The EDGES experiment is a very small radio telescope, 2 meter long and 1 meter high, located in the radio quiet zone in western Australia. The equipment consists in three broad-band antennas that cover a range of frequencies from 50 to 200 MHz. The low-band antenna (operating from 50 to 100 MHz) has been designed to observe a spectral distortion in the 21-cm energy band at cosmological redshift of 20 due to the absorption of CMB photons by the IGM. However, the detection of the 21-cm signal is very challenge because of the very large foregrounds of galactic diffuse synchrotron emission. The full-sky maps of the diffuse synchrotron emission at 45 MHz and 408 MHz can be found, for example, in [2] and [3] respectively. Before subtracting the foregrounds to the data is important to stress that: i) the brightness temperature in the frequency window of EDGES is always above 100 K even in region far away from the galactic center; ii) the galactic synchrotron emission is spectrally smooth above 50 MHz but might need several terms to model it in a proper way as discussed in details in [4]; iii) the synchrotron emission features a large spatial gradient especially in the region close to the galactic center where the activity is much larger (see e.g. [2, 3]).
Fig. 1 of [1] shows the EDGES detection in terms of brightness temperature as a function of the frequency obtained by looking at high galactic latitudes. It is evident from panel a. that the galactic synchrotron emission dominates the observed sky noise, yielding to an almost perfect power-law profile that decreases from about 5000 K at 50 MHz to about 1000 K at 100 MHz. Fitting and removing the galactic synchrotron emission from the spectrum with a physically motivated 5-terms polynomial the collaboration gets the residual in panel b.. This residual is not flat and it has a root-mean-square of 87 mK. Repeating the same exercise by adding to the 5-terms polynomial a template of the signal like the one in panel d., the collaboration gets the residuals in panel c.. This new residual is now flat and the fit substantially ameliorates with a root-mean-square of only 25 mK. Adding the template to the residual the 21-cm signal is finally reported in panel d.. Fig. 2 of Ref. [1] summarizes the detected signal obtained by using different experimental configurations. As one can see this is a signal in absorption because the brightness temperature is negative. It extends from redshift 20 to redshift 15 and it has an amplitude of 500+200 −500 mK at 99% CL. The value of the plateau, centered at a frequency of 78 MHz, translating to a redshift of 17.2, is quite surprising because is 3.8σ away from the prediction of standard cosmology. As I am going to discuss in Sec. 3, the global 21-cm signal predicted from ΛCDM can not ever be below −230 mK. If the measured amplitude is correct, BSM physics is required.
