Probing the Global 21 cm Background by Velocity-induced Dipole and Quadrupole Anisotropies

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 improve 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. We find that future 21 cm experiments can in principle detect the 21 cm dipole to high significance, potentially improving measurement accuracy of Earth velocity with respect to the Milky Way, galactic, and extragalactic foreground monopole spectra by an order of magnitude, as well as improving 21 cm astrophysical parameters by 2%–5%.