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

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 $x_e^{\min }$, and a late reionization step at $z_{\mathrm{re}}$. We find that future high signal-to-noise measurements of the multipoles $10⩽\ell <20$ are especially important for breaking the degeneracy between $x_e^{\min }$ and $z_{\mathrm{re}}$. In addition, we show that the uncertainties on these parameters determined from a map with sensitivity $10\mu \mathrm{K}\mathrm{arcmin}$ 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.

13 pages, 10 figures, matches version accepted by ApJ, corrected author information

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