Emergent Closed Universes in Symmetric Orbifold CFTs
We identify closed universe sectors in large $N$ symmetric orbifold CFTs with holographic duals. Starting from tensor product states built out of a finite dimensional low energy subspace of the seed theory, we show that the large $N$ Hilbert space decomposes into superselection sectors labeled by occupation number distributions. Before imposing the orbifold gauge constraint, these sectors have exponentially large dimensions, and the maximally entropic sector dominates the ungauged Hilbert space. We argue that this sector exhibits several characteristic features expected of a closed universe Hilbert space: pure states become indistinguishable from a mixed state at the level of simple correlation functions, and the associated operator algebra is naturally a hyperfinite type II$_1$ von Neumann algebra. We then impose the $S_N$ gauge constraint. The large gauge redundancy drastically reduces the number of independent states. In particular, in the large $N$ limit, the dimension of the physical Hilbert space grows only polynomially with $N$. Consequently, each superselection sector after imposing the constraint is one dimensional in this limit. This reproduces the qualitative behavior suggested by gravitational path integral calculations with wormholes. We then show why, in this setup, the Hartle-Hawking type semiclassical approximation for the dominant closed universe fails to reproduce the CFT results. Nevertheless, the dominant saddle point approximation for gravitational path integral calculation is reconstructed once the CFT degrees of freedom are coupled to external observer degrees of freedom.