arising from J. D. Bowman, A. E. E. Rogers, R. A. Monsalve, T. J. Mozdzen & N. Mahesh Nature 555, 67–70 (2018); https://doi.org/10.1038/nature25792

It is predicted1 that the spectrum of radio emission from the whole sky should show a dip arising from the action of the light from the first stars on the hydrogen atoms in the surrounding gas, which causes the 21-cm line to appear in absorption against the cosmic microwave background. Bowman et al.2 identified a broad flat-bottomed absorption profile centred at 78 MHz, which could be this feature, although the depth of the profile is much larger than expected. We have examined the modelling process they used and find that their data implies unphysical parameters for the foreground emission and also that their solution is not unique, in the sense that we found other simple formulations for the signal that are different in shape but that also fit their data. We argue that this calls into question the interpretation of these data as an unambiguous detection of the cosmological 21-cm absorption signature. There is a Reply to this Comment by Bowman, J. D. et al. Nature 564, https://doi.org/10.1038/s41586-018-0797-4 (2018).

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