Probing the Global 21-cm Signal via the Integrated Sachs-Wolfe Effect on the 21-cm Background

We propose a novel method to probe the global 21-cm background. This background experiences the integrated Sachs-Wolfe effect (ISW) as the cosmic microwave background does. The 21-cm ISW is modulated by the spectral shape of the global 21-cm signal, and thus the measure of the 21-cm ISW will be a probe of the evolution of the global signal. With the phase-1 SKA telescope, probing the global 21-cm background would be feasible with 10000-hour observation, enabling consistency checks with existing measures of the global 21-cm signal by EDGES and SARAS that are conflicting with each other.Comment: submitted to PRL; 6 pages, 2 figures + supplemental materia