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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Pritchard, J. R. & Loeb, A. 21 cm cosmology in the 21st century. Rep. Prog. Phys. 75, 086901 (2012)
Cohen, A., Fialkov, A., Barkana, R. & Lotem, M. Charting the parameter space of the global 21-cm signal. Mon. Not. R. Astron. Soc. 472, 1915–1931 (2017)
Barkana, R. Possible interaction between baryons and dark-matter particles revealed by the first stars. Nature 555, https://doi.org/10.1038/nature25791 (2018)
Monsalve, R. A., Rogers, A. E. E., Bowman, J. D. & Mozdzen, T. J. Results from EDGES high-band. I. Constraints on phenomenological models for the global 21 cm signal. Astrophys. J. 847, 64 (2017)
Alves, M. I. R. et al. The HIPASS survey of the Galactic plane in radio recombination lines. Mon. Not. R. Astron. Soc. 450, 2025–2042 (2015)
Rogers, A. E. E., Bowman, J. D., Vierinen, J., Monsalve, R. A. & Mozdzen, T. Radiometric measurements of electron temperature and opacity of ionospheric perturbations. Radio Sci. 50, 130–137 (2015)
Sokolowski, M. et al. The impact of the ionosphere on ground-based detection of the global epoch of reionization signal. Astrophys. J. 813, 18 (2015)
Zaldarriaga, M., Furlanetto, S. R. & Hernquist, L. 21 centimeter fluctuations from cosmic gas at high redshifts. Astrophys. J. 608, 622–635 (2004)
Ali-Haïmoud, Y. & Hirata, C. M. HyRec: a fast and highly accurate primordial hydrogen and helium recombination code. Phys. Rev. D 83, 043513 (2011)
Shaw, J. R. & Chluba, J. Precise cosmological parameter estimation using COSMOREC. Mon. Not. R. Astron. Soc. 415, 1343–1354 (2011)
Furlanetto, S. R. The global 21-centimeter background from high redshifts. Mon. Not. R. Astron. Soc. 371, 867–878 (2006)
Planck Collaboration. Planck 2015 results. XIII. Cosmological parameters. Astron. Astrophys. 594, A13 (2016)
Chen, X. & Miralda-Escude, J. The spin-kinetic temperature coupling and the heating rate due to Lyα scattering before reionization: predictions for 21 centimeter emission and absorption. Astrophys. J. 602, 1–11 (2004)
Tashiro, H., Kadota, K. & Silk, J. Effects of dark matter-baryon scattering on redshifted 21 cm signals. Phys. Rev. D 90, 083522 (2014)
Lopez-Honorez, L., Mena, O., Moliné, Á., Palomares-Ruiz, S. & Vincent, A. C. The 21 cm signal and the interplay between dark matter annihilations and astrophysical processes. J. Cosmol. Astropart. Phys. 8, 4 (2016)
Tashiro, H. & Sugiyama, N. The effect of primordial black holes on 21-cm fluctuations. Mon. Not. R. Astron. Soc. 435, 3001–3008 (2013)
Schleicher, D. R. G., Banerjee, R. & Klessen, R. S. Influence of primordial magnetic fields on 21 cm emission. Astrophys. J. 692, 236–245 (2009)
Biermann, P. L . et al. Cosmic backgrounds due to the formation of the first generation of supermassive black holes. Mon. Not. R. Astron. Soc. 441, 1147–1156 (2014)
Cline, J. M. & Vincent, A. C. Cosmological origin of anomalous radio background. J. Cosmol. Astropart. Phys. 2, 11 (2013)
Seiffert, M. et al. Interpretation of the Arcade 2 absolute sky brightness measurement. Astrophys. J. 734, 6 (2011)
Vernstrom, T., Norris, R. P., Scott, D. & Wall, J. V. The deep diffuse extragalactic radio sky at 1.75 GHz. Mon. Not. R. Astron. Soc. 447, 2243–2260 (2015)
Bernardi, G. et al. Bayesian constraints on the global 21-cm signal from the cosmic dawn. Mon. Not. R. Astron. Soc. 461, 2847–2855 (2016)
Voytek, T. C., Natarajan, A., Jáuregui García, J. M., Peterson, J. B. & López-Cruz, O. Probing the dark ages at z ~ 20: the SCI-HI 21 cm all-sky spectrum experiment. Astrophys. J. 782, L9 (2014)
Singh, S. et al. First results on the epoch of reionization from first light with SARAS 2. Astrophys. J. 845, L12 (2017)
Switzer, E. R. & Liu, A. Erasing the variable: empirical foreground discovery for global 21 cm spectrum experiments. Astrophys. J. 793, 102 (2014)
Vedantham, H. K. et al. Chromatic effects in the 21 cm global signal from the cosmic dawn. Mon. Not. R. Astron. Soc. 437, 1056–1069 (2014)
Burns, J. O. et al. A space-based observational strategy for characterizing the first stars and galaxies using the redshifted 21 cm global spectrum. Astrophys. J. 844, 33 (2017)
DeBoer, D. R. et al. Hydrogen Epoch of Reionization Array (HERA). Publ. Astron. Soc. Pac. 129, 045001 (2017)
Patil, A. H. et al. Upper limits on the 21 cm epoch of reionization power spectrum from one night with LOFAR. Astrophys. J. 838, 65 (2017)
Beardsley, A. P. et al. First season MWA EoR power spectrum results at redshift 7. Astrophys. J. 833, 102 (2016)
Dowell, J., Taylor, G. B., Schinzel, F. K., Kassim, N. E. & Stovall, K. The LWA1 low frequency sky survey. Mon. Not. R. Astron. Soc. 469, 4537–4550 (2017)
Bowman, J. & Rogers, A. E. E. VHF-band RFI in geographically remote areas. In Proc. RFI Mitigation Workshop id.30, https://pos.sissa.it/107/030/pdf (Proceedings of Science, 2010)
