Dipole of the Epoch of Reionization 21-cm signal

TL;DR

Measuring the dipole of the Epoch of Reionization 21-cm signal, caused by our solar system's motion, offers advantages in understanding cosmic velocity and mitigating systematics, despite the signal's weaker strength.

Contribution

This paper highlights the potential benefits of detecting the EoR 21-cm dipole, emphasizing its unique directional information and systematic advantages over monopole measurements.

Findings

Dipole signal is modulated by solar system motion.

Dipole measurement can be more robust against systematics.

Direction of cosmic velocity is precisely known from CMB data.

Abstract

The motion of the solar system with respect to the cosmic rest frame modulates the monopole of the Epoch of Reionization 21-cm signal into a dipole. This dipole has a characteristic frequency dependence that is dominated by the frequency derivative of the monopole signal. We argue that although the signal is weaker by a factor of $\sim100$, there are significant benefits in measuring the dipole. Most importantly, the direction of the cosmic velocity vector is known exquisitely well from the cosmic microwave background and is not aligned with the galaxy velocity vector that modulates the foreground monopole. Moreover, an experiment designed to measure a dipole can rely on differencing patches of the sky rather than making an absolute signal measurement, which helps with some systematic effects.