I think in Lewis’ model, those sometimes interacting/sometimes not interacting Universes would be considered part of ONE Universe, operating under multiverse rules.
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The flexibility afforded by this view is amazing to me. All the pressure goes away.
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I’d still put them as part of one Universe operating under those rules.
That one Universe would contain a multiverse of non-interacting Universes, like two cells with thick walls that only briefly interacting under certain conditions within a larger cell.
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So with this view, there’s philosophical room for the Bulk but also for Hawking’s no-boundary universe.
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The Bulk is theoretically the dimension higher than our universe from which the Big Bang originated upon the interaction of two boundaries, or “branes”. Also called M-brane theory.
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