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...

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