EDGES: Experiment to Detect the Global EoR Signature

Aims and results

EDGES is a long-term collaboration between MIT Haystack Observatory and Arizona State University, funded by the National Science Foundation (NSF). It is located at the Murchison Radio-astronomy Observatory (MRO) in a radio-quiet zone in western Australia, with onsite infrastructure support provided by Australia’s CSIRO. The antennas and portions of the receiver were designed and constructed by MIT Haystack Observatory’s Dr. Alan Rogers and the Haystack team, working with project lead Judd Bowman, Raul Monsalve, and the ASU team.

A ground-breaking and widely cited EDGES result, “An absorption profile centred at 78 megahertz in the sky-averaged spectrum,” was published in Nature. This result, which must be independently confirmed by other research groups, has profound implications for our understanding of the early universe.

If you’re new to cosmology, watch this excellent NSF video that explains the significance of the EDGES observations:

EDGES: The birth of the first stars (video: National Science Foundation)

The MIT News article on EDGES is available at http://news.mit.edu/2018/astronomers-detect-earliest-evidence-yet-hydrogen-universe-0228

For more technical details about EDGES, please see the MIT Haystack EDGES memo series, and the ASU EDGES memo series.

The project’s goal is radio detection of characteristic hydrogen signatures from the periods in the early history of the universe known as Cosmic Dawn—when the first stars were born, and the Epoch of Reionization (EoR)—when stars and galaxies generated radiation that eroded and eventually destroyed the neutral hydrogen. For an extensive scientific explanation of the cosmological mysteries that EDGES is designed to probe, see this article from ASU.

EDGES technology

The EDGES instrument comprises a small, simple antenna sitting on a large metal sheet (or “ground plane”), with a receiver system, spectrometer, and highly precise calibration components. The measurement is easily corrupted by radio frequency interference, so the system is sited in remote western Australia, in a radio-quiet reserve known as the Murchison Radio-astronomy Observatory (MRO). The signal being sought is a tiny perturbation against a sky that is extremely bright at the observing frequencies of interest, and can be detected only if the instrument is exquisitely well calibrated. The technological innovation is almost entirely focused on this formidable calibration challenge.

The signal is caused by neutral hydrogen, which naturally emits and absorbs radiation at a frequency of 1420 MHz, or a wavelength of 21 cm. Due to the expansion of the universe, signals from Cosmic Dawn and the EoR are red-shifted to low frequencies and wavelengths of a few meters, which is where the EDGES system operates. There are multiple versions of the system—”low-band,” “mid-band,” and “high-band”—that together cover the 50–200 MHz range. The low-band system broadband antenna is about 2 meters long and 1 meter high, and the metal ground plane is 30 by 30 meters. There are three generations of the system—the Nature result was obtained with EDGES-II, and the new EDGES-III system promises increased precision.

EDGES III antenna in the field
EDGES III prototype antenna being field-tested at a remote, radio-quiet location

The EDGES calibration system is based on switching known sources of noise in and out of the signal path, and precise measurement of the strength and properties of internal reflections in the various system components. One innovation in the EDGES-III system is to perform all these functions in the field, instead of the EDGES-II approach of doing the reflection measurements in the laboratory, before and after each deployment. The data analysis is complex, and depends on extreme smoothness of the instrument response with frequency. Detection relies on the signal being “not smooth” in frequency, and on persistence against a broad range of instrumental and observational configurations and parameters.

All technical details of the project are described in great detail in the MIT Haystack EDGES memo series.

The antennas and portions of the receiver were designed and constructed by MIT Haystack Observatory’s Dr. Alan Rogers and the Haystack team; Judd Bowman, Raul Monsalve, and the ASU team added an automated antenna reflection measurement system to the receiver, outfitted the control hut with the electronics, constructed the ground plane, and conducted the field work for the project. Australia’s Commonwealth Scientific and Industrial Research Organization (CSIRO) provides on-site infrastructure for the EDGES project.

There is a great technical challenge to making this detection. Sources of noise can be a thousand times brighter than the signal they are looking for. It is like being in the middle of a hurricane and trying to hear the flap of a hummingbird’s wing.

