Papageorgiou, Alexandros2021-09-242021-09-242021-05https://hdl.handle.net/11299/224627University of Minnesota Ph.D. dissertation. May 2021. Major: Physics. Advisors: Jorge Vinals, Marco Peloso. 1 computer file (PDF); ix, 179 pages.The present thesis explores various effects that arise as a consequence of axion-gauge couplings in the early universe. Axions (or Axion-Like Particles) are pseudo-scalar particles that enjoy an approximate shift symmetry which protects the flatness of their potential from obtaining large radiative corrections. This property, as well as the fact that axions are abundantly predicted by high energy theories such as Supergravity and String Theory, makes the study of the phenomenology of axions in the early universe particularly interesting. Assuming that axions are present and cosmologically relevant in the early universe, it is a natural question to ask what effects may arise from couplings of axions to other fields such as boson or fermions. There is a unique shift symmetric, five-dimensional axion-gauge coupling which is expected in any axion theory. The gauge fields that are studied in the present work are either Abelian U(1) or non-Abelian SU(2) gauge fields. In both cases, the presence of the axion-gauge interaction modifies the dispersion relation of the various gauge field perturbations and under certain conditions one helicity of the perturbation degrees of freedom may become tachyonically unstable. These unstable perturbations are produced exponentially rapidly and their abundance can backreact on scalar and tensor perturbations leading to modified predictions for inflationary models compared to the predictions that are neglecting such contributions. An additional effect of these enhanced perturbations, is the production of a primordial lepton asymmetry which could in principle account for the matter-antimatter asymmetry observed today. Finally such enhanced perturbations could play a role in theories of Quintessential Dark Energy. In such theories, the Dark Energy component is a scalar field in a slow-roll configuration. The possibility that the Quintessence field is an axion field with a steep potential is explored. In this case slow-roll is maintained as a consequence of the axion producing the unstable perturbations at the expense of its own kinetic energy.enAxionsCosmologyGravitational WavesInflationLeptogenesisUniverseThesis or Dissertation