Definition
An absolute horizon is only defined in an asymptotically flat spacetime — a spacetime which approaches flat space as one moves far away from any massive bodies. Examples of asymptotically flat spacetimes include Schwarzschild and Kerr black holes. The FRW universe — which is believed to be a good model for our universe — is generally not asymptotically flat. Nonetheless, we can think of an isolated object in an FRW universe as being nearly an isolated object in an asymptotically flat universe.
The particular feature of asymptotic flatness which is needed is a notion of "future null infinity". This is the set of points which are approached asymptotically by null rays (light rays, for example) which can escape to infinity. This is the technical meaning of "external universe". These points are only defined in an asymptotically flat universe. An absolute horizon is defined as the boundary of a region from which null rays cannot escape to future null infinity.
Read more about this topic: Absolute Horizon
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