The CMB is polarized: the CMB light waves do not vibrate in random directions. (This is due to an effect called Thomson scattering and it was discussed in this astrobite.) In the figure to the right, each line represents the direction of vibration of the light of the CMB. At the top, as an example, the light is polarized creating a pattern known as E-mode. At the bottom, the CMB presents B-mode polarization. When the CMB is lensed, the polarization patterns change shape: some of the patterns at the top of the figure above (E-modes) turn into patterns like the ones at the bottom (B-modes). Up to now, B-modes had not been detected, only upper limits were possible. In this letter, the South Pole Telescope collaboration, has measured B-modes in CMB polarization for the first time.How did they measure this? First, the CMB polarization map has to be decomposed into the contributions from E-modes and B-modes. Then, the effect of gravitational lensing on the CMB, represented by a deflection potential, Φ, needs to be estimated. This is done using a map of the Cosmic Infrared Background from the Herschel telescope, which traces the galaxies that are causing the lensing by observing their dust emission. The lensing potential is responsible for transferring the polarization E-modes into B-modes. By modelling how efficient this transfer should be, it is possible to construct the expected B-modes in the map from a combination of the measured E-modes, the lensing potential and the expected transfer between E- and B-modes. Then one can compare the expected B-modes to the measured ones, as shown in the figure below. The yellow, black and green points are measurements of this correlation between expected B-modes and measured B-mode polarization. The figure shows that B-modes have been detected (the points are very different from the null test, the gray bars) and the measurements are consistent with the expectation (black dashed line) from our current understanding of the Universe.
B-mode polarization from gravitational lensing can help set constraints on the amount of matter in the Universe (the source of gravitational lensing) and on cosmological parameters. However, B-modes can also arise from gravitational waves in the early Universe. In this letter, the South Pole Telescope collaboration have focused their efforts on measuring B-modes due to lensing. They suggest that these could, in principle, be subtracted from polarization maps of the CMB to look for B-modes from gravitational waves with the hope to learn more about the physics of the early Universe.