Dark Begets Light: Exploring Physics Beyond the Standard Model with Cosmology

Abstract

After nearly a century of inquiry, the particle nature of dark matter remains unknown. As a wide array of Dark Matter (DM) phenomenologies map onto similar variations in cosmological observ- ables, cosmology places far-reaching constraints on the theoretical parameter space of Dark Matter models beyond the Standard Model (SM) of particle physics. This dissertation presents the results of several inquiries that demonstrate the use of cosmology to measure the parameter space for differ- ent classes of DM models assuming only gravitational interactions. This dissertation first considers an extension of ΛCDM involving additional particle degrees of freedom of non-zero mass in early thermal contact with the SM, which are called Light but Massive Relics (LiMRs). LiMRs introduce a characteristic scale into the cosmology through their free-streaming while relativistic. Effects on the distribution of galaxies and the Cosmic Microwave Background (CMB) are explored and used to impose constraints on the mass, abundance, and degrees of freedom of LiMRs. Implications for measuring the mass and hierarchy of massive neutrinos, a special case of LiMR, are considered. Representing another class of DM models, ultralight axion-like particles (ALPs), like LiMRs, in- troduce characteristic scales into the cosmology: one set by the oscillation time of the field; another characterized by the macroscopic wavelength of the field, the Jeans Scale. The nonlinear connec- tion between matter perturbations and halo perturbations, the halo bias, is modelled in the presence LiMRs, neutrinos, and ultralight ALPs. Gravitational waves (GWs), a burgeoning source of cosmo- logical information, motivate the concluding study of this dissertation which considers the degree to which mHz-Hz frequency GW detectors can infer the formation channels of black holes - a process which may be influenced by the introduction of new physics to the SM and ΛCDM.