The event horizon is a boundary beyond which information will never reach an observer. An event horizon is a mathematical construct and not a physical object and a person passing through an event horizon will not notice any odd behavior. From an outside observer however, an object passing though an event horizon will appear redder and dimmer and will appear to freeze at the moment the object passes the event horizon.
An event horizon can form around a gravitational singularity where its gravitational pull exceeds the ability of any form of mass or energy, including light, to escape. Outside the event horizon, there is a region where light is bent because of the gravitational pull. However it has been demonstrated that a naked singularity where a gravitational singularity exists without an event horizon is possible.
An event horizon can also form in an expanding universe. When two objects expand faster than the speed of light relative to each other, they cannot exchange information, they are beyond each other's event horizon.
See also gravity, general relativity, black hole, gravitational singularity and quantum physics.
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