Detection of CMB B-mode Polarization by SPT

Last week, members of the South Pole Telescope (SPT) collaboration reported the first detection of B-mode polarization in the Cosmic Microwave Background (CMB). Scientists have been searching for B-mode polarization for over a decade because it offers an opportunity to study all the matter in the Universe and the cause of the Big Bang. So far, the measurements agree with the predictions of the existing standard cosmological models, but upcoming more sensitive measurements will allow us to refine and test those models.

The CMB is relic radiation from the time when electrons and protons combined to form hydrogen atoms 400,000 years after the Big Bang (about 14 billion years ago). Although the CMB is mostly uniform, tiny nano-Kelvin variations in its temperature across the sky are clues to cosmology. CMB polarization (the same principle that works in polarized sunglasses) comes in two patterns called E-modes and B-modes. The B-modes are important because there are only two known sources in cosmology: inflation and gravitational lensing.

The B-modes found by SPT were created by gravitational lensing. Matter in the Universe (stars, galaxies, etc.) creates gravitational fields, and these gravitational fields deflect light as it travels to us. This includes the light of the CMB, and the deflection distorts the pattern of the CMB, creating polarization of the B-mode type. By analyzing the pattern of deflection, scientists can calculate the distribution of matter that would cause that pattern, eventually leading to a map of all the matter in the Universe. So far, SPT scientists have used data from one year of observation to confirm that the amount of lensing is as expected. Future expected results from SPT and other similar telescopes include measurement of the mass of the neutrino and constraints on dark energy. Because gravitational lensing is a contaminating signal for measurements of inflationary B-modes, high-quality lensing maps will eventually lead to cleaner inflationary B-mode measurements.