EDGES is a pioneering ground-based experiment operating since 2006 in Western Australia that has set limits on the global redshifted 21-cm signal (Bowman & Rogers 2010). EDGES observes from the Murchison Radio-astronomy Observatory (MRO) in Western Australia, which is an officially recognized radio-quiet site that hosts large interferometers such as ASKAP, MWA, and the future SKA-Low. EDGES is a collaboration between Arizona State University and MIT Haystack Observatory.
EDGES conducts total power single-antenna measurements throughout the day as the sky drifts above the zenith-pointing antenna. Between 2006 and mid 2015 the antenna corresponded to a Fourpoint model over a metal ground plane, selected due to its low reflections and low frequency-dependence of the beam (or beam chromaticity). This antenna and instrument were tuned to the range 100-200 MHz, aiming to probe the 21-cm signal from the Epoch of Reionization (EoR) (13 > z > 6). In 2015 the Fourpoint antenna was replaced by a Blade model with a larger (> 10 m x 10 m) ground plane, which produces smoother reflections and lower beam chromaticity (Mozdzen et al. 2016). Also in 2015, EDGES deployed a second, completely independent Low-Band instrument that operates in the range 50-100 MHz. This instrument aims to detect the absorption trough predicted from the Cosmic Dawn (27 > z > 13).
The sky measurements are calibrated by, first, switching continuously in the field between the antenna and two smooth reference noise levels injected at the input of the front-end low-noise amplifier. This step represents a relative calibration that removes the time-dependent system gain and offset. Next, EDGES applies an absolute calibration obtained from lab measurements, which brings the relative calibration provided by the internal switching, to an absolute temperature scale at the receiver input. This absolute calibration accounts for the imperfect impedance match between the antenna and the receiver. Details of this process are provided in Monsalve et al (2017). The final step in the calibration consists of compensating for the effect of antenna losses and beam chromaticity. These effects are estimated from measurements, as well as from analytical and EM modeling (Rogers & Bowman 2008, 2012).
Measurements from EDGES have enabled to put constraints on the duration of the EoR. Specifically, they rule out durations shorter than 0.06 (Bowman & Rogers 2010). In addition, they have been used to determine the spectral index of the diffuse foregrounds (Rogers & Bowman 2008; Mozdzen et al 2017). They have also yielded an estimate for the fluctuations in the ionospheric absorption above the MRO (Rogers et al. 2015). Work is underway to derive much stronger constraints on the global 21-cm signal from data taken by both instruments during the 2015-2016 season.
Link to EDGES page at Arizona State University:
loco.lab.asu.edu/edges/
Link to EDGES memo series at Haystack Observatory:
www.haystack.mit.edu/ast/arrays/Edges/EDGES_memos/EdgesMemo.html
