Impact of reionization history on constraining primordial gravitational waves in future all-sky cosmic microwave background experiments

Abstract: We explore the impact of the reionization history on examining the shape of the power spectrum of the primordial gravitational waves (PGWs) with the cosmic microwave background (CMB) polarization. The large-scale CMB generated from the reionization epoch is important in probing the PGWs from all-sky experiments, such as LiteBIRD. The reionization model has been constrained by several astrophysical observations. However, its uncertainty could impact constraining models of the PGWs if we use large-scale CMB polarization. Here, by expanding the analysis of Mortonson & Hu (2007), we estimate how reionization uncertainty impacts constraints on a generic primordial tensor power spectrum. We assume that CMB polarization is measured by a LiteBIRD-like experiment and the tanh model is adopted for a theoretical template when we fit data. We show that constraints are almost unchanged even if the true reionization history is described by an exponential model, where all parameters are within 68% Confidence Level (CL). We also show an example of the reionization history that the constraints on the PGWs are biased more than 68% CL. Even in that case, using E-mode power spectrum on large scales would exclude such a scenario and make the PGW constraints robust against the reionization uncertainties.