Cosmic Echoes of the Early Universe: From Primordial Black Holes to Gravitational Waves

Abstract

Our framework for describing physics on quantum scales, the Standard Model of particle physics, is both gloriously successful and self-evidently incomplete. Generations of particle physicists have hoped to fill the gaps in our framework by looking for new physics in a few specific ways. For instance, it was hoped that collider experiments would reveal evidence of supersymmetry and that this would elucidate the nature of dark matter. Though these experiments have made numerous important discoveries, they have excluded or failed to furnish evidence for many previously leading theories, leaving the biggest questions unanswered. In the face of these unknowns, we are driven to search for new physics in new ways, and it is precisely in this context that the era of observational and theoretical cosmology is coming into its own. In this thesis, we investigate gravitational and particle phenomena in the early universe. The power of this cosmological approach lies in the fact that, in the early universe, temperatures and pressures were so extreme that now-rare high energy processes occurred frequently, and all forms of matter underwent dramatic changes as the universe cooled and expanded, leaving a record of these processes in a background of gravitational waves, cosmic neutrinos, large-scale structure, and the cosmic microwave background. Thus, the early universe functions as a laboratory for studying rare phenomena in which both ultra-small and ultra-large scale physics come into play, providing a window into otherwise inaccessible physical regimes. In this thesis, we aim to think globally about cosmological models. Rather than study the idiosyncratic features of a particular model, we have tried to identify generic features within large classes of well-motivated theoretical models to determine the testable predictions of these models that are within the range of existing and forthcoming experimental sensitivities.

This thesis is structured as follows: In Chapter 1, we present, for completeness, some background material and motivation for the questions we will address in the body of the thesis. In Chapter 2, we consider inflationary models that incorporate realistic features from high-energy physics—including multiple interacting scalar fields and nonminimal couplings to the spacetime Ricci scalar—that could produce PBHs with masses in the range required to address the present-day dark matter abundance. In Chapter 3, we perform a Markov Chain Monte Carlo (MCMC) analysis of a simple yet generic multifield inflation model characterized by two scalar fields coupled to each other and nonminimally coupled to gravity, fit to Planck 2018 cosmic microwave background (CMB) data. Chapter 4 proposes a formalism to describe relativistic hydrodynamics in spherical symmetry of a mixture of collisionless particles and a thermal equilibrium radiation gas, extending the formalisms of Refs.[213, 162] to include the effects of neutrino decoupling during the radiation dominated epoch on the formation and predicted abundance of SMBH seeds. In App. A, we present a detailed discussion of the evolution of adiabatic and isocurvature modes in multifield inflationary models. App. B delves into the supergravity embedding of the multifield inflationary model discussed in Chs. 2 and 3. In App. C we derive exact field-space trajectories of non-minimally coupled two-field inflation with a potential featuring a near-inflection point feature. App. D discusses the gravitational waves induced by scalar perturbations at second order, while App. E investigates the subtleties of adiabatic mode evolution during a transient period of ultra-slow-roll inflation. In App. F and G we present the derivations and definitions underlying the results of Ch. 4. Finally, we give concluding remarks and discuss ongoing and upcoming research directions in Ch. 5.