Bicep Array: Searching for Signals of Inflation From The South Pole

Abstract

The $\Lambda$CDM cosmological model posits a universe that began with a big bang - like singularity, which contains mostly cold dark matter, and which is experiencing an accelerating expansion due to a dark energy component. This model has experienced resounding success and is consistently upheld by experiments across the globe. The model is incomplete however, it cannot describe how the specific initial conditions required to create the universe we see today came about. Inflation is an extension to the $\Lambda$CDM model which hypothesizes that the universe underwent a period of exponential expansion just after the big bang, sufficient to set up the required initial conditions. Most inflationary theories predict a stochastic gravitational wave background generated as a result of this expansion which would have imprinted a characteristic B-mode signal into the polarization pattern of the Cosmic Microwave Background. Detecting this primordial gravitational wave signal would provide direct evidence for inflation The \textsc{Bicep}/{\it Keck} program constitutes a series of polarization sensitive microwave telescopes situated at the geographic South Pole targeting the degree-angular scale $B$-modes and searching for a primordial signal. Over the last two decades this program has consistently reported the tightest constraints on this signal, with the most recent analysis of data through $2018$ providing an upper limit on the tensor-to-scalar ratio $r<0.036$ at $95\%$ confidence. {\sc Bicep} Array is the latest experiment in the series and replaces the {\it Keck Array}, expanding the frequency coverage to two new low-frequency bands and, once fully operational, increasing the detector count by over an order of magnitude. {\sc Bicep} Array is expected to achieve $\sigma(r) = 0.002 - 0.004$ depending on foreground complexity and the degree of lensing removal. In this dissertation I cover the design of this new experiment -- with a focus on the design and performance of the cryogenics down to $4$\,K -- and the first year's observations. I analyze the first year of new low frequency data in combination with the recently release BK18 results and find that the new data provides no improvement on $\sigma(r)$. However, it provides significant constraining power on galactic synchrotron radiation resulting in a factor of two decrease in the uncertainty on the amplitude of this foreground signal.