Baby universes in string theory

We argue that the holographic description of four-dimensional BPS black holesnaturally includes multi-center solutions. This suggests that the holographicdual to the gauge theory is not a single AdS_2 times S^2 but a coherentensemble of them. We verify this in a particular class of examples, where thetwo-dimensional Yang-Mills theory gives a holographic description of the blackholes obtained by branes wrapping Calabi-Yau cycles. Using the free fermionicformulation, we show that O(e^{-N}) non-perturbative effects entangle the twoFermi surfaces. In an Euclidean description, the wave-function of themulti-center black holes gets mapped to the Hartle-Hawking wave-function ofbaby universes. This provides a concrete realization, within string theory, ofeffects that can be interpreted as the creation of baby universes. We findthat, at least in the case we study, the baby universes do not lead to a lossof quantum coherence, in accord with general arguments.Comment: 39 pages, 7 figure