Observational Cosmology Seminar

Cosmic inflation is a theorized burst of exponential expansion that occurred approximately 10−35 s after the Big Bang. Inflation resolves the fine-tuned initial conditions in standard hot big bang cosmology. However, generic inflationary models make a prediction that has not yet been observed: the existence of a background of primordial gravitational waves (PGW), which would imprint a characteristic "B-mode" polarization pattern in the Cosmic Microwave Background (CMB). Detecting this inflationary B-mode signal would therefore provide compelling evidence for the inflation theory and open a rare observational window into grand unified theory scale physics. In this way, it is one of the most important pursuits of modern physics. We are now in a golden era of CMB observations, with B-mode experiments like BICEP/Keck leading the field, ruling out previously-favored inflation models and advancing our understanding of high energy physics. BA4-90/150, the fourth receiver to be deployed in the BICEP Array series, is a dichroic 90/150 GHz instrument targeting the frequency space where sensitivity to CMB polarization is maximized. The receiver will be deployed in the 2026–2027 austral summer, and is set to position BICEP Array to achieve exceptionally precise measurements of CMB polarization and strengthen constraints on inflationary models. I will present the hardware development program underway ahead of its commissioning. This includes the integration and optical characterization of a prototype focal plane module, alongside electromagnetic simulations of the feedhorn-coupled wafer stack to quantify and mitigate optical crosstalk between neighboring detectors. I will also discuss detector-level upgrades motivated by recent measurements in existing BA receivers.