Seeing Sound Waves in the Early Universe

Temperature and polarization power spectra of the cosmic microwave background can provide essentially incontrovertible evidence for coherent acoustic oscillations in the early universe. A simple model calculation demonstrates explicitly how polarization couples to velocities at the surface of last scatter and is nearly independent of gravitational or density perturbations. For coherent acoustic oscillations, peaks in the temperature and polarization power spectra are precisely interleaved. If observed, such a signal would provide strong support for initial density perturbations on scales larger than the horizon, and thus for inflation.