Baby Pictures of the Universe: Developing Low-Frequency Detector Modules for Cosmic Microwave Background Studies

Abstract

Although much progress has been made in the field of cosmology over the course of the past few decades, relatively little is known about the physics of the early universe. Specifically, inflation–the idea that the universe rapidly expanded soon after the Big Bang–is a popular and promising theory which remains to be definitely proven. Luckily, inflation would produce observable consequences: inflationary gravitational waves would leave an imprint on the Cosmic Microwave Background, or CMB, in the form of B-mode polarization patterns. In order to accurately identify the presence of inflationary B-modes, broad frequency coverage is crucial in order to remove spurious B-modes produced by thermal dust and by synchrotron radiation. So far, no inflationary B-modes have been observed. More precise CMB telescopes are necessary in order to hunt for existing B-modes, or, alternatively, to tighten constraints on the size of the B-mode polarization signal. The Simons Observatory (SO), a group of new telescopes to be sited in the Atacama Desert of Northern Chile, aims to fill this need by employing a record number of detectors to measure the CMB in multiple frequency bands. In this thesis, we first outline the scientific motivation for studying CMB polarization patterns; then explain the inner workings of the Simons Observatory, with a particular focus on the detector readout system; and finally describe the design and fabrication of a low-frequency routing wafer for the Simons Observatory. The routing wafer is a key component of the SO low-frequency detector modules, which observe at 30 and 40 GHz and which serve to subtract off B-modes produced by synchrotron radiation. We have proven the routing wafer fabrication process by fabricating and testing a fully functional low-frequency routing wafer. The wafer has been successfully tested with the detector readout system, but has yet to be tested with the detectors themselves, because the low-frequency detectors are still in a research and development phase. The routing wafer works well as a prototype, and the now-proven design and fabrication process opens up multiple avenues for future work.