Strong Hydrogen Absorption at Cosmic Dawn: the Signature of a Baryonic Universe

Abstract

The recently reported detection of redshifted 21cm absorption at $z \approx 17$ is a significant advance in the exploration of the cosmic dark ages. The observed signal ($T_{\mathrm{21}} \approx -0.5$ K with the limit $T_{\mathrm{21}} < -0.3$ K at 99\% c.l.) is anomalously strong for $\Lambda$CDM, which predicts at most $-0.24$ K. Here I show that the strong observed signal is expected in a purely baryonic universe.