Marcell Howard Dissertation Defense

Title: Issues of Gravitation in Cosmology Abstract: This thesis is the culmination of work focused on studying the various of issues present in the study of modern cosmology as well as how the solution to these various problems can reside within our understanding of gravity. We summarize some previous results spanning quantum cosmology to the scattering of cosmological gravitational wave backgrounds by primordial black holes as dark matter. We then pivot to a discussion of ongoing work involving the gravitational production of dark matter. Due to the lack of direct detections for weakly interacting massive particles, theorists have had to think about more exotic scenarios for producing dark matter in the early Universe. One viable alternative is gravitational particle production, i.e. the creation of particles due to the presence of a sufficiently strong gravitational field. Previous studies have shown that gravitational production of a light, minimally-coupled scalar field initially prepared in the Bunch-Davies vacuum can make up 100% of the dark matter, provided their mass was m = 1.5 x 10-5 eV. In this work, we extend that previous analysis to include self-interacting scalar fields and find the region in the (m, λ) parameter space where gravitational production coupled with parametric self-resonance can lead to a saturation of the dark matter bound i.e. Ω_{PP} = Ω_{DM} for a family of parameters by finding the level curve of constant Ω_{PP} = Ω_{DM}.