Probing the global 21-cm background by velocity-induced dipole and quadrupole anisotropies

TL;DR

This paper explores how observer motion induces measurable dipole and quadrupole anisotropies in the cosmic 21-cm background, offering new ways to probe early universe processes and improve spectral measurements.

Contribution

It analyzes the feasibility of detecting velocity-induced anisotropies in the 21-cm background to enhance understanding of cosmic dawn and reionization.

Findings

Velocity-induced anisotropies can be used to probe the spectral shape of the 21-cm background.

Measurement of these anisotropies can help mitigate spectral ambiguities in global 21-cm observations.

The study discusses observational strategies and science drivers for detecting the dipole and quadrupole signals.

Abstract

The motion of an observer in the rest frame of the cosmic 21-cm background induces an anisotropy in the observed background, even when the background is isotropic. The induced anisotropy includes a dipole and a quadrupole, in the order decreasing in amplitude. If observed, these multipole anisotropies can be used as additional probes of the spectral shape of the global 21-cm background for mitigating the ambiguity in the monopole spectrum probed by single-element radio telescopes such as EDGES and SARAS. This could also help with understanding the astrophysical and cosmological processes that occurred during the cosmic dawn and the epoch of reionization, and even improving on the estimation of the solar velocity and the foreground spectra. Here, we study the feasibility of such observations and present science drivers for the measurement of the 21-cm dipole and quadrupole.