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

Understand

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.