Horizon (general relativity)

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