The gravity of particle physics: dark matter, black holes, and axions

Perhaps chief among the mysteries of physics is the nature of dark matter (DM). In this thesis, I explore the physics of two DM candidates—primordial black holes (PBHs) and axions. Central to the thesis was the tension between gravity and particle physics, and the consequences that only arise when both are modelled. PBHs form in the early universe and so could be the DM. However, the Schwarzschild metric describes black holes in an empty background. In the early universe this is not applicable—the PBH solution must be cosmologically embedded. I explore the physics of cosmological black holes generally, before choosing a particularly viable metric—the Thakurta metric—for study. This PBH has an effective mass roughly proportional to the cosmological scale factor. We found that this greatly affects the landscape of PBH constraints. Binary formation in the early universe is significantly suppressed, evading the gravitational wave bounds on PBH abundance. In addition, these PBHs evaporate much more rapidly. As a result, we close entirely the previously-unconstrained asteroid mass range for PBHs. In the second half of my thesis, I examine a different DM candidate—the axion. This hypothetical particle was proposed to solve the strong-CP problem. However, it was shown recently that the inclusion of gravity spoils the axion solution to the strong-CP problem. The most natural solution to this new strong-gravity-CP problem is the introduction of a second coupled axion, which we called the `companion' axion. We recomputed the axion-photon constraints for the companion axion model before recomputing a number of cosmological considerations related to axions—perhaps most importantly, the misalignment mechanism. Notably, the `favored' DM regime was at lighter masses than currently probed by haloscopes. We also found that the domain-wall problem which plagues the standard axion scenario is automatically avoided