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. This strategy naturally mitigates the impact of the Milky Way foreground, which is a common and most significant challenge in conventional 21 -cm background probes. With the phase -1 SKA telescope, probing the global 21 -cm background would be feasible with a few 1000 hours of observation, enabling consistency checks with existing measures of the global 21 -cm signal by EDGES and SARAS that are conflicting with each other.