July, 2013: B-mode polarization detected with SPTpol.

Scientists from the South Pole Telescope (SPT) project have detected subtle distortions referred to as 'B-modes' in the polarization of the Cosmic Microwave Background (CMB, preprint here). These distortions---detected for the first time in data from SPTpol, the polarization-sensitive receiver installed on the SPT in January, 2012---are thought to have only two sources: gravitational lensing of the more conventional 'E-mode' CMB polarization, and a background of gravitational waves from the epoch of cosmic inflation. The signals detected by SPTpol are due to gravitational lensing, and a sufficiently sensitive measurement of these signals will help us learn about neutrinos through their impact on the growth of structure in the universe. Successfully detecting this tiny B-mode signal also represents a major milestone along the way to using the CMB to learn about the earliest moments of the universe with the inflationary B modes.