Contamination from instrumental effects interacting with bright astrophysical sources is the primary impediment to measuring Epoch of Reionization and BAO 21 cm power spectra---an effect called mode-mixing. In this paper we identify four fundamental power spectrum shapes produced by mode-mixing that will affect all upcoming observations. We are able, for the first time, to explain the wedge-like structure seen in advanced simulations and to forecast the shape of an 'EoR window' that is mostly free of contamination. Understanding the origins of these contaminations also enables us to identify calibration and foreground subtraction errors below the imaging limit, providing a powerful new tool for precision observations.

Four Fundamental Foreground Power Spectrum Shapes for 21 cm Cosmology Observations

Abstract The detection of the accelerated expansion of the Universe has been one of the major breakthroughs in modern cosmology. Several cosmological probes (Cosmic Microwave Background, Supernovae Type Ia, Baryon Acoustic Oscillations) have been studied in depth to better understand the nature of the mechanism driving this acceleration, and they are being currently pushed to their limits, obtaining remarkable constraints that allowed us to shape the standard cosmological model. In parallel to that, however, the percent precision achieved has recently revealed apparent tensions between measurements obtained from different methods. These are either indicating some unaccounted systematic effects, or are pointing toward new physics. Following the development of CMB, SNe, and BAO cosmology, it is critical to extend our selection of cosmological probes. Novel probes can be exploited to validate results, control or mitigate systematic effects, and, most importantly, to increase the accuracy and robustness of our results. This review is meant to provide a state-of-art benchmark of the latest advances in emerging “beyond-standard” cosmological probes. We present how several different methods can become a key resource for observational cosmology. In particular, we review cosmic chronometers, quasars, gamma-ray bursts, standard sirens, lensing time-delay with galaxies and clusters, cosmic voids, neutral hydrogen intensity mapping, surface brightness fluctuations, stellar ages of the oldest objects, secular redshift drift, and clustering of standard candles. The review describes the method, systematics, and results of each probe in a homogeneous way, giving the reader a clear picture of the available innovative methods that have been introduced in recent years and how to apply them. The review also discusses the potential synergies and complementarities between the various probes, exploring how they will contribute to the future of modern cosmology. 

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Unveiling the Universe with emerging cosmological probes

The Murchison Widefield Array (MWA) team has derived new upper limits on the spherically averaged power spectrum of the 21-cm signal at six redshifts in the range z approximate to 6.5-8.7. We use t ...

Constraining the state of the intergalactic medium during the Epoch of Reionization using MWA 21-cm signal observations

Radio interferometer arrays such as HERA consist of many close-packed dishes arranged in a regular pattern, giving rise to a large number of `redundant' baselines with the same length and orientation. Since identical baselines should see an identical sky signal, this provides a way of finding a relative gain/bandpass calibration without needing an explicit sky model. In reality, there are many reasons why baselines will not be exactly identical, giving rise to a host of effects that spoil the redundancy of the array and induce spurious structure in the calibration solutions if not accounted for. In this paper, we perform a wide range of simulations for a small HERA-like array to understand how different types of non-redundancy manifest in the observed interferometric visibilities and their resulting frequency (delay-space) power spectra. We focus in particular on differences in the primary beam response between antennas, including variations in the main lobe, sidelobes, ellipticity, and orientation. We find that different types of non-redundancy impart characteristic patterns into the redundant gain solutions, which in turn introduce additional structure into the calibrated visibilities and therefore the delay spectra. We show that the most severe effects of primary beam non-redundancy are induced by the brightest sources passing through the beam, while diffuse emission has a lesser (but non-negligible) effect. We also find that redundant baseline groups with `outlier' antennas (where only one antenna deviates from perfect redundancy) sustain the largest gain errors, while even $\mathcal{O}(1)$ non-redundancies in the sidelobes seem to have a relatively minor impact in comparison.

