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are bubbles of spacetime pinching-off allowed solutions to general relativity? With "pinch-off bubble" i really mean a finite 3D volume of space whose 2D boundary decreases until it reaches zero and it disconnects from the mother spacetime, becoming a compact "child" spacetime

I've read this as a way in eternal cosmologies to produce new universes, but i don't know if that is something something allowed in general relativity or is something from a more advanced theory (i.e: string theory)

In particular, unlike unitary black holes, this does not seem to conserve information, unless someone believes that complementarity can do strange things as clone the information (including the quantum one). Does that automatically make this an unphysical solution?

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Well, with topology change in classical general relativity, a lot of wacky stuff happens. See arxiv.org/abs/gr-qc/9406053 for example. – Alex Nelson Jul 29 '12 at 15:46
In what sense do you mean "valid"? They would require unphysical stress-energy tensors, which I would consider invalid. – John Rennie Jul 29 '12 at 19:26
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@JohnRennie, if it is true that with enough unphysical matter-energy you would be able to do topology change then you should write that as an answer. But I was hoping for an answer that addressed the kinematical limitations of the theory, rather than the dynamical ones. For instance, maybe the question of topology change is ill-defined with only metric degrees of freedom and no extrinsic curvature fields. The paper pointed by Alex Nelson seems to suggest that is not accurate though – user56771 Jul 30 '12 at 14:13
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@JohnRennie: Why are you so sure this requires unphysical stress-energy? – Ron Maimon Jul 30 '12 at 15:40

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