The astrophysics of the cosmic dawn, when star formation commenced in the first collapsed objects, is predicted to be revealed by spectral and spatial signatures in the cosmic radio background at long wavelengths. The sky-averaged redshifted 21 cm absorption line of neutral hydrogen is a probe of the cosmic dawn. The line profile is determined by the evolving thermal state of the gas, radiation background, Lyman α radiation from stars scattering off cold primordial gas, and relative populations of the hyperfine spin levels in neutral hydrogen atoms. We report a radiometer measurement of the spectrum of the radio sky in the 55–85 MHz band, which shows that the profile found by Bowman et al. in data taken with the Experiment to Detect the Global Epoch of Reionization Signature (EDGES) low-band instrument is not of astrophysical origin; their best-fitting profile is rejected with 95.3% confidence. The profile was interpreted to be a signature of the cosmic dawn; however, its amplitude was substantially higher than that predicted by standard cosmological models. Our non-detection bears out earlier concerns and suggests that the profile found by Bowman et al. is not evidence for new astrophysics or non-standard cosmology. 

/pdf/on-the-detection-of-a-cosmic-dawn-signal-in-the-radio-1p0e2hf6.pdf

On the detection of a cosmic dawn signal in the radio background

ABSTRACT We present CosmoPower, a suite of neural cosmological power spectrum emulators providing orders-of-magnitude acceleration for parameter estimation from two-point statistics analyses of Large-Scale Structure (LSS) and Cosmic Microwave Background (CMB) surveys. The emulators replace the computation of matter and CMB power spectra from Boltzmann codes; thus, they do not need to be re-trained for different choices of astrophysical nuisance parameters or redshift distributions. The matter power spectrum emulation error is less than $0.4{{\ \rm per\ cent}}$ in the wavenumber range $k \in [10^{-5}, 10] \, \mathrm{Mpc}^{-1}$ for redshift z ∈ [0, 5]. CosmoPower emulates CMB temperature, polarization, and lensing potential power spectra in the 5-σ region of parameter space around the Planck best-fitting values with an error ${\lesssim}10{{\ \rm per\ cent}}$ of the expected shot noise for the forthcoming Simons Observatory. CosmoPower is showcased on a joint cosmic shear and galaxy clustering analysis from the Kilo-Degree Survey, as well as on a Stage IV Euclid-like simulated cosmic shear analysis. For the CMB case, CosmoPower is tested on a Planck 2018 CMB temperature and polarization analysis. The emulators always recover the fiducial cosmological constraints with differences in the posteriors smaller than sampling noise, while providing a speed-up factor up to O(104) to the complete inference pipeline. This acceleration allows posterior distributions to be recovered in just a few seconds, as we demonstrate in the Planck likelihood case. CosmoPower is written entirely in python, can be interfaced with all commonly used cosmological samplers, and is publicly available at: https://github.com/alessiospuriomancini/cosmopower. 

/pdf/scp-cosmopower-scp-emulating-cosmological-power-spectra-for-3s0skyi6.pdf

<scp>CosmoPower</scp>: emulating cosmological power spectra for accelerated Bayesian inference from next-generation surveys

The HI 21cm absorption line is masked by bright foregrounds and systematic distortions that arise due to the chromaticity of the antenna used to make the observation coupling to the spectral inhomogeneity of these foregrounds. We demonstrate that these distortions are sufficient to conceal the 21cm signal when the antenna is not perfectly achromatic and that simple corrections assuming a constant spatial distribution of foreground power are insufficient to overcome them. We then propose a new physics-motivated method of modelling the foregrounds of 21cm experiments in order to fit the chromatic distortions as part of the foregrounds. This is done by generating a simulated sky model across the observing band by dividing the sky into $N$ regions and scaling a base map assuming a distinct uniform spectral index in each region. The resulting sky map can then be convolved with a model of the antenna beam to give a model of foregrounds and chromaticity parameterised by the spectral indices of the $N$ regions. We demonstrate that fitting this model for varying $N$ using a Bayesian nested sampling algorithm and comparing the results using the evidence allows the 21cm signal to be reliably detected in data of a relatively smooth conical log spiral antenna. We also test a much more chromatic conical sinuous antenna and find this model will not produce a reliable signal detection, but in a manner that is easily distinguishable from a true detection.

A general Bayesian framework for foreground modelling and chromaticity correction for global 21 cm experiments

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. The Radio Experiment for the Analysis of Cosmic Hydrogen (REACH) aims to detect the sky-averaged 21-cm neutral hydrogen line from the early Universe by jointly analysing the cosmological signal, foreground emission and systematic effects. 

