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Exotic Aspects of Extended Field Theories

Extended field theories (ExFTs) are a relatively young class of theories that lie at the intersection of Kaluza-Klein theory and the remarkable dualities of string- and M-theory. Whereas the original Kaluza-Klein construction unified the local symmetries of an Einstein-Maxwell-dilaton theory into diffeomorphisms in one dimension higher, ExFTs aim for a much more ambitious goal: to unify the local symmetries of supergravity fields into a single symmetry manifest on a higher-dimensional space. Depending on whether we start with Type II or 11-dimensional supergravity, we obtain double and exceptional field theory respectively which we collectively refer to as ExFTs. At the cost of being forced into a generalised notion of diffeomorphisms that fail to close onto an algebra, ExFTs embody a powerful paradigm built on the idea of unification-of symmetries, of fields and of solutions. However, ExFTs are much more than just a rewriting of supergravities. They have been found to contain much more than was originally put into their construction and, in this thesis, we discuss some of the more exotic aspects of these theories. We describe a novel solution in exceptional field theory that unifies a whole family of so-called 'exotic branes' into a single solution on the extended space. We follow this with the construction of a maximally non-Riemannian solution whose reduction to the usual spacetime is free of the scalar moduli that typically plague dimensional reductions. In the final part, we consider reductions between exceptional field theories and illustrate, amongst other things, that we can have ExFTs defined on local patches that nevertheless cannot be related by even the duality transformations of the lower-dimensional theory.