FPO - Kevin T. Crowley - "In Search of Inflation: Tools for Cosmic Microwave Background Polarimetry"

Fri, Jul 27, 2018, 10:00 am
Location: 
Jadwin Hall room 202, Chair's confernece room

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 CDM 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 (Adv ACT) project, high­density 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.