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

TL;DR

This paper reports the detection of a 78 MHz absorption profile in the sky-averaged spectrum, consistent with early star formation effects, but with an amplitude larger than standard predictions, implying new physics or conditions in the early Universe.

Contribution

First detection of a sky-averaged 78 MHz absorption profile linked to early Universe star formation, challenging existing models with a larger-than-expected amplitude.

Findings

Absorption profile centered at 78 MHz with 19 MHz width

Amplitude of 0.5 Kelvin exceeds predictions by over a factor of two

Indicates early star activity and possible new physics in the early Universe

Abstract

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. This discrepancy suggests that either the primordial gas was much colder than expected or the background radiation temperature was hotter than expected. Astrophysical phenomena (such as radiation from stars and stellar remnants) are unlikely to account for this discrepancy, of the proposed extensions to the standard model of cosmology and particle physics, only cooling of the gas as a result of interactions between dark matter and baryons seems to explain the observed amplitude. The low-frequency edge of the observed profile indicates that stars existed and had produced a background of Lyman-alpha photons by 180 million years after the Big Bang. The high-frequency edge indicates that the gas was heated to above the radiation temperature less than 100 million years later.