Horizon (general relativity)

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There are several types of spacetime horizons that play a role in Einstein's theory of general relativity:

• Absolute horizon, a boundary in spacetime in general relativity inside of which events cannot affect an external observer
• Apparent horizon, a surface defined in general relativity
• Cauchy horizon, a surface found in the study of Cauchy problems
• Celestial horizon, a great circle parallel to the horizon
• Cosmological horizon, a limit of observability
• Event horizon, a boundary in spacetime beyond which events cannot affect the observer
• 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