Observations of temperature fluctuations in the Cosmic Microwave Background (CMB) have played a central role in establishing our current model of cosmology. The next frontier in CMB observations is the measurement of polarized fluctuations, which could determine (and may have already have) if an epoch of cosmic inflation put the Bang in the Big Bang. Also, measurements of polarized CMB gravitational lensing have the potential to weigh the sum of neutrino masses by their effect on structure formation.
The POLARBEAR experiment has made some of the first measurements of the faint and hard-to-detect polarized gravitational lensing effect of the CMB. I will describe our new measurements and the detector technology we developed to achieve the required sensitivity, as well as the design aspects of the experiment to control systematic errors.
POLARBEAR is also searching for the signature of the Inflation in the early Universe. I will describe the current status of the field, including the recent BICEP2 results and the importance of separating a cosmic signal from galactic foreground emission. In the next few years, the single-telescope POLARBEAR experiment will be expanded to become the Simons Array, consisting of three telescopes. The Simons Array will have higher sensitivity than any current generation experiment, broad frequency coverage to separate galactic foreground emission, and stringent control of systematic errors. Finally, I will discuss long-term plans for CMB polarization measurements focusing on the ground-based CMB-S4 experiment and the LiteBIRD space mission.