Constraining exotic dark matter models with the dark ages 21-cm signal

Abstract

The dark ages 21-cm signal is a powerful tool for precision cosmology and probing new physics. We study two non-standard models: an excess radio background (ERB) model (possibly generated by dark matter decay) and the millicharged dark matter (mDM) model. These models were inspired by the possible EDGES detection of a strong global 21-cm absorption during cosmic dawn, but more generally they provide a way to anticipate the potential discovery space. During the dark ages the 21-cm global signal in the ERB model reaches a saturated form for an amplitude Ar = 0.4, where Ar is the radio background intensity at cosmic dawn relative to the cosmic microwave background. This amplitude is one fifth of the minimum required to explain the EDGES signal, and corresponds to just 0.1 per cent of the observed extragalactic background; it would give a signal that can be detected at 5.9σ significance (compared to 4.1σ for the standard signal) and can be distinguished from the standard (no ERB) signal at 8.5σ, all with a 1000 h global signal measurement. The 21-cm power spectrum has potentially more information, but far greater resources would be required for comparable constraints. For the mDM model, over a range of viable parameters, the global signal detection significance would be 4.7 − 7.2 σ, and it could be distinguished from standard at 2.2 − 9.3 σ. With an array of global signal antennas achieving an effective 100 000 h integration, the significance would be 10× better. Our analysis helps motivate the development of lunar and space-based dark ages experiments.