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    Generating deepest-yet cosmic microwave background polarization maps with the BICEP/Keck experiments
    (2024-09) Cheshire, James
    The ΛCDM concordance model of cosmology has seen considerable success in recent decades, describing the energy budget of the Universe as being dominated by a cosmological constant and a cold dark matter component, and having evolved from an initial hot Big Bang. Despite the success of this model, there are some questions which it does not address, namely that of the Universe's initial conditions. This puzzle, along with a few others, may be neatly accounted for via the inclusion of a period of exponential expansion, "cosmic inflation", early in the Universe's history. The generic paradigm of cosmic inflation has seen a wealth of indirect evidence, with the Gaussianity, adiabaticity, and near scale-invariance of the primordial perturbation power spectrum all matching inflationary predictions to exquisite precision. However, there exist other models which may produce such phenomena, and a unique signature of inflation is yet to be seen. Such a signature may arise via a stochastic background of primordial gravitational waves, a generic prediction of inflation. These gravitational waves would imprint themselves as a characteristic odd-parity "B-mode" polarization pattern in the cosmic microwave background, their amplitude parameterized through the tensor-to-scalar ratio, r. Fielding a series of small-aperture refracting polarimeters from the geographic South Pole, the BICEP/Keck program has set the most stringent constraints to-date on primordial gravitational waves from cosmic inflation via measurements of B-mode polarization in the cosmic microwave background using data through the 2018 observing season ("BK18"), σ(r)=0.009 (r < 0.036, 95% C.L.). BICEP/Keck has continued to collect a significant additional volume of data in the years since, including from the new BICEP Array instrument. The next planned release, containing data up to and including the 2023 observing season (dubbed "BK23"), expands the program's observing frequency coverage on both the low- and high-frequency sides to better understand foreground emission, more than doubles the data volume from BICEP3 at 95 GHz, and will be the product of a from-scratch reanalysis of both new and archival BK data which itself yields significant improvements. This dissertation describes the BICEP/Keck experiments and their recent progress, particularly detailing the reanalysis effort which has yielded the BK23 map set, and the still-ongoing procedure of validating those maps against an ensemble of simulations. The BK23 95 GHz polarization maps reach an effective depth of 1.75 µK-arcmin., making them the deepest maps of microwave background polarization made to-date. The unprecedented sensitivity of the BK23 map set, along with efforts to mitigate the effects of gravitational lensing-induced B-modes together with the South Pole Telescope, promise significant improvements in σ(r) and thus in constraints on the dynamics of cosmic inflation.