Optical depth to reionization from perturbative 21cm clustering

TL;DR

Future 21cm reionization experiments, with precise large-scale measurements, could tightly constrain the optical depth τ, offering competitive insights into reionization timing and duration, but face challenges from astrophysical uncertainties and foregrounds.

Contribution

This work introduces a symmetries-based bias expansion approach to forecast τ constraints from 21cm clustering data, highlighting the importance of large-scale anisotropies and parametric models.

Findings

Future 21cm experiments can constrain reionization timing and duration.

Measurement of large-scale anisotropies is crucial for τ constraints.

Astrophysical uncertainties and foregrounds limit the precision of τ measurements.

Abstract

The optical depth $\tau$ is the least well determined parameter in the standard model of cosmology, and one whose precise value is important for both understanding reionization and for inferring fundamental physics from cosmological measurements. We forecast how well future epoch of reionization experiments could constraint $\tau$ using a symmetries-based bias expansion that highlights the special role played by anisotropies in the power spectrum on large scales. Given a parametric model for the ionization evolution inspired by the physical behavior of more detailed reionization simulations, we find that future 21cm experiments could place tight constraints on the timing and duration of reionization and hence constraints on $\tau$ that are competitive with proposed, space-based CMB missions provided they can measure $k\approx 0.1\,h\,\text{Mpc}^{-1}$ with a clean foreground wedge across redshifts spanning the most active periods of reionization, corresponding to ionization fractions $0.2 \lesssim x \lesssim 0.8$. Significantly improving upon existing CMB-based measurements with next-generation 21cm surveys would require substantially longer observations ($\sim5$ years) than standard $\mathcal{O}(1000 \,\,\text{hour})$ integration times. Precise measurements of smaller scales will not improve constraints on $\tau$ until a better understanding of the astrophysics of reionization is achieved. In the presence of noise and foregrounds even future 21cm experiments will struggle to constrain $\tau$ if the ionization evolution deviates significantly from simple parametric forms.