Constraining Primordial Gravitational Waves with the BICEP/Keck Series of CMB Polarization Experiments*

The BICEP/Keck series of experiments have set leading limits on

primordial gravitational waves (PGW) though over 15 years of cosmic

microwave background (CMB) polarization measurements from the geographic

South Pole. BICEP/Keck consists of a series of small-aperture

refracting telescopes observing the CMB at a range of frequencies from 30 to

270 GHz. Measurements of the "B-mode" polarization of the CMB provide

a unique observational tool through which to characterize PGW, and

thus understand the physics of cosmic inflation in the very early

Universe. Such measurements have allowed BICEP/Keck to set the most

stringent limits to date on the primordial tensor-to-scalar ratio,

r, and thus on models of cosmic inflation. The most recent

publication, containing data up through 2018, has set the limit

r_0.05<0.036 (95% C.L.), with an uncertainty σ(r)=0.009. In

this talk, I will detail the progress of the BICEP/Keck program, our

current constraints, efforts at astrophysical foreground and polarized

systematic characterization and mitigation, limitations, and future

directions including the development and analysis of data from the new

BICEP Array and delensing of CMB B-mode data in conjunction with

SPT-3G.