Large field excursion of axion-like field and its cosmological implications

Abstract

We discuss a large field excursion of axion-like fields and its cosmological implications. This thesis consists of two parts: The first part is devoted to discussions about the inflationary expansion of the universe, and the role of an axion-like field in large field inflation. We discuss difficulties for realizing large field inflation, and consider a possibility that a pseudo Nambu-Goldstone boson (PNGB), especially an axion-like field, is identified as an inflaton field. Even if we identify a PNGB as an inflaton, it is difficult to realize trans-Planckian field excursion because of radiative corrections to the inflaton potential. In this regard, we introduce several models that make use of multiple number of axions to realize trans-Planckian field range of the inflaton field. We also discuss an issue related to weak gravity conjecture. When requiring the weak gravity conjecture, the inflaton potential may contain small modulations. We discuss how these modulations modify the primordial curvature power spectrum and how they are constrained by the observational data. In the second part of the thesis, we extend the discussion. We investigate the cosmological evolution of an axion-like field after the end of inflation. We show that the evolution of the axion-like field produces gauge bosons to which the axion couples under certain conditions. We identify such conditions, and discuss how it can be applied to the cosmological relaxion scenario, which is recently proposed to solve the hierarchy problem in the Standard Model.