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

Understand

After stars formed in the early Universe, their ultraviolet light is expected, eventually, to have penetrated the primordial hydrogen gas and altered the excitation state of its 21-centimetre hyperfine line.

• This alteration would cause the gas to absorb photons from the cosmic microwave background, producing a spectral distortion that should be observable today at radio frequencies of less than 200 megahertz.
• Here we report the detection of a flattened absorption profile in the sky-averaged radio spectrum, which is centred at a frequency of 78 megahertz and has a best-fitting full-width at half-maximum of 19 megahertz and an amplitude of 0.5 kelvin.
• The profile is largely consistent with expectations for the 21-centimetre signal induced by early stars, however, the best-fitting amplitude of the profile is more than a factor of two greater than the largest predictions.