E-modes and B-modes in the CMB polarisation from SPT and Herschel data
A small portion of the Cosmic Microwave Background (CMB) is polarised, and the pattern observed in the polarised fraction can be split in two components, called E-modes and B-modes. These carry very different and complementary information about both the early and the late Universe. This image shows the data from the National Science Foundation's South Pole Telescope (SPT) and ESA's Herschel Space Observatory that were used to achieve the first detection of B-modes in the CMB polarisation.
When the CMB photons travel through the large-scale structure of the Universe, they get deflected by large concentrations of mass such as galaxies, galaxy clusters and the dark matter halos in which these are embedded. One of the effects of this distortion is a mixing of E- and B-modes: part of the signal contained in E-modes is transferred to the B-modes.
The left panel shows the E-mode component of the polarised CMB as detected by SPT. The E-modes are affected by gravitational lensing, and the effect of such distortion is encrypted in the image.
The central panel shows the projected gravitational potential of the large-scale distribution of matter present on the line of sight to the CMB in the same field as the one observed by SPT. The gravitational potential has been inferred using data from Herschel, which probed the light emitted by stars and re-radiated by cosmic dust in all galaxies across cosmic history.
Since gravitational lensing turns part of the E-modes into B-modes, it is possible to combine the observed (and distorted) E-modes with the intervening gravitational potential that distorts them, to estimate the resulting B-modes caused by the gravitational lensing effect. The right panel shows the B-modes of the CMB polarisation estimated in this way.