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Towards underlying quantum gravity constraints on string inflation
Towards underlying quantum gravity constraints on string inflation
Inflation is a widely accepted concept in cosmology proposing an accelerated expansion of the very early universe. For the class of large-field inflation models the energy driving the expansion arises from a scalar inflaton field that traverses trans-Planckian distances in a suitable potential. This thesis aims to discuss whether there exist underlying string theory or quantum gravity principles constraining/forbidding large-field inflation. Our framework is axion inflation and its interplay with moduli stabilization in string theory. Axionic inflaton fields appear naturally in string compactifications and are protected from UV corrections due to their shift symmetry. The thesis is basically organized as follows: first, attempting to engineer a fully-fledged model of large-field inflation within string theory and second, analyzing possible underlying quantum gravity reasons to explain the ubiquitous control issues. More precisely, we investigate aligned inflation in the vicinity of a conifold in the complex structure moduli space as well as axion monodromy inflation for a D7-brane position modulus. The ultimate failure of all scenarios boils down to the violation of a sophisticated mass hierarchy that is required to justify the employed effective field theories. These obstacles can be traced back to the swampland conjectures which had been claimed to hold generically for effective theories deduced from quantum gravity. In order to gather more evidence for these conjectures we investigate geodesic distances in moduli spaces of various Calabi-Yau manifolds. Our results strongly support one of the swampland conjectures that predicts a break down of the effective theory of inflation as soon as one moves trans-Planckian distances. If true, parametrically controllable models of large single field inflation seem to be impossible in string theory.
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Wolf, Florian
2018
English
Universitätsbibliothek der Ludwig-Maximilians-Universität München
Wolf, Florian (2018): Towards underlying quantum gravity constraints on string inflation. Dissertation, LMU München: Faculty of Physics
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Abstract

Inflation is a widely accepted concept in cosmology proposing an accelerated expansion of the very early universe. For the class of large-field inflation models the energy driving the expansion arises from a scalar inflaton field that traverses trans-Planckian distances in a suitable potential. This thesis aims to discuss whether there exist underlying string theory or quantum gravity principles constraining/forbidding large-field inflation. Our framework is axion inflation and its interplay with moduli stabilization in string theory. Axionic inflaton fields appear naturally in string compactifications and are protected from UV corrections due to their shift symmetry. The thesis is basically organized as follows: first, attempting to engineer a fully-fledged model of large-field inflation within string theory and second, analyzing possible underlying quantum gravity reasons to explain the ubiquitous control issues. More precisely, we investigate aligned inflation in the vicinity of a conifold in the complex structure moduli space as well as axion monodromy inflation for a D7-brane position modulus. The ultimate failure of all scenarios boils down to the violation of a sophisticated mass hierarchy that is required to justify the employed effective field theories. These obstacles can be traced back to the swampland conjectures which had been claimed to hold generically for effective theories deduced from quantum gravity. In order to gather more evidence for these conjectures we investigate geodesic distances in moduli spaces of various Calabi-Yau manifolds. Our results strongly support one of the swampland conjectures that predicts a break down of the effective theory of inflation as soon as one moves trans-Planckian distances. If true, parametrically controllable models of large single field inflation seem to be impossible in string theory.