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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's boundary and the boundary of an expanding universe
- Apparent horizon, a surface defined in general relativity
- Cauchy horizon, 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 vector field
- Particle horizon, the maximum distance from which particles can have travelled to an observer in the age of the universe