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Journal Article

Central extensions of mapping class groups from characteristic classes


Valentino,  Alessandro
Max Planck Institute for Mathematics, Max Planck Society;

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Fiorenza, D., Schreiber, U., & Valentino, A. (2018). Central extensions of mapping class groups from characteristic classes. Cahiers de Topologie et Géométrie Différentielle Catégoriques, 59(3), 260-298.

Cite as: https://hdl.handle.net/21.11116/0000-0003-E26C-5
Tangential structures on smooth manifolds, and the extension of mapping class groups they induce, admit a natural formulation in terms of higher (stacky) differential geometry. This is the literal translation of a
classical construction in differential topology to a sophisticated language, but it has the advantage of emphasizing how the whole construction naturally emerges from the basic idea of working in slice categories. We characterize, for every higher smooth stack equipped with tangential structure, the induced higher group extension of the geometric realization of its higher automor-
phism stack. We show that when restricted to smooth manifolds equipped with higher degree topological structures, this produces higher extensions of homotopy types of diffeomorphism groups. Passing to the groups of connected components, we obtain abelian extensions of mapping class groups
and we derive sufficient conditions for these being central. We show as a special case that this provides an elegant re-construction of Segal’s approach to $\mathbb{Z}$ -extensions of mapping class groups of surfaces that provides the anomaly
cancellation of the modular functor in Chern-Simons theory. Our construction generalizes Segal’s approach to higher central extensions of mapping class groups of higher dimensional manifolds with higher tangential structures, expected to provide the analogous anomaly cancellation for higher dimensional TQFTs.