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An event horizon is a type of boundary such that any information past this boundary is inaccessible to the observer it is defined for. Common examples are the Schwarzschild radius of a black hole (which is defined commonly for all observers outside this radius) and the cosmological event horizon (which is defined as a radius from an observer)

An event horizon is a type of boundary such that any information past this boundary is inaccessible to the observer it is defined for. Common examples are the boundaries of (this boundary being defined commonly for all observers outside this radius) and cosmological event horizons (which is defined as a radius from an observer) present, for example, in .