Evidence for horizon-scale power from CMB polarization

The CMB temperature power spectrum offers ambiguous evidence for the existence of horizon-scale power in the primordial power spectrum due to uncertainties in spatial curvature and the physics of cosmic acceleration as well as the observed low quadrupole. Current polarization data from WMAP provide evidence for horizon-scale power that is robust to these uncertainties. Polarization on the largest scales arises mainly from scattering at z<6 when the universe is fully ionized, making the evidence robust to ionization history variations at higher redshifts as well. A cutoff in the power spectrum is limited to C=k_C/10^{-4} Mpc^{-1}<5.2 (95% CL) by polarization, only slightly weaker than joint temperature and polarization constraints in flat LCDM (C<4.2). Planck should improve the polarization limit to C<3.6 for any model of the acceleration epoch and ionization history as well as provide tests for foreground and systematic contamination.