specific types of horizon - absolute horizon - fall def. 8 |

The absolute horizon is defined only in asymptotically flat space – space-time, which is suitable for flat space, as one goes far from any massive body. Examples of asymptotically flat space-time include the Schwarzschild and Kerr black holes. In FRW Universe, which is believed to be a good model for our Universe – is generally not asymptotically flat. However, we can think about an isolated object in the Universe is almost FRW as a single object in an asymptotically flat Universe.

The feature of asymptotic flatness, which is essential to the concept of "future null infinity". This is a set of points which are approached asymptotically by null rays, light rays, for example, which can escape to infinity. This technical meaning of "external Universe". These points are only defined in asymptotically flat Universe. The absolute horizon is defined as the boundary of a region from which null rays cannot escape to future null infinity. 

 an absolute horizon is a boundary in spacetime, defined with respect to the external universe, inside of which events cannot affect an external observer. The light emitted inside the horizon can never reach the observer, and everything that passes through the horizon from the observer is not seen by the observer. The absolute horizon is the boundary of a black hole.

In the context of black holes, the absolute horizon is almost exclusively referred to as the event horizon, although 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, an important distinction must be made between absolute horizons and the horizons of the visible, the concept of horizon in General relativity is subtle, and depends on fine distinctions. 

Def. 8 Dr. Karam Ouharou 

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