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Introduction
Introduction
My research program focuses on the interplay among string theory, duality symmetries, generalized geometry, and quantum gravity, with particular emphasis on Double Field Theory, non-geometric backgrounds, and the low-energy effective structures that emerge from string theory. The central motivation of this research direction is to understand how genuinely stringy phenomena — such as T-duality, winding modes, doubled coordinates, and non-geometric fluxes — can be systematically incorporated into target-space field theories and cosmological frameworks. These studies aim to extend conventional geometric descriptions of spacetime and provide deeper insights into the fundamental structure of quantum gravity and unified theories. The research accomplishments described below are primarily based on three representative works concerning Double Field Theory, cosmological implications of manifest O(D, D) symmetry, and the quantum consistency of double sigma models.
In Double Field Theory, the mass-squared of doubled fields associated with bosonic closed string states is proportional to $N_L+N_R-2$. Massless states are therefore not only the graviton, anti-symmetric, and dilaton fields with ($N_L=1$, $N_R=1$) such theory is focused on, but also the symmetric traceless tensor and the vector field relative to the states ($N_L=2$, $N_R=0$) and ($N_L=0$, $N_R=2$) which are massive in the lower-dimensional non-compactified space. While they are not even physical in the absence of compact dimensions, they provide a sample of states for which both momenta and winding numbers are non-vanishing, differently from the states ($N_L=1$, $N_R=1$). A quadratic action is therefore here built for the corresponding doubled fields. It results that its gauge invariance under the linearized double diffeomorphisms is based on a generalization of the usual weak constraint, giving rise to an extra mass term for the symmetric traceless tensor field, not otherwise detectable: this can be interpreted as a mere stringy effect in target space due to the simultaneous presence of momenta and windings. Furthermore, in the context of the generalized metric formulation, a non-linear extension of the gauge transformations is defined involving the constraint extended from the weak constraint that can be uniquely defined in triple products of fields. Finally, we show that the above mentioned stringy effect does not appear in the case of only one compact doubled space dimension.
Double field theory offers a manifest T-duality formulation for massless closed string field theory with momentum and winding excitations. In this work, we solve and study the properties of Friedmann-Robertson-Walker and doubled spherically symmetric metric of Double Field Theory.
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