- Absolute horizon
In

general relativity , an**absolute horizon**is a boundary inspacetime , defined with respect to the external universe, inside of which events cannot affect an external observer. Light emitted inside the horizon can never reach the observer, and anything that passes through the horizon from the observer's side is never seen again. An absolute horizon is the boundary of ablack hole by definition.In the context of black holes, the absolute horizon is almost exclusively referred to as an

, though this is often used as a more general term for all types of horizons. The absolute horizon is just one type of horizon. For example, important distinctions must be made between absolute horizons andevent horizon apparent horizon s. See also the articles onergosphere ,Cauchy horizon , theReissner-Nordström solution ,photon sphere ,Killing horizon andnaked singularity ; the notion of a horizon in general relativity is subtle, and depends on fine distinctions.**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. [*cite book | title=The large scale structure of space-time | author=S. W. Hawking and G. F. R. Ellis | publisher=Cambridge University Press | year=1975*] [*cite book | author=Wald, Robert M. | title=General Relativity | location=Chicago | publisher=University of Chicago Press | year = 1984*] [*cite book | author=Thorne, Kip S.; Misner, Charles; Wheeler, John | title=Gravitation | publisher=W. H. Freeman and Company | year=1973*]**Nature of the absolute horizon**The definition of an absolute horizon is sometimes referred to as "teleological", meaning that it cannot be known where the absolute horizon is without knowing the entire evolution of the universe, including the future. This is both an advantage and a disadvantage. The advantage is that this notion of a horizon is very geometrical, and does not depend on the observer, unlike

apparent horizon s, for example. The disadvantage is that it requires the full history (all the way into the future) of the spacetime to be known. In the case ofnumerical relativity , where a spacetime is simply being evolved into the future, only a finite portion of the spacetime can be known.**ee also***

Event horizon

*Apparent horizon

*Black hole

*Schwarzschild metric

*Particle horizon

*Cosmological horizon **References****Further reading***cite book | title=Black Holes and Time Warps | author=Kip Thorne | publisher=W. W. Norton | year=1994 This is a popular book, aimed at the lay reader, containing good discussion of horizons and black holes.

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