Background image: Illustration of the directions of polarization of the cosmic microwave background as seen by the WMAP satellite.
Credit: NASA / WMAP Science Team
The cosmic background radiation is also polarized, as first discovered by the Degree Angular Scale Interferometer (DASI) experiment in 2002. The polarization can be divided up in two components: one carries additional information about the particle interactions in the hot plasma (E-modes), whereas the other carries information about primordial gravitational waves thought to be generated during the inflationary epoch (B-modes). The E-mode polarization spectrum was first obtained in 2004 by the Cosmic Background Imager (CBI). Detection of the far more elusive B-modes were announced by the South Pole Telescope (SPT) in July 2013.