DEPARTMENT OF PHYSICS DISSERTATION DEFENSE: Tongtian Liu

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Title:

Searching for Beyond ΛCDM Physics with BICEP/Keck CMB Polarization Data

Abstract:

The ΛCDM model provides a robust framework for our understanding of the universe, yet persistent large-scale anomalies in the Cosmic Microwave Background (CMB) suggest that it is incomplete. Since 2006, the BICEP/Keck Array experiments at the South Pole have led the search for primordial B-mode polarization, while simultaneously providing powerful constraints on physics beyond the standard model. As the program transitions to the BICEP Array era, achieving higher sensitivity necessitates increasing detector counts, which in turn requires optimized readout systems and hardware integration to maintain this trajectory of constraining power. This dissertation details the development of the BICEP Array receivers and the subsequent analysis of BICEP/Keck data for signatures beyond the standard cosmological model. We focus the discussion on the quality assurance of SQUID systems and the integration of high-sensitivity detector modules, which are essential for measuring the signals of thousands of transition-edge sensors. Using this data for cosmological analysis, we investigate the statistical significance of large-scale anomalies, specifically the CMB Cold Spot and non-Gaussianity in the BBB bispectrum. In particular, we focus on a search for multipole-dependent cosmic birefringence: a rotation of the CMB polarization plane that varies with angular scale. We implement a model-independent step-function estimator to identify potential ell-dependence, providing a first-order test for early dark energy and axion-like particle couplings while minimizing the number of free parameters. We validate our pipelines using extensive suites of simulations, providing new constraints on the parity-violating properties of the early universe.