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:
MIT News published a great article on EDGES, available at http://news.mit.edu/2018/astronomers-detect-earliest-evidence-yet-hydrogen-universe-0228. See also 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.
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.
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.