Raising the reionization optical depth with inflationary CMB features
Within the highly successful $Λ$CDM paradigm established with cosmic microwave background (CMB) anisotropy measurements, the optical depth through reionization $τ$ is the most uncertain due both to the difficulty in measuring large-angle polarization and the assumptions made in their interpretation. Currently, for the Planck primary data in the flat $Λ$CDM cosmology with slow-roll inflation and standard reionization, the one-sided 95% upper limit for $τ$ is $τ_{\rm max}=0.0696$. Yet when all current CMB measurements excluding large-angle polarization are combined with baryon acoustic oscillation (BAO) measurements, the one-sided 95% lower limit is an incompatible $τ_{\rm min}=0.074$. If the long-standing low-power feature of the temperature measurements is interpreted as physically originating from inflation then $τ$ inferred from large-angle polarization becomes larger. Marginalizing over templates of the low-power feature based on the generalized slow-roll formalism of inflation raises the Planck maximum to a more compatible $τ_{\rm max}=0.075$ which further increases to $τ_{\rm max} = 0.082$ with the inclusion of all CMB+BAO data. This marginalization does not assess the statistical significance of the low-power feature itself; rather, it shows that allowing a higher $τ$ is a consequence of interpreting the anomaly as a physical feature instead of a statistical fluctuation.