cosmological models without an event horizon, universes are dominated by matter or radiation. Conversely, a universe dominated by the cosmological constant, a de Sitter universe, is an example of a model with an event horizon. The speeds of these cosmological horizons were given in a research paper on the FLRW cosmological model.
The apparent horizon of an accelerated particle is another relevant concept, offering insights into the complex interplay of speed, gravity, and observation. """