Duality Symmetric String and M-Theory

TL;DR

The paper surveys duality-symmetric formulations of string theory and M-theory, emphasizing how T-duality and U-duality motivate extending spacetime to include dual coordinates. It develops the worldsheet doubled formalism and the spacetime Double Field Theory (DFT), detailing their gauge structures, generalised geometry foundations, and extensions to RR fields, fermions, and branes. It then extends these ideas to M-theory with exceptional generalised geometry, presenting SL(5) as a concrete example and showing how Scherk-Schwarz reductions yield gauged supergravities, thereby linking extended geometry to lower-dimensional gaugings. The work demonstrates the quantum consistency of doubled/formulated theories (modular invariance, beta-functions) and highlights the role of non-geometric backgrounds (T-folds, U-folds) in broadening the string/M-theory landscape and its phenomenological potential.

Abstract

We review recent developments in duality symmetric string theory. We begin with the world sheet doubled formalism which describes strings in an extended space time with extra coordinates conjugate to winding modes. This formalism is T-duality symmetric and can accommodate non-geometric T-fold backgrounds which are beyond the scope of Riemannian geometry. Vanishing of the conformal anomaly of this theory can be interpreted as a set of spacetime equations for the background fields. These equations follow from an action principle that has been dubbed Double Field Theory (DFT). We review the aspects of generalised geometry relevant for DFT. We outline recent extensions of DFT and explain how, by relaxing the so-called strong constraint with a Scherk Schwarz ansatz, one can obtain backgrounds that simultaneously depend on both the regular and T-dual coordinates. This provides a purely geometric higher dimensional origin to gauged supergravities that arise from non-geometric compactification. We then turn to M-theory and describe recent progress in formulating an E_{n(n)} U-duality covariant description of the dynamics. We describe how spacetime may be extended to accommodate coordinates conjugate to brane wrapping modes and the construction of generalised metrics in this extend space that unite the bosonic fields of supergravity into a single object. We review the action principles for these theories and their novel gauge symmetries. We also describe how a Scherk Schwarz reduction can be applied in the M-theory context and the resulting relationship to the embedding tensor formulation of maximal gauged supergravities.