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

TL;DR

This study investigates how uncertainties in the reionization history affect the ability of future CMB experiments like LiteBIRD to constrain primordial gravitational waves, finding that constraints are generally robust but can be biased by certain reionization scenarios.

Contribution

The paper extends previous analyses to quantify the impact of reionization uncertainties on primordial gravitational wave constraints from large-scale CMB polarization.

Findings

Constraints are stable under typical reionization models within 68% CL.

Certain reionization histories can bias PGW constraints beyond 68% CL.

Large-scale E-mode polarization can exclude biased reionization scenarios.

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.