Impact of reionization on CMB polarization tests of slow-roll inflation

TL;DR

This paper investigates how incorrect assumptions about reionization history can bias inflationary parameter estimates from CMB B-mode polarization data, and proposes a principal component approach to mitigate this bias.

Contribution

It introduces a principal component method to account for reionization uncertainties, reducing bias in inflationary parameters from CMB polarization measurements.

Findings

Incorrect reionization assumptions can bias inflationary parameters.

Principal component parametrization effectively removes bias.

Method maintains precision with minimal degradation.

Abstract

Estimates of inflationary parameters from the CMB B-mode polarization spectrum on the largest scales depend on knowledge of the reionization history, especially at low tensor-to-scalar ratio. Assuming an incorrect reionization history in the analysis of such polarization data can strongly bias the inflationary parameters. One consequence is that the single-field slow-roll consistency relation between the tensor-to-scalar ratio and tensor tilt might be excluded with high significance even if this relation holds in reality. We explain the origin of the bias and present case studies with various tensor amplitudes and noise characteristics. A more model-independent approach can account for uncertainties about reionization, and we show that parametrizing the reionization history by a set of its principal components with respect to E-mode polarization removes the bias in inflationary parameter measurement with little degradation in precision.