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An absorption profile centred at 78 megahertz in the sky-averaged spectrum

Title	An absorption profile centred at 78 megahertz in the sky-averaged spectrum
Authors	Judd D. Bowman, Alan E. E. Rogers, Raul A. Monsalve, Thomas J. Mozdzen, Nivedita Mahesh
Magazine	Nature, Volume 555, Issue 7694, pp. 67-70
Date	10/13/2018
DOI	10.1038/nature25792
Introduction	The early Universe, following the formation of stars, saw their ultraviolet emissions permeate primordial hydrogen gas, modifying the excitation state of its 21-centimetre hyperfine line. This interaction led to the gas absorbing photons from the cosmic microwave background, resulting in a distinct spectral distortion detectable at radio frequencies below 200 megahertz. This study presents the observation of a flattened absorption profile within the sky-averaged radio spectrum. Centre at 78 megahertz, this profile exhibits a best-fitting full-width at half-maximum of 19 megahertz and an amplitude of 0.5 kelvin. While largely aligning with theoretical predictions for the 21-centimetre signal from early stars, the observed amplitude surpasses the highest predictions by over a factor of two. This divergence suggests that the primordial gas was either significantly colder than anticipated or the background radiation temperature was unexpectedly higher. Conventional astrophysical explanations, such as radiation from stars or stellar remnants, are insufficient to account for this difference. Among proposed extensions to the standard cosmological and particle physics models, only gas cooling due to dark matter-baryon interactions appears to explain the observed amplitude. The profile's low-frequency boundary indicates the presence of stars and Lyman-alpha photon production by 180 million years post-Big Bang, with the high-frequency boundary signifying gas heating above the radiation temperature within the subsequent 100 million years.
Quote	Bowman, J. D.; Rogers, A. E. E.; Monsalve, R. A.; Mozdzen, T. J.; Mahesh, N. An absorption profile centred at 78 megahertz in the sky-averaged spectrum. Nature 2018, 555, 67–70. https://doi.org/10.1038/nature25792
Element	Hydrogen (H)
Industry	Space , Research & Laboratory

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