Comments on wormholes and factorization

In AdS/CFT partition functions of decoupled copies of the CFT factorize. In bulk computations of such quantities contributions from spacetime wormholes which link separate asymptotic boundaries threaten to spoil this property, leading to a factorization puzzle. Certain simple models like JT gravity have wormholes, but bulk computations in them correspond to averages over an ensemble of boundary systems. These averages need not factorize. We can formulate a toy version of the factorization puzzle in such models by focusing on a specific member of the ensemble where partition functions will again factorize. As Coleman and Giddings-Strominger pointed out in the 1980s, fixed members of ensembles are described in the bulk by $alpha$-states in a many-universe Hilbert space. In this paper we analyze in detail the bulk mechanism for factorization in such $alpha$-states in the topological model introduced by Marolf and Maxfield (the MM model) and in JT gravity. In these models geometric calculations in $alpha$ states are poorly controlled. We circumvent this complication by working in $textit{approximate}$ $alpha$ states where bulk calculations just involve the simplest topologies: disks and cylinders. One of our main results is an effective description of the factorization mechanism. In this effective description the many-universe contributions from the full $alpha$ state are replaced by a small number of effective boundaries. Our motivation in constructing this effective description, and more generally in studying these simple ensemble models, is that the lessons learned might have wider applicability. In fact the effective description lines up with a recent discussion of the SYK model with fixed couplings arXiv:2103.16754. We conclude with some discussion about the possible applicability of this effective model in more general contexts.