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 (~10 mK), and contamination by strong (>100 K) 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 200 MHz, 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.
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- 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.