Newly sighted microwave-background polarization mode

The cosmic microwave background (CMB) has been streaming freely since the dawn of transparency, when the universe was only 4 × 10⁵ years old and still incandescent. Now cooled to 2.7 K by 10¹⁰ years of cosmic expansion, the CMB still carries valuable imprints of the Big Bang, especially in its tiny departures from perfect isotropy. Harder to observe than the temperature anisotropies, which have for decades been a mainstay of cosmology’s standard model (see Physics Today, June 2013, page 18 ), are the much weaker spatial fluctuations in the CMB’s polarization field—in particular their divergence-free B mode. (The designation B is meant to recall the divergence-free B field of electromagnetism.) The B mode is prized as a unique probe of primordial gravity waves thought to have been generated in the Big Bang. Now a collaboration using data from new polarization-sensitive detectors on the South Pole Telescope (shown in the photo) has reported the first detection of the CMB’s B mode. The surveyed field of view, 10° × 10°, is still too small to reveal the large-angular-scale primordial gravity waves. But the team’s analysis of the polarization data, in conjunction with the Herschel Space Observatory’s far-IR survey of the same patch of sky, has revealed the expected B-mode signature of gravitational lensing, a valuable probe of the mass distribution of the later cosmos. Furthermore, such lensing maps can be subtracted from larger B-mode surveys to facilitate the ongoing search for the primordial gravity waves. (D. Hanson et al., Phys. Rev. Lett.111, 141301, 2013, doi:10.1103/PhysRevLett.111.141301 .)