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So when enough matter to create a black hole falls towards the center of its collective mass, at one point the mass/area ratio becomes high enough to form the event horizon of the black hole. Because this would yield a sphere rather than a point, it has volume and because the event horizon can't fall any further inside the black hole (since for matter just inside the event horizon, time would have stopped), that would make the event horizon an impenetrable shell.

Now the stuff at the exact center of the black hole would experience the same pull towards every direction and thus would still be able to travel through time and space, right? In that case, it would travel along the shortest path towards the event horizon (since its ulikely to be at the exact center, until the pull gets strong enough to stop time for that matter as well.

By that reasoning, all black holes should be hollow, right?

And now the mind bending bit - after the big rip, when space is expanding faster than the speed of light, it should also do so inside black holes. Does that mean that black holes will be expanding at the speed of light? Up until the point where the mass density gets low enough for it not to be a black hole anymore?

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closed as unclear what you're asking by ACuriousMind, CuriousOne, AccidentalFourierTransform, David Z Apr 25 '16 at 12:08

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    $\begingroup$ There is no gravity inside a hollow sphere. Things don't fall outward to the shell. I don't know what happens inside hollow black holes, but it seems unlikely that things will fall upward towards the horizon. $\endgroup$ – Peter Shor Apr 25 '16 at 2:05
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For the simple case of a spherically symmetric, uniform, pressureless ball we have an analytic solution to describe the collapse called the Oppenheimer-Snyder metric. In this case the event horizon starts at the centre before the singularity is formed and grows outwards. The matter remains as a uniform sphere that eventually shrinks to a point. At no point does it form a shell like structure. The Oppenheimer-Snyder metric is a highly simplified model, but we expect it to capture the general features of a real collapse.

A black hole in an expanding universe is described by the de Sitter-Schwarzschild metric. In the case of a Big Rip type expansion the event horizon disappears to leave a naked singularity.

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  • $\begingroup$ Google on frozen star Oppenheimer, remember what you said here, and remember what Einstein said in the 2nd paragraph here. A gravitational field is a place where the speed of light is spatially variable. The event horizon starts at the centre and moves outward like the hailstone. But light can't go slower than stopped. Gravity vanishes. $\endgroup$ – John Duffield Apr 25 '16 at 19:03

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