Toward a Complete Interpretation of Global 21 cm Absorption Signals Using the Radio CMB

Abstract

The global 21 cm signal is among the most direct probes of cosmic dawn, encoding information about when the first stars formed and how efficiently they heated their surroundings. However, such measurements are limited by bright Galactic foregrounds, ionospheric effects, and instrumental systematics. Even a confident detection would face a further obstacle: the brightness temperature depends on the spin temperature of the gas and the temperature of the radio background only through a single combination, so anomalous signals such as the EDGES (Bowman et al. 2018) absorption trough admit both cold gas and excess radio background interpretations. Recently, Holder et al. 2026 showed that resonant 21 cm absorption by high-redshift neutral hydrogen imprints an excess optical depth on CMB anisotropies at radio wavelengths, which depends on the spin temperature alone. Here we propose combining a measurement of the CMB hydrogen optical depth with the global signal to break this degeneracy, isolating the radio background temperature without the need for astrophysical modelling. Forecasting four interferometric configurations, we find that a compact $55 \times 55$ array of 6 m dishes observing for 4000 hours can establish an EDGES-like optical depth at $4.2σ$, while fiducial cosmic dawn histories require roughly an order of magnitude more collecting area. This constitutes a model-independent route to interpreting any global 21 cm detection.