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

A Brief Communications Arising to this article was published on 19 December 2018


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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Figure 1: Summary of detection.
Figure 2: Best-fitting 21-cm absorption profiles for each hardware case.

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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.

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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.

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Correspondence to Judd D. Bowman.

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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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Extended data figures and tables

Extended Data Figure 1 Block diagram of the low-band system.

The inset images show: a, the capacitive tuning bar that feeds the dipoles at the top of the balun; b, the SubMiniature version A connector at the bottom of the balun coaxial transmission line where the receiver connects; c, the low-1 receiver that is installed under its antenna with the ground-plane cover plate removed; and d, the inside of the low-1 receiver. The LNA is contained in the secondary metal enclosure in the lower-left corner of the receiver. SW, switch.

Extended Data Figure 2 Low-band antennas.

a, The low-1 antenna with the 30 m × 30 m mesh ground plane. The darker inner square is the original 10 m × 10 m mesh. The control hut is 50 m from the antenna. b, A close view of the low-2 antenna. The two elevated metal panels form the dipole-based antenna and are supported by fibreglass legs. The balun consists of the two vertical brass tubes in the middle of the antenna. The balun shield is the shoebox-sized metal shroud around the bottom of the balun. The receiver is under the white metal platform and is not visible.

Extended Data Figure 3 Antenna and simulator reflection coefficients.

a, b, Measurements of the reflection coefficient magnitude (a) and phase (b) are plotted for hardware configurations H2 (blue), H4 (red) and H6 (yellow). The antennas are designed identically (except H6 has the balun shield removed), but are tuned manually during installation by adjusting the panel separation and the height of the small metal plate that connects one panel to the centre conductor of the balun transmission line on the other. The measurements were acquired in situ. c, d, The red curve is the 10,000-K artificial antenna noise source and the blue curve is the 300-K mismatched load.

Extended Data Figure 4 Antenna beam model

a, Beam cross-sections showing the gain in the plane containing the electric field (dashed) and in the plane containing the magnetic field (solid) from FEKO for the H2 antenna and ground plane over soil. Cross-sections are plotted at 50 MHz (red), 70 MHz (yellow) and 100 MHz (blue). b, Frequency dependence of the gain at zenith angle Θ = 0° (solid) and the 3-dB points at 70 MHz in the electric-field plane (dashed) and magnetic-field plane (dotted). c, Small undulations with frequency, after a five-term polynomial (equation (2)) has been subtracted from each of the curves, are plotted as fractional changes in the gain. Simulated observations with this model yield residuals of 0.015 K (0.001%) to the five-term fit over the frequency range 52–97 MHz at GHA = 10 and residuals of 0.1 K (0.002%) at GHA = 0, showing that the cumulative beam yields less chromaticity than the approximately 0.5% variations in the individual points plotted.

Extended Data Figure 5 Calibration parameter solutions.

ag, Solutions for the low-1 receiver at its fixed 25 °C operating temperature. It was calibrated on three occasions spanning two years, bracketing all of the low-1 observations reported. The first calibration was in August 2015 before commencing cases H1 and H2 (solid), the next was in May 2017 before H3 (dotted), and the final was in September 2017 after the conclusion of H3 (dashed). hn, Solutions for the low-2 receiver controlled to three different temperatures: 15 °C (blue), 25 °C (black) and 35 °C (red). The parameters C1 and C2 are scale and offset factors, respectively; TC, TS and TU are noise-wave parameters (TS is not the spin temperature here; see ref. 34 for details); S11 is the LNA input reflection coefficient.

Extended Data Figure 6 Raw and processed spectra.

a, Typical raw 13-s spectra from H2 for each of the receiver’s ‘three position’ switch states. The small spikes on the right of the antenna spectrum are FM radio stations. b, The spectrum has been partially calibrated (‘Three position’) using the three raw spectra to correct gain and offset contributions in the receiver and cables, then fully calibrated (Calib.) by applying the calibration parameter solutions from the laboratory to yield the sky temperature. c, Residuals to a fit of the fully calibrated spectrum with the five-term polynomial foreground model (equation (2)). In b and c, the frequencies listed in the legend give the binning used for each curve.

Extended Data Figure 7 Normalized channel weights.

The fraction of data integrated for each 390.6-kHz spectral bin are shown. a, The FM band causes the low weights above 87 MHz, because many 6.1-kHz raw spectral channels in this region are removed for all times. The weights are nearly identical across all hardware cases (H1–H6). b, A close-up showing the weights below the FM band, where there is little RFI to remove.

Extended Data Figure 8 Residuals to the 21-cm profile model.

The black curve shows the best-fitting 21-cm profile model derived from the observations. The solid blue and yellow curves show fits to the model profile using the physical (equation (1)) and five-term polynomial (equation (2)) foreground models, respectively. The dashed lines show the residuals after subtracting the fits from the model. These residuals are similar to those found when fitting the observations using only a foreground model (Fig. 1b).

Extended Data Figure 9 Residual r.m.s. as a function of integration time.

The curves show the residual r.m.s. after a best-fitting model is removed at each integration time for the H2 dataset.

Extended Data Figure 10 Parameter estimation.

Likelihood distributions for the foreground and 21-cm model parameters are shown for the H2 dataset. Contours are drawn at the 68% and 95% probability levels. The foreground polynomial coefficients (an) are highly correlated with each other, whereas the 21-cm model parameters are largely uncorrelated, except for the profile amplitude (A) and flattening (τ). Systematic uncertainties from the verification hardware cases are not presented here.

Extended Data Table 1 Best-fitting parameter values for the 21-cm absorption profile for representative verification tests
Extended Data Table 2 Recovered 21-cm profile amplitudes for various GHAs

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Bowman, J., Rogers, A., Monsalve, R. et al. An absorption profile centred at 78 megahertz in the sky-averaged spectrum. Nature 555, 67–70 (2018).

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