Observations of the 21-cm line from primordial hydrogen promise to be one of the best tools to study the early epochs of the Universe: the dark ages, the cosmic dawn and the subsequent epoch of reionization. In 2018, the Experiment to Detect the Global Epoch of Reionization Signature (EDGES) caught the attention of the cosmology community with a potential detection of an absorption feature in the sky-averaged radio spectrum centred at 78 MHz. The feature is deeper than expected, and, if confirmed, would call for new physics. However, different groups have re-analysed the EDGES data and questioned the reliability of the signal. The Radio Experiment for the Analysis of Cosmic Hydrogen (REACH) is a sky-averaged 21-cm experiment aiming at improving the current observations by tackling the issues faced by current instruments related to residual systematic signals in the data. The novel experimental approach focuses on detecting and jointly explaining these systematics together with the foregrounds and the cosmological signal using Bayesian statistics. To achieve this, REACH features simultaneous observations with two different antennas, an ultra-wideband system (redshift range about 7.5 to 28) and a receiver calibrator based on in-field measurements. Simulated observations forecast percent-level constraints on astrophysical parameters, potentially opening up a new window to the infant Universe.
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The REACH collaboration acknowledges the Kavli Institute for Cosmology in Cambridge (www.kicc.cam.ac.uk), Stellenbosch University (www.sun.ac.za), the National Research Foundation of South Africa (www.nrf.ac.za) and the Cambridge–Africa ALBORADA Research Fund (www.cambridge-africa.cam.ac.uk/initiatives/the-alborada-research-fund/) for their financial support of the project. E.d.L.A. wishes to acknowledge the support of the Science and Technology Facilities Council (STFC) through grant number ST/V004425/1 (Ernest Rutherford Fellowship). G.B. and M.S. acknowledge support from the Ministero degli Affari Esteri della Cooperazione Internazionale–Direzione Generale per la Promozione del Sistema Paese Progetto di Grande Rilevanza ZA18GR02 and the National Research Foundation of South Africa (Grant Number 113121) as part of the ISARP RADIOSKY2020 Joint Research Scheme. The research of D.I.L.d.V. and O.S. is supported by the South African Research Chairs Initiative of the Department of Science and Technology and National Research Foundation. M.S. acknowledges funding from the INAF PRIN-SKA 2017 project 1.05.01.88.04 (FORECaST). This work is based on the research supported in part by the National Research Foundation of South Africa (Grant Number 75322). H.T.J.B. acknowledges the support of the STFC through grant number ST/T505997/1.
The authors declare no competing interests.
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Re-analysis of the publicly available EDGES data restricting the foregrounds to physical parameters (purple) and with unrestricted foreground parameters (orange) for a flatten Gaussian EDGES-style signal model (top-left) and a 21cmGEM standard physical model from  (bottom-left). On the right column we show the corresponding residuals after subtraction of the posterior average fitted foreground and signal models.
Top: Outline of the calibration algorithm. Blue blocks represent data to be taken, red blocks represent calculations and green blocks represent calculation outputs. Bottom: Plot showing the joint posteriors for two noise wave parameters used for calibration of the receiver; TL and TNS. Posteriors are derived using a single room-temperature ‘cold’ load as a calibrator, a single ‘hot’ load heated to 373 K and both loads used in tandem shown in grey, red and blue respectively. The black cross hairs mark the known values of the calibration parameters. A zoom-in of the posterior intersection is provided to illustrate the constraint on parameter values attributed to the correlation between parameters that is considered by our algorithm when deriving the blue, dual-load posterior.
Top: Plot showing the subdivision of the sky in galactic coordinates into a number of regions N=6 of similar spectral index. Bottom: Plot comparing the residuals from fitting simulated 21-cm data. The plots shows the results of fitting data with a 5th order log-polynomial model (left), fitting data corrected by (A1) with a 5th order log-polynomial model (centre) and fitting the data with the REACH pipeline, using N=9 (right). The residuals after subtraction of the foreground models are shown in red. The signal model and true signal inserted into the simulated data, are shown in blue and green respectively, where visible. These results are simulated using a conical log-spiral antenna and a hexagonal dipole antenna.
Top-left: Plot of the recovered 21-cm signal in purple, compared to the true inserted 21-cm signal in green, for simulated data sets of a log spiral and hexagonal dipole antenna. Each data set consisted of three time bins, 20 minutes apart. The lower plots show the results of fitting an integration of the three bins to a single foreground model and the upper plots show the results of fitting the separate bins jointly to corresponding models in a single fit. The rightmost plots show the results of fitting the data sets from both antenna simultaneously in the same fit. Top-right: Plot of the optimum numbers of foreground regions, determined using the Bayesian evidence, for the model fits shown in the top-left plot. Bottom-left: Plot showing a run of the pipeline where the antenna model included the presence of the finite 20x20 m metallic ground plane underneath the spiral antenna. This plot shows that the chromaticity introduced by reflections at the edge of the REACH ground plane, if properly modelled, would not severely affect the ability of the pipeline to recover the cosmological signal. Bottom-right: Plot showing the result of running the data pipeline when a sinusoidal systematic arising from the presence of the 6 m cable connecting the spiral antenna feed point to the receiver has been introduced in the data. The additive systematic signal is shown as a black-solid line in this plot. In the simulated analysis we included a sinusoidal model to fit for this systematic signal simultaneously with the foregrounds and the cosmological signal. This result shows that a detection of the true signal could be achieved in this case.
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de Lera Acedo, E., de Villiers, D.I.L., Razavi-Ghods, N. et al. The REACH radiometer for detecting the 21-cm hydrogen signal from redshift z ≈ 7.5–28. Nat Astron 6, 984–998 (2022). https://doi.org/10.1038/s41550-022-01709-9
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