Topological Models on the Lattice and a Remark on String Theory Cloning
  The addition of a topological model to the matter content of a conventional closed-string theory leads to the appearance of many perturbatively-decoupled space-time worlds. We illustrate this by classifying topological vertex models on a triangulated surface. We comment on how such worlds could have been coupled in the Planck era. 
Costas Bachas and Marios Petropoulos