BICEP Array Upgrades, Primordial Gravitational Wave Constraint Forecasts, and Low-Frequency Receiver Performance

The recently deployed BA1 receiver will allow BICEP Array (BA) to set competitive limits on galactic synchrotron as a contaminant to cosmic microwave background (CMB) polarization observations, and the full BA will continue the BICEP/Keck program's legacy of leading constraints on primordial gravitational waves (PGW). BA is an array of CMB polarimeters at the South Pole, seeking to constrain the amplitude of PGW via "B-mode" polarization. B-modes are now the path to improving constraints on the tensor-to-scalar ratio, r, given the cosmic-variance saturation of temperature-based probes. The first 30+40 GHz "BA1" receiver began observations in 2020, and the BA2/3/4 receivers at 150/95/220+270 GHz, respectively, are under construction and set to deploy imminently. These will allow the full BA to constrain r < 0.003 by 2027. BA will work with SPT-3G to account for the B-mode signal from gravitationally lensed E-modes, which has recently become BICEP/Keck's most limiting factor in constraining r. In this presentation, I will discuss the performance of BA1 in its first two observing seasons, ongoing efforts to upgrade BA to its full configuration, and forecasts for constraints on primordial gravitational waves.