{{dabconcept
A horizon is a
boundary in
spacetime satisfying prescribed conditions.
There are several types of horizons that play a role in
Albert Einstein's theory of
general relativity:
*
Absolute horizon, a boundary in spacetime in general relativity inside of which events cannot affect an external observer
*
Event horizon, a boundary in spacetime beyond which events cannot affect the observer, thus referring to a
black hole
A black hole is a region of spacetime where gravitation, gravity is so strong that nothing, including light or other Electromagnetic radiation, electromagnetic waves, has enough energy to escape it. The theory of general relativity predicts t ...
's boundary and the boundary of an expanding
universe
*
Apparent horizon
In general relativity, an apparent horizon is a surface that is the boundary between light rays that are directed outwards and moving outwards and those directed outward but moving inward.
Apparent horizons are not invariant properties of spacetim ...
, a surface defined in general relativity
*
Cauchy horizon
In physics, a Cauchy horizon is a light-like boundary of the domain of validity of a Cauchy problem (a particular boundary value problem of the theory of partial differential equations). One side of the horizon contains closed space-like geodesics ...
, a surface found in the study of
Cauchy problems
*
Cosmological horizon, a limit of observability
*
Killing horizon, a null surface on which there is a
Killing
Killing, Killings, or The Killing may refer to:
Arts, entertainment, and media Films
* ''Killing'' (film), a 2018 Japanese film
* ''The Killing'' (film), a 1956 film noir directed by Stanley Kubrick Television
* ''The Killing'' (Danish TV serie ...
vector field
*
Particle horizon, the maximum distance from which particles can have travelled to an observer in the age of the universe
General relativity