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These notes are an expanded version of lectures given at the 2022 TASI summer school in
Boulder, Colorado. One goal of these lecture notes is to (partially) bridge the gap between …

A bstract We study the self-energies of weakly interacting scalar fields in de Sitter space with
one field much lighter than the Hubble scale. We argue that self-energies drastically simplify …

A bstract The classification of 4D reflexive polytopes by Kreuzer and Skarke allows for a
systematic construction of Calabi-Yau hypersurfaces as fine, regular, star triangulations …

Axion strings are horizon-size topological defects that may be produced in the early
Universe. Ultralight axion-like particles may form strings that persist to temperatures below …

A bstract It is widely believed that string theory easily allows for a QCD axion in the
cosmologically favored mass range. The required small decay constant, fa≪ MP, can be …

We study axion-photon couplings in compactifications of type IIB string theory. We find that
these couplings are systematically suppressed compared to the inverse axion periodicity, as …

A bstract The quantum chromodynamics axion with a decay constant near the Grand
Unification (GUT) scale has an ultralight mass near a neV. We show, however, that axion …

A bstract We study the dynamics of scalar fields with compact field spaces, or axions, in de
Sitter space. We argue that the field space topology can qualitatively affect the physics of …

A bstract An axion-like-particle (ALP) in the post-inflationary scenario with domain wall
number N> 1 can be dark matter if the residual ℤ N symmetry has a small explicit breaking …