"The search for primordial gravitational waves, part 2 -- experimental perspective"
Abstract: The BICEP/Keck Array cosmic microwave background (CMB) polarization experiments located at the South Pole are a series of small-aperture refracting telescopes designed to probe the degree-scale B-mode signature of primordial gravitational waves. These highly-targeted experiments have produced the world's deepest maps of CMB polarization, leading to the most stringent constraints on the tensor-to-scalar ratio to date: r < 0.09 from B-modes alone, and r < 0.07 in combination with other datasets. These limits are rapidly improving with ongoing measurements at the multiple frequencies needed to separate Galactic foregrounds from the CMB, and in combination with higher-resolution experiments to remove B-modes induced by gravitational lensing. I will discuss the challenges that will be confronted as these measurements and planned "Stage 4" CMB efforts strive for sensitivity to primordial gravitational waves at levels r ~ 0.01 down to r < 0.001 in coming years.