The cross-correlation of the CMB polarisation and the 21-cm line fluctuations from cosmic reionisation

TL;DR

This paper explores how the cross-correlation between CMB polarisation and 21 cm line fluctuations can provide detailed insights into the epoch and duration of cosmic reionisation, offering a new observational probe.

Contribution

It introduces a framework to analyze the cross-correlation's oscillations and damping effects, enhancing understanding of reionisation history through this novel approach.

Findings

The cross-correlation exhibits oscillations at all multipoles.

The first peak corresponds to the quadrupole at the probed redshift.

Damping on small scales constrains reionisation duration.

Abstract

The cosmic microwave background (CMB) polarisation and the 21 cm line fluctuations are powerful probes of cosmological reionisation. We study how the cross-correlation between the CMB polarisation (E-modes) and the 21 cm line fluctuations can be used to gain further understanding of the reionisation history, within the framework of inhomogeneous reionisation. Since the E-mode polarisation reflects the amplitude of the quadrupole component of the CMB temperature fluctuations, the angular power spectrum of the cross-correlation exhibits oscillations at all multipoles. The first peak of the power spectrum appears at the scale corresponding to the quadrupole at the redshift that is probed by the 21 cm line fluctuations. The peak reaches its maximum value in redshift when the average ionisation fraction of the universe is about half. On the other hand, on small scales, there is a damping that depends on the duration of reionisation. Thus, the cross-correlation between the CMB polarisation and the 21 cm line fluctuations has the potential to constrain accurately the epoch and the duration of reionisation.