Detecting the integrated Sachs-Wolfe effect with high-redshift 21-cm surveys

TL;DR

This paper explores detecting the integrated Sachs-Wolfe effect by cross-correlating high-redshift 21-cm surveys with galaxies, proposing methods to confirm the effect using different observational techniques and instruments.

Contribution

It introduces a novel approach to detect the ISW effect using high-redshift 21-cm signals and analyzes the feasibility with current and future radio arrays.

Findings

Detection of unscattered photons is feasible with planned radio arrays.

Scattered photon detection requires advanced radio interferometers.

High correlation expected between 21-cm signals and CMB at peak frequencies.

Abstract

We investigate the possibility to detect the integrated Sachs-Wolfe (ISW) effect by cross-correlating 21-cm surveys at high redshifts with galaxies, in a way similar to the usual CMB-galaxy cross-correlation. The high-redshift 21-cm signal is dominated by CMB photons that travel freely without interacting with the intervening matter, and hence its late-time ISW signature should correlate extremely well with that of the CMB at its peak frequencies. Using the 21-cm temperature brightness instead of the CMB would thus be a further check of the detection of the ISW effect, measured by different instruments at different frequencies and suffering from different systematics. We also study the ISW effect on the photons that are scattered by HI clouds. We show that a detection of the unscattered photons is achievable with planned radio arrays, while one using scattered photons will require advanced radio interferometers, either an extended version of the planned Square Kilometre Array or futuristic experiments such as a lunar radio array.