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 \lesssim 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 \Lambda CDM ( 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 .