Letter | Published:

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

Nature volume 555, pages 6770 (01 March 2018) | Download Citation

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 megahertz1. 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 predictions2. 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 amplitude3. The low-frequency edge of the observed profile indicates that stars existed and had produced a background of Lyman-α 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.

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Acknowledgements

We thank CSIRO for providing site infrastructure and access to facilities. We thank the MRO Support Facility team, especially M. Reay, L. Puls, J. Morris, S. Jackson, B. Hiscock and K. Ferguson. We thank C. Bowman, D. Cele, C. Eckert, L. Johnson, M. Goodrich, H. Mani, J. Traffie and K. Wilson for instrument contributions. We thank R. Barkana for theory contributions and G. Holder, T. Vachaspati, C. Hirata and J. Chluba for exchanges. We thank C. Lonsdale, H. Johnson, J. Hewitt and J. Burns. We thank C. Halleen and M. Halleen for site and logistical support. This work was supported by the NSF through awards AST-0905990, AST-1207761 and AST-1609450. R.A.M. acknowledges support from the NASA Ames Research Center (NNX16AF59G) and the NASA Solar System Exploration Research Virtual Institute (80ARC017M0006). This work makes use of the Murchison Radio-astronomy Observatory. We acknowledge the Wajarri Yamatji people as the traditional owners of the site of the observatory.

Author information

Affiliations

  1. School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287, USA

    • Judd D. Bowman
    • , Raul A. Monsalve
    • , Thomas J. Mozdzen
    •  & Nivedita Mahesh
  2. Haystack Observatory, Massachusetts Institute of Technology, Westford, Massachusetts 01886, USA

    • Alan E. E. Rogers
  3. Center for Astrophysics and Space Astronomy, University of Colorado, Boulder, Colorado 80309, USA

    • Raul A. Monsalve
  4. Facultad de Ingeniería, Universidad Católica de la Santísima Concepción, Alonso de Ribera 2850, Concepción, Chile

    • Raul A. Monsalve

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Contributions

J.D.B., R.A.M. and A.E.E.R. contributed to all activities. N.M. and T.J.M. modelled instrument properties, performed laboratory calibrations and contributed to the preparation of the manuscript.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Judd D. Bowman.

Reviewer Information Nature thanks S. Weinreb and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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https://doi.org/10.1038/nature25792

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