On the detection of a cosmic dawn signal in the radio background

The astrophysics of the cosmic dawn, when star formation commenced in the first collapsed objects, is predicted to be revealed by spectral and spatial signatures in the cosmic radio background at long wavelengths. The sky-averaged redshifted 21 cm absorption line of neutral hydrogen is a probe of the cosmic dawn. The line profile is determined by the evolving thermal state of the gas, radiation background, Lyman alpha radiation from stars scattering off cold primordial gas, and relative populations of the hyperfine spin levels in neutral hydrogen atoms. We report a radiometer measurement of the spectrum of the radio sky in the 55-85 MHz band, which shows that the profile found by Bowman et al. in data taken with the Experiment to Detect the Global Epoch of Reionization Signature (EDGES) low-band instrument is not of astrophysical origin; their best-fitting profile is rejected with 95.3% confidence. The profile was interpreted to be a signature of the cosmic dawn; however, its amplitude was substantially higher than that predicted by standard cosmological models. Our non-detection bears out earlier concerns and suggests that the profile found by Bowman et al. is not evidence for new astrophysics or non-standard cosmology. The EDGES team reported a measurement of the redshifted 21 cm absorption line of neutral hydrogen from the cosmic dawn. However, the SARAS 3 measurement of the radio sky spectrum now suggests that the EDGES detection might not have a cosmological source.

Automated summary

The astrophysics of the cosmic dawn, when star formation began, can be revealed by observing the cosmic radio background. However, recent measurements suggest that the previously reported signal is not of astrophysical origin and may be due to an anomaly in the standard cosmological models.