/pdf/the-reach-radiometer-for-detecting-the-21-cm-hydrogen-signal-1qrrv5lq.pdf

The REACH radiometer for detecting the 21-cm hydrogen signal from redshift z ≈ 7.5–28

We modelled the two major layer of Earth's ionosphere, the F-layer and the D-layer, by a simplified spatial model with temporal variance to study the chromatic ionospheric effects on global 21-cm observations. From the analyses, we found that the magnitude of the ionospheric disruptions due to ionospheric refraction and absorption can be greater than the expected global 21-cm signal, and the variation of its magnitude can differ, depending on the ionospheric conditions. Within the parameter space adopted in the model, the shape of the global 21-cm signal is distorted after propagating through the ionosphere, while its amplitude is weakened. It is observed that the ionospheric effects do not cancel out over time, and thus should be accounted for in the foreground calibration at each timestep to account for the chromaticity introduced by the ionosphere.

Quantifying ionospheric effects on global 21-cm observations

Maximally Smooth Functions (MSFs) are a form of constrained functions in which there are no inflection points or zero crossings in high order derivatives. Consequently, they have applications to signal recovery in experiments where signals of interest are expected to be non-smooth features masked by larger smooth signals or foregrounds. They can also act as a powerful tool for diagnosing the presence of systematics. The constrained nature of MSFs makes fitting these functions a non-trivial task. We introduce maxsmooth, an open source package that uses quadratic programming to rapidly fit MSFs. We demonstrate the efficiency and reliability of maxsmooth by comparison to commonly used fitting routines. We show that by using quadratic programming we can reduce the fitting time by approximately two orders of magnitude. maxsmooth features a built-in library of MSF models and allows the user to define their own. We also introduce and implement with maxsmooth Partially Smooth Functions, which are useful for describing elements of non-smooth structure in foregrounds. This work has been motivated by the problem of foreground modelling in 21-cm cosmology for which MSFs have been shown to be a viable alternative to polynomials. We discuss applications of maxsmooth to 21-cm cosmology and highlight this with examples using data from the Experiment to Detect the Global Epoch of Reionization Signature (EDGES) and the Large-aperture Experiment to Detect the Dark Ages (LEDA) experiments. MSFs are applied to data from LEDA for the first time in this paper. maxsmooth is pip installable and available for download at:this https URL

MAXSMOOTH: rapid maximally smooth function fitting with applications in Global 21-cm cosmology

http://academic.oup.com/mnras/advance-article-pdf/doi/10.1093/mnras/stab2737/40440260/stab2737.pdf

globalemu: a novel and robust approach for emulating the sky-averaged 21-cm signal from the cosmic dawn and epoch of reionization

Following the reported detection of an absorption profile associated with the 21~cm sky-averaged signal from the Cosmic Dawn by the EDGES experiment in 2018, a number of experiments have been set up to verify this result. This paper discusses the design process used for global 21~cm experiments, focusing specifically on the Radio Experiment for the Analysis of Cosmic Hydrogen (REACH). This experiment will seek to understand and compensate for systematic errors present using detailed modelling and characterization of the instrumentation. There is detailed the quantitative figures of merit and numerical modelling used to assist the design process of the REACH dipole antenna (one of the 2 antenna designs for REACH Phase I). This design process produced a 2.5:1 frequency bandwidth dipole. The aim of this design was to balance spectral smoothness and low impedance reflections with the ability to describe and understand the antenna response to the sky signal to inform the critically important calibration during observation and data analysis.

Radio antenna design for sky-averaged 21 cm cosmology experiments: the REACH case

Emulation of the Global (sky-averaged) 21-cm signal from the Cosmic Dawn and Epoch of Reionization with neural networks has been shown to be an essential tool for physical signal modelling. In this paper we present globalemu, a Global 21-cm signal emulator that uses redshift as a character defining variable along side a set of astrophysical parameters to estimate the brightness temperature of the 21-cm signal. Combined with a physically motivated pre-processing of the data this makes for a reliable and fast emulator that is relatively insensitive to the neural network design. A single high resolution signal can be emulated in 1.3 ms when using globalemu in comparison to 133 ms, a factor of 102 improvement, when using the existing public state of the art emulator 21cmGEM evaluated with the same computing power. We illustrate, with the same training and test data used for 21cmGEM, that globalemu is almost twice as accurate as 21cmGEM and for 95% of models in a test set of $\approx1,700$ we can achieve a RMSE of $\leq 5.37$ mK and a mean RMSE of 2.52 mK across the band z = 7 -28 (approximately 10% the expected noise of 25 mK for the Radio Experiment for the Analysis of Cosmic Hydrogen (REACH)). Further, globalemu provides a flexible framework in which the neutral fraction history and Global signal models with updated astrophysics can be emulated easily. The emulator is pip installable and available at: this https URL. globalemu will be used by the REACH collaboration to perform physical signal modelling inside a Bayesian nested sampling loop.

H T J Bevins

Papers

MAXSMOOTH: rapid maximally smooth function fitting with applications in Global 21-cm cosmology

globalemu: a novel and robust approach for emulating the sky-averaged 21-cm signal from the cosmic dawn and epoch of reionization

Radio antenna design for sky-averaged 21 cm cosmology experiments: the REACH case

GLOBALEMU: A novel and robust approach for emulating the sky-averaged 21-cm signal from the cosmic dawn and epoch of reionisation