From Models of the Early Universe to Developing Experiments for Axionic Dark Matter Detection

Abstract

Modern physical cosmology is thriving with abundant data and new observations from the James Webb Space Telescope, multiple CMB experiments, and new surveys of galaxies from the ground and space. This thesis uses theoretical and experimental approaches to discuss two important topics of cosmology: early universe curvature perturbations and dark matter. One of the critical questions posed to theoretical cosmology is to explain the origin of temperature fluctuations seen on the cosmic microwave background. In the first part of the thesis, we explore a new entropic mechanism for generating curvature perturbations in the context of non-singular bouncing cosmologies. A gauge-invariant hydrodynamic and a microphysical approach are used to demonstrate the mechanism. We show that the curvature perturbations can be generated from the entropic ones in a cosmological model with two kinetically coupled scalar fields, an arbitrary potential, and a higher-order (quartic) kinetic term. We also present a concrete example of a cosmological scenario in which a nearly scale-invariant spectrum of curvature perturbations is generated on super-Hubble scales at the end of the phase of the universe contraction in accordance with the current observations. In the second part of the thesis, we discuss an experimental search for dark matter. Multiple pieces of indirect evidence suggest that about one-fourth of the universe’s energy budget consists of dark matter, but it has yet to be observed. In particular, we focus on sub-ueV axion as one of the most promising dark matter candidates. We discuss experimental approaches in the search for sub-ueV axions and argue that developing a high-Q superconducting lumped-element resonator is one of the key elements of the upcoming dark matter search experiments. We then demonstrate our current development of a new state-of-the-art resonator with a Q factor of 710,000 at 301 kHz, the highest value recorded near this frequency.