nLab T-fold



Riemannian geometry

String theory




A T-fold (Hull 04) is supposed to be a kind of space that locally has charts which are Riemannian manifolds equipped with a B-field (i.e. a circle 2-bundle with connection or bundle gerbe with connection) but where the charts are glued together not just by diffeomorphisms (as an ordinary smooth manifold is) but also by T-duality transformations along some torus fibers.

The idea is that a T-fold is a target space for a string sigma-model that is only locally a Riemannian manifold but globally a more general kind of geometry, due due duality in string theory. In the literature sometimes the term non-geometric backgrounds is used for such “generalized geometric” backgrounds.

It is expected that T-folds should have a description in terms of spaces that locally are fiber products of one torus fiber bundle with its T-dual, as the correspondence spaces considered in topological T-duality. In the rational/infinitesimal approximation this is derived from analysis of super p-brane WZ-terms in FSS 16. A proposal for a (non-supersymmetric) global description of T-folds as total spaces of principal 2-bundles for the T-duality 2-group is in Nikolaus-Waldorf 18.

One may then consider local field theory on these double torus fibrations, and this should be closely related to what is called double field theory (Hull 06).



The original idea:

The relation to double field theory goes back to:

Further developments:

Discussion for nonabelian T-duality:

Comments in relation to topological T-duality:

Via higher geometry

A global definition of T-folds (with T-duality understood as topological T-duality) in higher differential geometry, concretely as principal 2-bundles for the T-duality 2-group (as described in T-Duality and Differential K-Theory) is proposed in

The local superspace supergeometry of T-folds compatible with this proposal is derived from fundamental super p p -brane structure in:

The automorphism 2-group of the T-duality 2-group (and hence potentially the full structure of T-folds) is determined in:

Introduction and review:

Proposals for further generalization (2-connections and further non-geometric backgrounds):

Last revised on June 8, 2024 at 12:50:37. See the history of this page for a list of all contributions to it.