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In astrophysics, an event horizon is a boundary beyond which events cannot affect an outside observer. Wolfgang Rindler coined the term in the 1950s. [1]In 1784, John Michell proposed that gravity can be strong enough in the vicinity of massive compact objects that even light cannot escape. [2]
The particle horizon is the boundary between two regions at a point at a given time: one region defined by events that have already been observed by an observer, and the other by events which cannot be observed at that time. It represents the furthest distance from which we can retrieve information from the past, and so defines the observable ...
For example, Schwarzschild spacetime contains an event horizon and so can be associated a temperature. In the case of Schwarzschild spacetime this is the temperature T {\displaystyle T} of a black hole of mass M {\displaystyle M} , satisfying T ∝ M − 1 {\displaystyle T\propto M^{-1}} (see also Hawking radiation ).
Although charged black holes with r Q ≪ r s are similar to the Schwarzschild black hole, they have two horizons: the event horizon and an internal Cauchy horizon. [8] As with the Schwarzschild metric, the event horizons for the spacetime are located where the metric component diverges; that is, where + = =
Between 20° and 5° of the horizon the temperature gradient becomes the dominant factor and numerical integration, using a method such as that of Auer and Standish [12] and employing the temperature gradient of the standard atmosphere and the measured conditions at the observer, is required. Closer to the horizon, actual measurements of the ...
Within an apparent horizon, light does not move outward; this is in contrast with the event horizon. In a dynamical spacetime, there can be outgoing light rays exterior to an apparent horizon (but still interior to the event horizon). An apparent horizon is a local notion of the boundary of a black hole, whereas an event horizon is a global notion.
If the universe started with even slightly different temperatures in different places, the CMB should not be isotropic unless there is a mechanism that evens out the temperature by the time of decoupling. In reality, the CMB has the same temperature in the entire sky, 2.726 ± 0.001 K. [3]
The curvature of the horizon is easily seen in this 2008 photograph, taken from a Space Shuttle at an altitude of 226 km (140 mi). The horizon is the apparent curve that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. This curve divides all ...