Eliminating the optical depth nuisance from the CMB with 21 cm cosmology

TL;DR

This paper proposes using 21 cm cosmology observations to independently predict the optical depth parameter $ au$, thereby improving constraints on cosmological parameters and reducing degeneracies in CMB data analysis.

Contribution

It develops two novel methods leveraging 21 cm data to predict or constrain $ au$, enhancing the precision of cosmological parameter estimation from CMB observations.

Findings

Significant reduction in $ au$ error estimates with upcoming 21 cm observations.

Improved constraints on the primordial fluctuation amplitude $A_s$ by up to a factor of four.

Enhanced CMB constraints on neutrino mass sum, potentially down to 12 meV.

Abstract

Amongst standard model parameters that are constrained by cosmic microwave background (CMB) observations, the optical depth $\tau$ stands out as a nuisance parameter. While $\tau$ provides some crude limits on reionization, it also degrades constraints on other cosmological parameters. Here we explore how 21 cm cosmology---as a direct probe of reionization---can be used to independently predict $\tau$ in an effort to improve CMB parameter constraints. We develop two complementary schemes for doing so. The first uses 21 cm power spectrum observations in conjunction with semi-analytic simulations to predict $\tau$. The other uses global 21 cm measurements to directly constrain low redshift (post-reheating) contributions to $\tau$ in a relatively model-independent way. Forecasting the performance of the upcoming Hydrogen Epoch of Reionization Array, we find that significant reductions in the errors on $\tau$ can be achieved. These results are particularly effective at breaking the CMB degeneracy between $\tau$ and the amplitude of the primordial fluctuation spectrum $A_s$, with errors on $\ln (10^{10} A_s)$ reduced by up to a factor of four. Stage 4 CMB constraints on the neutrino mass sum are also improved, with errors potentially reduced to $12\,\textrm{meV}$ regardless of whether CMB experiments can precisely measure the reionization bump in polarization power spectra. Observations of the 21 cm line are therefore capable of improving not only our understanding of reionization astrophysics, but also of cosmology in general.