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Theoretical Physics
Abstract:
Effective field theories are the way physics describes the world at different energies. Conversely, this means that any effective theory of our universe should couple to quantum gravity at high energies. Realizing that this is not always possible, the swampland program tries to delineate the conditions under which a low energy theory can be consistently completed with gravity in the UV. These conditions are called swampland conjectures. They give real predictions and bounds on the effective theories we should consider in beyond the standard model physics. For instance, it is conjectured that a long-lived de Sitter (dS) vacuum is simply impossible. In Anti-de Sitter (AdS), the magnitude of the cosmological constant is related to the mass of a tower of states, so that the energy cutoff of the effective theory goes to zero as we approach flat space. However, as the name “conjecture” already implies, these statements are not in general mathematically proven. Only a very few conjectures have been rigorously proven, and even then only in special subsectors of string theory. The bulk of the swampland conjectures takes inspiration and evidence from string theory and black hole physics. String theory as a natural theory of quantum gravity has a huge number of vacua, with many well characterized effective theories. This provides an enormous data set that swampland conjectures can be tested on. Unfortunately, the best understood vacua of string theory are not necessarily a representative set. With similar setups, only using standard D-branes, p-form fluxes and perturbative contributions, there is a danger that the swampland conjectures are a product of the lamppost effect. This is the motivation for the work presented in this thesis. We investigate the dS and AdS swampland conjectures in less explored regimes of string theory, in order to escape the lamppost. We introduce non-BPS branes and consider exotic string theories to try and get around various obstructions to finding dS vacua. Always we observe that while we do manage to circumvent obstructions, new problems appear. This confirms the no-dS conjecture also in exotic corners of the string landscape. Finally we consider non-perturbative contributions and find that here the AdS swampland conjectures have to be corrected by log-terms. In summary, we find that even in strange and new corners of string theory, and up to quantum log-corrections, the swampland conjectures still hold.
