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

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 ∼ 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 σðrÞ ∼ 4 × 10 −3 after ten years of observation and with a sky coverage f sky ∼ 0.7. The forecasts are dependent on the evolution of the Lyman-α 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.

A Multi-messenger view of Cosmic Dawn: Conquering the Final Frontier

“…Further, it is possible to place limits on the fraction of dark matter in different sub-species, such as axions [253], by using the spin of black holes in Active Galactic Nuclei which emit radiation in the [OIII] forbidden line transition. Similarly, inflationary gravitational waves can be probed through the circular polarisation of the 21-cm line (e.g., [254]). An important use of the gravitational wave measurements from standard sirens is to constrain the value of the Hubble parameter and its evolution with cosmic time, shedding light into the presently unresolved problem of the discrepancy between the local and CMB measurements of the Hubble parameter (e.g., [255]).…”

Polarisation of high energy gamma-rays after scattering

“…The Hydrogen excitation that generates the circular polarisation signal is produced owing to the interaction of the Hydrogen with the CMB quadrupole moment and could be measured in the future with an array of dipole antennas. Such a signal would indicate the existence of primordial gravitational waves [44].…”

Constraining primordial black holes as dark matter using the global 21-cm signal with X-ray heating and excess radio background

“…Beyond these modelling efforts that aim to understand the EDGES result are the attempts to use the EDGES result for constraining various physical processes. Thanks to the richness of 21-cm physics, several studies on 21-cm signal have been theorised (many of which have existed even before the EDGES detection) to be useful to probe primordial magnetic fields [31][32][33][34][35][36], gravitational waves [37][38][39], warm dark matter [40][41][42][43][44], dark matter decay and annihilation [45][46][47][48], viscous dark matter [49] and many other exotic processes. We address one such possibility in this paper by considering the effect of primordial black holes (PBHs) on the 21-cm signal and deriving constraints on the abundance of PBHs from the EDGES measurement.…”