A strong broadband 21 cm cosmological signal from dark matter spin-flip interactions

In the standard cosmology, it is believed that there are two weak and distinct band-limited absorption features, near 20 MHz (z ~ 70) and 90 MHz (z ~ 15) in the global cosmological 21 cm signal which are signatures of collisional gas dynamics in the cosmic dark ages and Lyman-alpha photons from the first stars at cosmic dawn, respectively. A similar prediction of two distinct band-limited, but stronger, absorption features is expected in models with excess gas cooling, which have been invoked to explain the anomalous EDGES signal. In this work, we explore a novel mechanism, where dark matter spin-flip interactions with electrons through a light axial vector mediator could directly induce a 21 cm signal which is characteristically different from either of these. We find generically, that our model predicts a strong, broadband absorption signal extending from frequencies as low as 1.4 MHz (z ~ 1000), from early in the cosmic dark ages where no conventional signal is expected, all the way up to 90 MHz, depending upon the epoch of star formation and X-ray heating. We will discuss a rich set of spectral features that could be probed in current and future experiments looking for the global 21 cm signal as well as some complementary laboratory tests of short range spin-dependent forces between electrons.