In Search of Inflation: Tools for Cosmic Microwave Background Polarimetry

Abstract

The pursuit of knowledge of the early universe via the properties of the cosmic microwave background (CMB) is now being guided by the need to perform measurements of large-scale polarization patterns at a part in 100 million. In addition to confirming the ΛCDM concordance model shaped by the CMB, such studies pursue evidence of primordial tensor perturbations, which themselves could elucidate a period of inflation in the early universe. These tensor perturbations are imprinted on the CMB polarization as divergence-free patterns, known as B-modes. To make these demanding polarization measurements, increased instrumental sensitivity and control of systematics is required. As part of the Advanced ACTPol (AdvACT) project, high-density detector arrays of thousands of highly-sensitive bolometers were deployed on the Atacama Cosmology Telescope (ACT). The Atacama B-mode Search (ABS) instrument featured a polarization modulator system to control systematics and gain access to large-scale anisotropy modes otherwise masked by changing signals in the atmosphere. We describe aspects of these technologies, and their impact on CMB polarization studies.

In this thesis, I begin by presenting the standard model of the universe and discuss the promise of CMB polarization measurements. This motivates a discussion of current technologies progressing to an introduction of arrays of multiplexed bolometers. I discuss generic bolometer models involving superconducting thermistors, known as transition-edge sensors (TESes). These models are then compared to data on bolometer sensitivity and response acquired for AdvACT devices. I next describe the principle of polarization modulation using a continuously-rotating half-wave plate (CRHWP), including the signal injected into bolometer data thereby. I present a pipeline developed to investigate and remove this signal. Initial results from the 2017 run of silicon metamaterial CRHWPs on ACT are shown. Finally, I describe the maximum-likelihood pipeline developed as part of the ABS collaboration to constrain the parameter describing the power in primordial tensor perturbations, the tensor-to-scalar ratio r. The final published results for ABS are discussed. I conclude by considering the future development of high-sensitivity focal planes in the context of systematic error control, specifically detector non-linearity, for the Simons Observatory set of instruments, which are in the design phase.