  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.
