Beyond optical depth: Future determination of ionization history from the CMB

TL;DR

This paper investigates the fundamental limits of constraining the universe's reionization history using large-scale CMB anisotropy measurements, highlighting the importance of TE data and high signal-to-noise E-mode observations.

Contribution

It introduces a detailed analysis of how different CMB polarization measurements can improve constraints on reionization history, including a two-step reionization model with high redshift ionization.

Findings

TE power spectrum adds 20% to optical depth precision.

High S/N measurements of low multipoles are crucial for parameter degeneracy.

Future measurements at 10 μK·arcmin sensitivity can achieve near noiseless constraints.

Abstract

We explore the fundamental limits to which reionization histories can be constrained using only large-scale cosmic microwave background (CMB) anisotropy measurements. The redshift distribution of the fractional ionization $x_e(z)$ affects the angular distribution of CMB polarization. We project constraints on the reionization history of the universe using low-noise full-sky temperature and E-mode measurements of the CMB. We show that the measured TE power spectrum, $\hat C_\ell^\mathrm{TE}$, has roughly one quarter of the constraining power of $\hat C_\ell^\mathrm{EE}$ on the reionization optical depth $\tau$, and its addition improves the precision on $\tau$ by 20% over using $\hat C_\ell^\mathrm{EE}$ only. We also use a two-step reionization model with an additional high redshift step, parametrized by an early ionization fraction $x_e^\mathrm{min}$, and a late reionization step at $z_\mathrm{re}$. We find that future high signal-to-noise measurements of the multipoles $10\leqslant\ell<20$ are especially important for breaking the degeneracy between $x_e^\mathrm{min}$ and $z_\mathrm{re}$. In addition, we show that the uncertainties on these parameters determined from a map with sensitivity $10\,\mathrm{\mu K\,arcmin}$ are less than 5% larger than the uncertainties in the noiseless case, making this noise level a natural target for future large sky area E-mode measurements.