The 21-cm global signal is cosmology's deepest accessible probe: the sky-averaged radio spectrum of neutral hydrogen at Cosmic Dawn, when the first luminous sources coupled the gas's spin temperature to its cold kinetic temperature and stamped an absorption trough against the background radiation. The EDGES experiment reported that trough at 78 MHz, redshift near 17, with a stunning amplitude of -500 mK, roughly twice the deepest absorption standard ΛCDM thermodynamics permits (Bowman et al. 2018).
The depth is the physics problem; the verification is the experimental one. Standard adiabatic cooling caps the trough near -230 mK, so a real EDGES signal requires either gas colder than adiabatic, the motivation for dark-matter-baryon scattering models, or a radio background at Cosmic Dawn brighter than the CMB alone, requiring early radio-luminous sources the standard inventory does not supply. Both options breach the model's boundaries. Meanwhile SARAS 3, a differently designed experiment, failed to confirm the absorption profile, leaving open whether EDGES detected cosmology or an instrumental artifact shaped by foreground modeling (Singh et al. 2022). The tension is therefore double: between EDGES and theory, and between EDGES and SARAS 3.
The standing awaits adjudication by the next experimental generation, REACH, MIST, and lunar-farside concepts, with the radio-background question independently pressured by the ARCADE 2 excess at low redshift.