Topological interfaces between universes and generalized families of 2d QFTs
Two-dimensional quantum field theories with topological point operators decompose into universes. Just as fusion categories have played an important role in understanding non-invertible symmetries, multi-fusion categories play an analogous role for the symmetries of such decomposing theories. We describe a finite symmetry as a multi-fusion category together with its action functor, which keeps track of trivially acting symmetries and of the topological interfaces between universes. Relative to a chosen symmetry, we classify pairs of universes by the topological interfaces it contains, which reflect the structure of the multi-fusion category: universes in different indecomposable summands are joined by no interface (Type~III), while within a summand they are related by invertible (Type~I) or only non-invertible (Type~II) interfaces. For orbifolds with a trivially acting abelian normal subgroup, we derive the full inherited symmetry, which characterizes the operators between universes found case by case by Robbins, Sharpe and Vandermeulen and yields pairs of Types~I and~II but never of Type~III. We construct Type~III pairs instead from extensions of $2$-group type and from renormalization group flows out of a common ultraviolet theory. Finally, we propose a correspondence with the generalized families of quantum field theories of Brennan and Intriligator, in which equivariant families correspond to Type~I pairs and categorical ones to Type~II pairs whose gauging preserves the symmetry, family anomalies appear as universes carrying different symmetry-protected topological phases, and Type~III pairs can realize families over a discrete parameter. An appendix by Xingyang Yu works out the higher-charge Schwinger model as a Lagrangian example.