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My question is do baby universes give us a satisfying explanation of the black hole information paradox without violating thermodynamics, or any other established/known physics?

My understanding is that it wouldn’t contradict unitarity because, while information is “lost” from our perspective outside of the black hole, unitarity is obeyed in the multiverse, of which observers only have partial knowledge. So for example in arxiv:0901.3156, Sabine Hossenfelder and Lee Smolin consider it a conservative solution.

So according to this scenario universes are spawned through black holes, they “emerge” on the other side through white holes. But if white holes are time reversed black holes (CPT symmetry), then since entropy increases as the area of a black hole increases then does entropy decrease in a white hole? If the area decrease coincides with the increase in entropy then surely that contradicts the holographic entropy bound?

I think there’s got to be a mistake in my comment but I’m not sure, thanks for any responses.

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    $\begingroup$ Why do you mention white holes? Why are they relevant to your question about baby universes? $\endgroup$ – PM 2Ring Jun 22 '19 at 20:56
  • $\begingroup$ Hey there, according to the “baby universes” solution to the BHIP, universes are spawned through black holes, they “emerge” on the other side through white holes. $\endgroup$ – Andrew Jun 22 '19 at 21:15
  • $\begingroup$ Oh, ok. In that case, I guess it's relevant then. :) But you should add that info to your question. Not all readers are familiar with Smolin's Fecund Universes theory. I read The Life of the Cosmos when it was new, but that was a few years ago now. ;) $\endgroup$ – PM 2Ring Jun 22 '19 at 21:25
  • $\begingroup$ But if white holes are time reversed black holes (CPT symmetry) I don't think this is quite right. GR doesn't really have discrete symmetries such as P and T. In the maximally extended Schwarzschild spacetime, what makes us say that the white hole part is time-reversed is that it's causally related to $\mathscr{I}^+$ but not to $\mathscr{I}^-$, which is the opposite of the case for a black hole. But its arbitrary what you call $\mathscr{I}^+$ and what you call $\mathscr{I}^-$. $\endgroup$ – Ben Crowell Jun 22 '19 at 23:26
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    $\begingroup$ [baby universes] “emerge” on the other side through white holes: could you give a reference? Usually (such as this paper) the de Sitter space is patched onto the near-singularity region of BH interior without any asymptotic regions for the white hole to be present. $\endgroup$ – A.V.S. Jun 23 '19 at 7:14

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