Peter Kurczynski, program director for Advanced Technologies and Instrumentation, in the Division of Astronomical Sciences at the National Science Foundation, which has provided funding for the project over the past several years

Haystack project team

Monsalve, R.A., A. Fialkov, J.D. Bowman, A.E.E. Rogers, T.J. Mozdzen, A. Cohen, R. Barkana, N. Mahesh, (2019), Results from EDGES High-Band. III. New Constraints on Parameters of the Early Universe, ApJ, 875, 67. https://doi.org/10.3847/1538-4357/ab07be

Bowman, Judd D., Alan EE Rogers, Raul A. Monsalve, Thomas J. Mozdzen, and Nivedita Mahesh. (2018). Reply to Hills et al. Nature 564: E32–E34. https://doi.org/10.1038/s41586-018-0797-4

Mozdzen, T.J., N Mahesh, R A Monsalve, A.E.E Rogers, J.D. Bowman, (2018). Spectral Index of the Diffuse Radio Background Between 50 and 100 MHz Monthly Notices of the Royal Astronomical Society, sty3410. https://doi.org/10.1093/mnras/sty3410

Monsalve, R.A., Bradley Greig, Judd D. Bowman, Andrei Mesinger, Alan E.E. Rogers and Thomas J. Mozdzen, Nicholas S. Kern and Nivedita Mahesh (2018). Results from EDGES High-band. II. Constraints on Parameters of Early Galaxies, The Astrophysical Journal 863, no 1. https://doi.org/10.3847/1538-4357/aace54

Monsalve, Raul A., Alan EE Rogers, Judd D. Bowman, and Thomas J. Mozdzen (2017). Results from EDGES High-band. I. Constraints on Phenomenological Models for the Global 21 cm Signal. The Astrophysical Journal 847, no. 1 (2017): 64. https://doi.org/10.3847/1538-4357/aa88d1

Bowman, Judd D., Rogers, Alan E.E., Monsalve, Raul A., Mozdzen, Thomas J., and Mahesh Nivedita. (2018). An absorption profile centred at 78 megahertz in the sky-averaged spectrum. Nature 555, 67–70. https://doi.org/10.1038/nature25792

Monsalve, R.A., Rogers, A.E.E., Bowman, J.D., and Mozden, T.J. (2017). Calibration of the EDGES high-band receiver to observe the global 21 cm from the Epoch of Reionization. Astrophysical Journal 835(1). https://doi.org/10.3847/1538-4357/835/1/49

Monsalve, R.A., Rogers, A.E.E., Mozdzen, T.J., and Bowman, J.D. (2016). One-port direct/reverse method for characterizing VNA calibration standards. IEEE Transactions on Microwave Theory and Techniques 64(8): 2631–2639. https://arxiv.org/pdf/1606.02446

Mozdzen, Thomas J., Judd D. Bowman, Raul A. Monsalve, and Alan E.E. Rogers (2016). Improved measurement of the spectral index of the diffuse radio background between 90 and 190 MHz. Monthly Notices of the Royal Astronomical Society 464(4): 4995–5002. https://doi.org/10.1093/mnras/stw2696

Rogers, A.E.E., Bowman, J.D., Vierinen, J., Monsalve, R. and Mozdzen, T. (2015). Radiometric measurements of electron temperature and opacity of ionospheric perturbations. Radio Sci., 50, 130–137. https://doi.org/10.1002/2014RS005599

Mozdzen, T.J., Bowman, J.D., Monsalve, R.A., Rogers, A.E.E. (2015). Limits on foreground subtraction from chromatic beam effects in global redshifted 21cm measurements. Monthly Notices of the Royal Astronomical Society 2015 455(4): 3890–3900. https://doi.org/10.1093/mnras/stv2601

Rogers, A.E.E., Bowman, J.D. (2012). Absolute calibration of a wideband antenna and spectrometer for accurate sky noise temperature measurements. Radio Science 47, RS0K06. https://doi.org/10.1029/2011RS004962

Bowman, J.D., Rogers, A.E.E. (2010). A lower limit of Δz > 0.06 for the duration of the reionization epoch. Nature 468, pp. 796-797. https://doi.org/10.1038/nature09601

Rogers, A.E.E., Bowman, J.D. (2008). Spectral Index of the Diffuse Radio Background Measured from 100 to 200 MHz. AJ 136(2), pp. 641-648. https://iopscience.iop.org/article/10.1088/0004-6256/136/2/64

Bowman, J.D., Rogers, A.E.E. Hewitt, J.N. (2008). Toward empirical constraints on the global redshifted 21 cm brightness temperature during the Epoch of Reionization. Ap.J. 676(1), 1–9. https://iopscience.iop.org/article/10.1086/528675