I have read this question:
Yes, the expansion of space itself is allowed to exceed the speed-of-light limit because the speed-of-light limit only applies to regions where special relativity – a description of the spacetime as a flat geometry – applies. In the context of cosmology, especially a very fast expansion, special relativity doesn't apply because the curvature of the spacetime is large and essential. When this v exceeds c, it means that the two places/galaxies are "behind the horizons of one another" so they can't observer each other anytime soon. But they're still allowed to exist.
Can space expand with unlimited speed?
The event horizon actually isn't a physical thing at all, just like any other border isn't a real thing. Remember that the horizon is just a 'border' which marks where the escape velocity raises above luminal speed.
Are black holes naked singularities for an observer within the event horizon?
So the first one says that space can expand at a speed that exceeds the speed of light, but that is not a violation of SR, because space is not a thing, no information, no particles or objects are actually moving faster then the speed of light. And that SR is not applicable where curvature is large and essential.
Now similarly, the event horizon of a black hole is not a thing, but a boundary, between two regions of spacetime (which similarly are casually disconnected, just like in the case of space expansion), so SR should not be violated if an event horizon would expand faster then the speed of light. Same way, spacetime curvature is large and essential in the case of the event horizon, so SR should not apply.
So there are two things that come to mind:
space is not a thing, and if it expands faster then the speed of light, that does not violate SR, because no information, no particles, no objects are locally moving actually faster then light (although they are moving away from us faster then the speed of light). Same applies for a black hole's event horizon expanding, which is not a physical thing, but a boundary.
SR only applies in regions where spacetime is flat, but if spacetime curvature is large and essential, like in the case of a black hole's event horizon, SR does not apply, so it is not a violation of SR if space expands faster then the speed of light (in the case of cosmology, where curvature is large and essential) or if a black hole's event horizon expands (where curvature is large and essential) faster then the speed of light.
Just to clarify, I am simply asking, as seen from a far away observer (may be inside or outside), how fast can the EH be seen to expand? Can this measured (from far away) speed exceed the speed of light?
- If spacetime can expand faster than the speed of light, then can a black hole do that too?