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As a non-physicist, mathematician or theoretician I would like to ask a question that may sound naive, so I apologize in advance.

How does the expansion of one universe relate to other universes under the theory of multiple universes? I understand that there is a theory that proposes universes which seem to have fixed limits - 'membranes'- that remain in contact with other universes, colliding and retreating as they expand and contract. Is it possible that the universes are more fluid? If so, is it possible that they flow into each other and if so how? (Do they permeate the adjacent universe or do they flow in directed - ie fixed boundaries- streams, for example?) Do the individual universes retain their integrity or are they essentially changed through the contact?

Perhaps a simpler way to pose the question is: what are the effects of contact between expanding universes and non-expanding universes? And, what role does sequence have, if any?