A lower limit of Δz>0.06 for the duration of the reionization epoch

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Observations of the 21-centimetre line of atomic hydrogen in the early Universe directly probe the history of the reionization of the gas between galaxies1. The observations are challenging, though, because of the low expected signal strength (~10mK), and contamination by strong (>100K) foreground synchrotron emission in the Milky Way and extragalactic continuum sources2. If reionization happened rapidly, there should be a characteristic signature2, 3, 4 visible against the smooth foreground in an all-sky spectrum. Here we report an all-sky spectrum between 100 and 200MHz, corresponding to the redshift range 6<z<13 for the 21-centimetre line. The data exclude a rapid reionization timescale of Δz<0.06 at the 95% confidence level.

At a glance


  1. Measured spectrum between 100 and 195[thinsp]MHz.
    Figure 1: Measured spectrum between 100 and 195MHz.

    The spectrum corresponds to redshifts 13>z>6. The grey spikes are spectral channels that experienced radio-frequency interference during the integration and are masked from the analysis. The shape and amplitude of the spectrum are dominated by Galactic synchrotron emission and modulated by the uncalibrated antenna bandpass, which causes the spectrum to roll off from the characteristic power-law form of the foregrounds at low and high frequencies. Any global 21-cm contribution in the spectrum is at the 20–30mK level, approximately four orders of magnitude below the visible foreground emission. Thermal noise in the spectrum is 6mK at 150MHz using 1-MHz binned spectral resolution. The thermal noise increases at lower frequencies owing to the larger sky noise and lowered transmission efficiency of the antenna. Any 20-MHz sub-band in this spectrum can be fitted by a fifth-order polynomial, leaving residuals at or below the thermal noise level.

  2. Lower confidence bounds on the duration of the reionization transition. Statistical and systematic uncertainties are included.
    Figure 2: Lower confidence bounds on the duration of the reionization transition. Statistical and systematic uncertainties are included.

    Grey indicates the 68% confidence bound and black the 95% bound. The white region is allowed by the data. The data rule out rapid reionization histories shorter than Δz0.1 for many redshifts between 6<z<13. The two large gaps at redshifts z9.5 (138MHz) and z10 (130MHz) are at frequencies that require extensive radio-frequency interference excision because they fall into satellite and aircraft communication bands, respectively.


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Author information

  1. These authors contributed equally to this work.

    • Judd D. Bowman &
    • Alan E. E. Rogers


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

    • Judd D. Bowman
  2. Massachusetts Institute of Technology, Haystack Observatory, Westford, Massachusetts 01886, USA

    • Alan E. E. Rogers


Both authors contributed equally to the work in this paper.

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The authors declare no competing financial interests.

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Supplementary information

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  1. Supplementary Information (1M)

    This file contains Supplementary Information comprising Description of the EDGES radio spectrometer, Analog-to-Digital Conversion, Compact, Planar, Dual-Octave Antenna and Ground Screen, Calibration and Data Interpretation, Selection of Observing Site, Supplementary Figures 1-2 with legends and additional references.

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