  The astrophysics of cosmic dawn, when star formation commenced in the first
collapsed objects, is predicted to be revealed as 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 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 the 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 cosmic dawn; however, its
amplitude was substantially higher than that predicted by standard cosmological
models. Explanations for the amplitude of the profile included non-standard
cosmology, additional mechanisms for cooling the baryons, perhaps via
interactions with millicharged dark matter and an excess radio background at
redshifts beyond 17. 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.
