Acoustic oscillations in the early universe, jumpstarted by superhorizon fluctuations left behind by inflation, give rise to a series of coherent peaks in the spatial power spectrum of the cosmic microwave background (CMB). An inevitable consequence of this scenario is that the CMB photons are left with small polarization anisotropies. Polarization patterns may be separated by parity into E-modes and B-modes; acoustic oscillations give rise only to E-modes. A recent detection of the E-modes agrees well with predictions. Many experiments are poised to make further measurements of E-modes, and to begin searching for B-modes, which should arise at subdegree scales because of weak lensing deflections of the original E-modes en route to the observer. At larger angular scales, gravitational waves from inflation could produce B-modes. Experimental prospects are outlined.
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