Detecting primordial gravitational waves with circular polarization of the redshifted 21 cm line: II. Forecasts

TL;DR

This paper forecasts the potential of future 21 cm line polarization measurements to detect primordial gravitational waves by constraining the tensor-to-scalar ratio, emphasizing the need for advanced experimental capabilities.

Contribution

It provides detailed sensitivity forecasts for a large-scale FFTT to measure the CMB quadrupole via 21 cm polarization, linking it to primordial gravitational wave detection.

Findings

A 100 km FFTT could achieve  imes 10^{-3} sensitivity to r after ten years.

Detection depends on the evolution of Lyman-lpha flux, which is currently unconstrained.

Foreground polarization levels are estimated, highlighting the importance of mitigation strategies.

Abstract

In the first paper of this series, we showed that the CMB quadrupole at high redshifts results in a small circular polarization of the emitted 21 cm radiation. In this paper we forecast the sensitivity of future radio experiments to measure the CMB quadrupole during the era of first cosmic light ($z\sim 20$). The tomographic measurement of 21 cm circular polarization allows us to construct a 3D remote quadrupole field. Measuring the $B$-mode component of this remote quadrupole field can be used to put bounds on the tensor-to-scalar ratio $r$. We make Fisher forecasts for a future Fast Fourier Transform Telescope (FFTT), consisting of an array of dipole antennas in a compact grid configuration, as a function of array size and observation time. We find that a FFTT with a side length of 100 km can achieve $\sigma(r)\sim 4\times 10^{-3}$ after ten years of observation and with a sky coverage $f_{\mathrm{sky}}\sim 0.7$. The forecasts are dependent on the evolution of the Lyman-$\alpha$ flux in the pre-reionization era, that remains observationally unconstrained. Finally, we calculate the typical order of magnitudes for circular polarization foregrounds and comment on their mitigation strategies. We conclude that detection of primordial gravitational waves with 21 cm observations is in principle possible, so long as the primordial magnetic field amplitude is small, but would require a very futuristic experiment with corresponding advances in calibration and foreground suppression techniques.