Constraining Primordial Gravitational Waves Using Present and Future CMB Experiments

Abstract

Cosmic inflation is our current best theory for what occurred in the universe in the first instances of time. It postulates a brief period of exponential expansion in which quantum fluctuations are magnified to cosmic size and become the seeds for the growth of all structure in the Universe. Inflation makes a number of predictions, the most unique of which is the production of primordial gravitational waves (PGWs). Most of the predictions have since been tested, but the discovery of PGWs has eluded us to this day. The polarized Cosmic Microwave Background (CMB) is a powerful probe of these predictions, including the exciting possible existence of PGWs. This thesis provides a detailed account of the development of an optimal multi-component spectral-based likelihood analysis framework for joint analyses of heterogeneous multi-frequency CMB datasets, and its subsequent use for joint analysis of BICEP/Keck, Planck and WMAP CMB polarization data to derive the tightest constraints available on PGWs, parametrized by the tensor-to-scalar ratio. The manuscript also details the development of a spectral-based Fisher projection framework, specifically targeted towards optimizing tensor-to-scalar parameter constraints in the presence of galactic foregrounds and gravitational lensing of the CMB, that directly uses information from current BICEP/Keck achieved performances, to robustly forecast the science reach of upcoming CMB-polarization endeavors. This methodology allows for rapid iteration over experimental configurations and offers a flexible way to optimize the design of future experiments given a scientific goal. We document the use of this framework to perform forecasts for the next iteration of BICEP/Keck instrument -- BICEP-Array, as well as the next generation ground-based CMB experiment -- CMB-S4.