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Abstract

In this thesis we study landscape constructions to challenge existing swampland conjectures and extend established ones in order to expand our understanding of quantum gravity and its possible consequences for phenomenology. We construct a new class of axions in Klebanov-Strassler throats including a four-dimensional supergravity formulation. We show that the axion and associated instantons violate the axionic weak gravity conjecture in its simplest form and that the instantons consistent with it populate very coarse charge sub-lattices. Based on the winding scenario, we show that light axionic directions of field space can posses tunable, positive minima in their scalar potential. This gives a controlled supersymmetry-breaking and uplifting mechanism which may be applied in known AdS vacua. We discuss stability of these solutions. We propose a bound on the quality of global symmetries that are derived from gauge symmetries. This involves a Stückelberg axion as well as instantons coupling to it. Integrating over instanton insertions leads to global-symmetry-violating operators. Using the axionic weak gravity conjecture the coefficient of the operator may be bounded from below. In the spirit of the weak gravity conjecture, we constrain attractive forces in the absence of gauge charges. We claim that the minimal size of a bound state is governed by gravitational forces alone and arrive at a bound state conjecture: In an EFT, there is a universal lower bound on the typical radius of any bound state.