The curved spacetime thing, I think. I don't have a firm view of what happens when this universe 'ends'...whether it leads back to the beginning of itself, or to the start of a 'new' universe. I think that both of those ideas are confused by a flawed conception of time though. If time ends at the end of the universe (or curves back to join up with its beginning), there isn't really any 'after' in the conventional sense. There seems to be no _continuity_ of time from one universe to the next so it may make as much sense to say that the next universe is before this one, after this one, and coexistent with this one. I like your picture of the universe ending when the last soul attains enlightenment though... it has a nice symmetry.