What's the difference between apparent horizon and event horizon?

In summary: Dynamical apparent horizon is a hypothetical horizon which would be created by the motion of matter within the spacetime. It is not the same as the cosmological event horizon, which is the boundary of the universe where the expansion of the universe has stopped.
  • #1
xuphys
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0
What's the difference between apparent horizon and event horizon? I checked Wikipedia but I still don't understand. Could anyone give a short explanation?

Thanks!
 
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  • #2
The apparent horizon is, roughly speaking, the surface at which light rays shined radially outward do not move radially outward (inside the apparent horizon, they move inwards.) This implies that the apparent horizon is a local concept, in the sense that it relies only on an experiment in some small region of spacetime. Furthermore, apparent horizons are coordinate dependent horizons which can change or even vanish depending on your slicing of spacetime.

Event horizons, by contrast, are the surfaces beyond which light can never propagate to null infinity. This definition requires you to know the entire history of the spacetime to determine whether or not the light can escape, and in this sense it is a fundamental causal property of the spacetime which does not depend on coordinates.

Other differences between the two are that apparent horizons, if they exist, are always inside event horizons. The two horizons also coincide for a static spacetime. The apparent horizon can evolve discontinuously, while the EH evolves smoothly.
 
  • #3
Nabeshin said:
The apparent horizon is, roughly speaking, the surface at which light rays shined radially outward do not move radially outward (inside the apparent horizon, they move inwards.) This implies that the apparent horizon is a local concept, in the sense that it relies only on an experiment in some small region of spacetime. Furthermore, apparent horizons are coordinate dependent horizons which can change or even vanish depending on your slicing of spacetime.

Event horizons, by contrast, are the surfaces beyond which light can never propagate to null infinity. This definition requires you to know the entire history of the spacetime to determine whether or not the light can escape, and in this sense it is a fundamental causal property of the spacetime which does not depend on coordinates.

Other differences between the two are that apparent horizons, if they exist, are always inside event horizons. The two horizons also coincide for a static spacetime. The apparent horizon can evolve discontinuously, while the EH evolves smoothly.

Thank you for responding. Do you know about dynamical apparent horizon? It seems related to cosmological event horizon. I am wondering whether the dynamical apparent horizon is the same thing with what you mentioned here...

Thank you again!
 

1. What is an apparent horizon?

An apparent horizon is a theoretical boundary in space-time where the escape velocity is equal to the speed of light. This means that anything within the apparent horizon cannot escape the gravitational pull of a massive object, such as a black hole.

2. What is an event horizon?

An event horizon is a theoretical boundary around a black hole beyond which nothing, including light, can escape. It is the point of no return, where the gravitational pull of the black hole is so strong that even light cannot escape.

3. How are the apparent horizon and event horizon different?

The main difference between the apparent horizon and event horizon is that the apparent horizon is a local boundary in space-time, while the event horizon is a global boundary. The apparent horizon can change depending on the observer's frame of reference, while the event horizon is always the same for all observers.

4. Can we see the apparent horizon or event horizon?

No, neither the apparent horizon nor the event horizon can be seen directly because they are theoretical boundaries. However, we can observe the effects of these boundaries, such as the bending of light and the distortion of space-time, around black holes.

5. How do the apparent horizon and event horizon relate to each other?

The apparent horizon is always located within the event horizon. As an observer approaches the event horizon, the apparent horizon will move closer and closer until it eventually coincides with the event horizon. Beyond the event horizon, the apparent horizon no longer exists as it is subsumed by the event horizon.

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