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Encyclopedia > Absolute horizon

In general relativity, an absolute horizon is a boundary in spacetime, 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 apparent horizon is the boundary of a black hole by definition. Image File history File links Circle-question-red. ... General relativity (GR) [also called the general theory of relativity (GTR) and general relativity theory (GRT)] is the geometrical theory of gravitation published by Albert Einstein in 1915/16. ... In physics, spacetime is a mathematical model that combines space and time into a single construct called the space-time continuum. ... A black hole is an object predicted by general relativity,[1] with a gravitational field so powerful that even electromagnetic radiation (such as light) cannot escape its pull. ...

In the context of black holes, the absolute horizon is almost exclusively referred to as an event horizon, though this is often used as a more general term for all types of horizons. Important distinctions must also be made between absolute horizons and apparent horizons. For the science fiction film, see Event Horizon (film). ...

See, however the articles on ergosphere, Cauchy horizon, the Reissner-Nordström solution, photon sphere, Killing horizon and naked singularity; the notion of a horizon in general relativity is subtle, and depends on fine distinctions. A rotating black hole (Kerr black hole or Kerr-Newman black hole) is a black hole that possesses angular momentum. ... In physics, a Cauchy horizon is a light_like boundary of the domain of validity of a Cauchy problem. ... A photon sphere is a spherical region of space surrounding extremely massive objects such as black holes. ... Astronomy A common case of a Killing horizon in astrophysics occurs as part of a rotating black hole and is a surface on which the rotational killing vector field becomes null. ... blah blah blah, some people believe God made the universe and that is all there is. ...

## Definition GA_googleFillSlot("encyclopedia_square");

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. This article is in need of attention from an expert on the subject. ... It has been suggested that Deriving the Schwarzschild solution be merged into this article or section. ... In general relativity, the Kerr metric (or Kerr vacuum) describes the geometry of spacetime around a rotating massive body, such as a rotating black hole. ... The Friedmann-LemaÃ®tre-Robertson-Walker (FLRW) metric describes a homogeneous, isotropic expanding/contracting 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.[1][2][3] In physics and mathematics, Minkowski space (or Minkowski spacetime) is the mathematical setting in which Einsteins theory of special relativity is most conveniently formulated. ... In topology, the boundary of a subset S of a topological space X is the set of points which can be approached both from S and from the outside of S. More formally, it is the set of points in the closure of S, not belonging to the interior of...

## 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 horizons, 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 of numerical relativity, where a spacetime is simply being evolved into the future, only a finite portion of the spacetime can be known. This article is in need of attention from an expert on the subject. ...

## See also

For the science fiction film, see Event Horizon (film). ... A black hole is an object predicted by general relativity,[1] with a gravitational field so powerful that even electromagnetic radiation (such as light) cannot escape its pull. ... It has been suggested that Deriving the Schwarzschild solution be merged into this article or section. ... It has been suggested that this article or section be merged into Observable universe. ... In cosmology, a cosmological horizon marks a limit to observability, and marks the boundary of a region that an observer cannot see into directly due to cosmological effects. ...

## References

1. ^ S. W. Hawking and G. F. R. Ellis (1975). The large scale structure of space-time. Cambridge University Press.
2. ^ Wald, Robert M. (1984). General Relativity. Chicago: University of Chicago Press.
3. ^ Thorne, Kip S.; Misner, Charles; Wheeler, John (1973). Gravitation. W. H. Freeman and Company.
• Kip Thorne (1994). Black Holes and Time Warps. W. W. Norton.  This is a popular book, aimed at the lay reader, containing good discussion of horizons and black holes.

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