Bowman, J. D. & Rogers, A. E. E. Lower Limit of Δz>0.06 for the duration of the reionization epoch. Nature 468, 796–798 (2010)
Monsalve, R. A., Rogers, A. E. E., Bowman, J. D. & Mozdzen, T. J. Calibration of the EDGES high-band receiver to observe the global 21 cm signature from the epoch of reionization. Astrophys. J. 835, 49 (2017)
Mozdzen, T. J., Bowman, J. D., Monsalve, R. A. & Rogers, A. E. E. Improved measurement of the spectral index of the diffuse radio background between 90 and 190 MHz. Mon. Not. R. Astron. Soc. 464, 4995–5002 (2017)
Raghunathan, A., Shankar, N. U. & Subrahmanyan, R. An octave bandwidth frequency independent dipole antenna. IEEE Trans. Anenn. Propag. 61, 3411–3419 (2013)
Ellingson, S. W. Antennas for the next generation of low-frequency radio telescopes. IEEE Trans. Antenn. Propag. 53, 2480–2489 (2005)
Roberts, W. K. A new wide-band balun. Proc. IRE 45, 1628–1631 (1957)
Bowman, J. D., Rogers, A. E. E. & Hewitt, J. N. Toward empirical constraints on the global redshifted 21 cm brightness temperature during the epoch of reionization. Astrophys. J. 676, 1–9 (2008)
Rogers, A. E. E., Pratap, P., Carter, J. C. & Diaz, M. A. Radio frequency interference shielding and mitigation techniques for a sensitive search for the 327 MHz line of deuterium. Radio Sci. 40, RS5S17 (2005)
Singh, S . et al. SARAS 2: a spectral radiometer for probing cosmic dawn and the epoch of reionization through detection of the global 21 cm signal. Preprint at https://arxiv.org/abs/1710.01101 (2017)
Price, D. C . et al. Design and characterization of the Large-Aperture Experiment to Detect the Dark Age (LEDA) radiometer systems. Preprint at https://arxiv.org/abs/1709.09313 (2017)
Rogers, A. E. E. & Bowman, J. D. Absolute calibration of a wideband antenna and spectrometer for accurate sky noise temperature measurement. Radio Sci. 47, RS0K06 (2012)
Hu, R. & Weinreb, S. A novel wide-band noise-parameter measurement. IEEE Trans. Microw. Theory Tech. 52, 1498–1507 (2004)
Belostotski, L. A calibration method for RF and microwave noise sources. IEEE Trans. Microw. Theory Tech. 59, 178–187 (2011)
Ramo, S. & Whinnery, J. R. Fields and Waves in Modern Radio Ch. 6 (Wiley, 1953)
Monsalve, R. A., Rogers, A. E. E., Mozdzen, T. J. & Bowman, J. D. One-port direct/reverse method for characterizing VNA calibration standards. IEEE Trans. Microw. Theory Tech. 64, 2631–2639 (2016)
Sutinjo, A. T. et al. Characterization of a low-frequency radio astronomy prototype array in Western Australia. IEEE Trans. Antenn. Propag. 63, 5433–5442 (2015)
Haslam, C. G. T., Salter, C. J., Stoffel, H. & Wilson, W. E. A 408 MHz all-sky continuum survey. II. The atlas of contour maps. Astron. Astrophys. Suppl. Ser. 47, 1–143 (1982)
Chandrasekhar, S. Radiative Transfer (Courier Dover, 1960)
de Oliveira-Costa, A. et al. A model of diffuse Galactic radio emission from 10 MHz to 100 GHz. Mon. Not. R. Astron. Soc. 388, 247–260 (2008)
Bernardi, G., McQuinn, M. & Greenhill, L. J. Foreground model and antenna calibration errors in the measurement of the sky-averaged λ21 cm signal at z ~ 20. Astrophys. J. 799, 90 (2015)
Mozdzen, T. J., Bowman, J. D., Monsalve, R. A. & Rogers, A. E. E. Limits on foreground subtraction from chromatic beam effects in global redshifted 21 cm measurements. Mon. Not. R. Astron. Soc. 455, 3890–3900 (2016)
McKinley, B. et al. Low-frequency observations of the Moon with the Murchison widefield array. Astron. J. 145, 23 (2013)
Seager, S., Sasselov, D. D. & Scott, D. How exactly did the Universe become neutral? Astrophys. J. Suppl. Ser. 128, 407–430 (2000)
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.
The authors declare no competing financial interests.
Reviewer Information Nature thanks S. Weinreb and the other anonymous reviewer(s) for their contribution to the peer review of this work.
Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Extended data figures and tables
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.
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.
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.
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.
a–g, 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). h–n, 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.
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.
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.
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).
The curves show the residual r.m.s. after a best-fitting model is removed at each integration time for the H2 dataset.
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.
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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). https://doi.org/10.1038/nature25792
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