The Global 21-Centimetre Signal with a Bounce Cutoff: A Shift in the Absorption Trough

The global 21-centimetre signal from cosmic dawn, an absorption trough in the radio background centred near redshift seventeen in standard models, is timed by the first stars: their ultraviolet light couples the hydrogen spin temperature to the gas temperature, and their X-rays later heat the gas and end the absorption. For a cutoff at 10 per megaparsec the trough centre moves to redshift 12.5, corresponding to an observed frequency of 104.8 megahertz, against the standard centre near redshift 17 and 79 megahertz. Future global-signal experiments aiming at few-megahertz precision on the trough position will therefore test the same cutoff range as the dwarf and variance probes, in an entirely different observational band, which is precisely the kind of cross-check a physical cutoff should invite. Computed from stated assumptions, not measured.