Non-semisimple Crane-Yetter theory varying over the character stack

TL;DR

This paper develops a relative 4D topological quantum field theory based on non-semisimple data, extending the Crane-Yetter framework to include character stacks and gauge theory insights.

Contribution

It introduces a relative invertibility property for a non-semisimple Crane-Yetter theory, generalizing key features of the original theory and connecting to gauge theory and skein algebra unicity.

Findings

Established a relative invertibility property for the new TQFT.

Connected the invertibility to a categorical unicity theorem for skein algebras.

Provided a canonical line bundle structure on the character stack for 3-manifolds.

Abstract

We construct a relative version of the Crane-Yetter topological quantum field theory in four dimensions, from non-semisimple data. Our theory is defined relative to the classical $G$-gauge theory in five dimensions -- this latter theory assigns to each manifold $M$ the appropriate linearization of the moduli stack of $G$-local systems, called the character stack. Our main result is to establish a relative invertibility property for our construction. This invertibility generalizes the key invertibility property of the original Crane-Yetter theory which allowed it to capture the framing anomaly of the celebrated Witten-Reshetikhin-Turaev theory. In particular our invertibilty statement at the level of surfaces implies a categorical, stacky version of the unicity theorem for skein algebras; at the level of 3-manifolds it equips the character stack with a canonical line bundle. Regarded as a topological symmetry defect of classical gauge theory, our work establishes invertibility of this defect by a gauging procedure.