CMB polarization features from inflation versus reionization

Abstract

The angular power spectrum of the cosmic microwave background temperature anisotropy observed by WMAP has an anomalous dip at $\ensuremath{\ell}\ensuremath{\sim}20$ and a bump at $\ensuremath{\ell}\ensuremath{\sim}40$. One explanation for this structure is the presence of features in the primordial curvature power spectrum, possibly caused by a step in the inflationary potential. The detection of these features is only marginally significant from temperature data alone. However, the inflationary feature hypothesis predicts a specific shape for the $E$-mode polarization power spectrum with a structure similar to that observed in temperature at $\ensuremath{\ell}\ensuremath{\sim}20--40$. Measurement of the CMB polarization on few-degree scales can therefore be used as a consistency check of the hypothesis. The Planck satellite has the statistical sensitivity to confirm or rule out the model that best fits the temperature features with $3\ensuremath{\sigma}$ significance, assuming all other parameters are known. With a cosmic variance limited experiment, this significance improves to $8\ensuremath{\sigma}$. For tests of inflationary models that can explain both the dip and the bump in temperature, the primary source of uncertainty is confusion with polarization features created by a complex reionization history, which, at most, reduces the significance to $2.5\ensuremath{\sigma}$ for Planck and $5\ensuremath{\sigma}--6\ensuremath{\sigma}$ for an ideal experiment. Smoothing of the polarization spectrum by a large tensor component only slightly reduces the ability of polarization to test for inflationary features, as does requiring that polarization is consistent with the observed temperature spectrum, given the expected low level of $TE$ correlation on few-degree scales. If polarized foregrounds can be adequately subtracted, Planck will supply valuable evidence for or against features in the primordial power spectrum. A future high-sensitivity polarization satellite would enable a decisive test of the feature hypothesis and provide complementary information about the shape of a possible step in the inflationary potential.