Baby universes

This paper reports that in a reasonable theory of quantum gravity, the topology of spacetime must be able to be different from that of flat space. Otherwise, the theory would not be able to describe closed universes, or black holes. Presumably, the theory should allow all possible spacetime topologies. In particular, it should allow closed universes to branch off, or join onto, our asymptotic flat region of spacetime. Of course, such behavior is not possible with a real, non singular, Lorentzian metric. However, we now all know that quantum gravity has to be formulated in the Euclidean domain. There, it is no problem: it is just a question of plumbing