UC Berkeley

From the moment after the Big Bang to the present day, the universe has continually cooled and expanded. The Standard Model of particle physics predicts that as the temperature dropped, the universe underwent a series of phase transitions including the development of mass, the confinement of quarks and gluons into composite particles, and the formation of nuclei and atoms. But new physics beyond the Standard Model may also contribute to this rich thermal tapestry, whether by altering predicted Standard-Model phenomena or by introducing entirely new phase transitions earlier in the universe's history. By studying the early thermal history of the universe, we therefore hope not only to look into the past, but also to search for evidence of new physics beyond the reach of our current particle detectors.

Space-based gravitational wave observatories planned for the coming decades propose to make this search a reality. In the first half of this dissertation I explore the landscape of new physics models with phase transitions that could potentially give rise to detectable gravitational wave signals, highlighting particularly interesting cases that also shed light on baryogenesis and the nature of dark matter. But identifying models from a signal depends, conversely, on our ability to accurately predict the gravitational wave signals that they would produce. In the second half of this paper, I detail a novel, non-perturbative method for predicting these signals, offering the chance to dramatically improve on existing perturbative methods for strongly-coupled or super-cooled cases where non-perturbative effects dominate. Together, these two pieces form a theoretical programme that is essential to make sense of experimental results, and in doing so lead us from signal to new insights into the fundamental laws of physics.