Patterns of primary beam non-redundancy in close-packed 21 cm array observations

The bispectrum can quantify the non-Gussianity present in the redshifted 21-cm signal produced by the neutral hydrogen (HI) during the epoch of reionization (EoR). Motivated by this, we perform a comprehensive study of the EoR 21-cm bispectrum using simulated signals. Given a model of reionization, we demonstrate the behaviour of the bispectrum for all unique triangles in $k$ space. For ease of identification of the unique triangles, we parametrize the $k$-triangle space with two parameters, namely the ratio of the two arms of the triangle ($n=k_2/k_1$) and the cosine of the angle between them ($\cos{\theta}$). Furthermore, for the first time, we quantify the impact of the redshift space distortions (RSD) on the spherically averaged EoR 21-cm bispectrum in the entire unique triangle space. We find that the real space signal bispectra for small and intermediate $k_1$-triangles ($k_1 \leq 0.6 \,{\rm Mpc^{-1}}$) is negative in most of the unique triangle space. It takes a positive sign for squeezed, stretched and linear $k_1$-triangles, specifically for large $k_1$ values ($k_1 \geq 0.6 \,{\rm Mpc^{-1}}$). The RSD affects both the sign and magnitude of the bispectra significantly. It changes (increases/decreases) the magnitude of the bispectra by $50-100\%$ without changing its sign (mostly) during the entire period of the EoR for small and intermediate $k_1$-triangles. For larger $k_1$-triangles, RSD affects the magnitude by $100-200\%$ and also flips the sign from negative to positive. We conclude that it is important to take into account the impact of RSD for a correct interpretation of the EoR 21-cm bispectra.

Redshifted 21-cm bispectrum – I. Impact of the redshift space distortions on the signal from the Epoch of Reionization

We compare various foreground removal techniques that are being utilised to remove bright foregrounds in various experiments aiming to detect the redshifted 21cm signal of neutral hydrogen from the Epoch of Reionization. In this work, we test the performance of removal techniques (FastICA, GMCA, and GPR) on 10 nights of LOFAR data and investigate the possibility of recovering the latest upper limit on the 21cm signal. Interestingly, we find that GMCA and FastICA reproduce the most recent 2σ upper limit of Δ221< (73)2 mK2 at k=0.075 hcMpc−1, which resulted from the application of GPR. We also find that FastICA and GMCA begin to deviate from the noise-limit at \textit{;k};-scales larger than ∼0.1 hcMpc−1. We then replicate the data via simulations to see the source of FastICA and GMCA's limitations, by testing them against various instrumental effects. We find that no single instrumental effect, such as primary beam effects or mode-mixing, can explain the poorer recovery by FastICA and GMCA at larger \textit{;k};-scales. We then test scale- independence of FastICA and GMCA, and find that lower \textit{;k};-scales can be modelled by a smaller number of independent components. For larger scales (k≳0.1 hcMpc−1), more independent components are needed to fit the foregrounds. We conclude that, the current usage of GPR by the LOFAR collaboration is the appropriate removal technique. It is both robust and less prone to overfitting, with future improvements to GPR's fitting optimisation to yield deeper limits.

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Comparing foreground removal techniques for recovery of the LOFAR-EoR 21 cm power spectrum

Numerous studies have established the theoretical potential of the 21cm bispectrum to boost our understanding of the Epoch of Reionization (EoR), and therefore early generation of stars and galaxies. In this paper we look at the impact of foregrounds and instrumental effects on the 21cm bispectrum and our ability to measure it. Unlike the power spectrum for which (in the absence of instrumental effects) there is a window clear of smooth-spectrum foregrounds in which the 21cm power spectrum may be detectable, there is no such "EoR window" for the bispectrum. Instead, on smaller scales EoR structure modulates that of the foregrounds (FG) to alter the EoR+FG bispectrum from that of the foregrounds in a complex manner. On larger scales the EoR structures are completely swamped by those of the foregrounds, and the EoR+FG bispectrum is entirely dominated by that of the foregrounds. It is therefore unlikely that the bispectrum will be useful in cases where we are observationally restricted to using foreground avoidance techniques. We also find that there is potential for instrumental effects to seriously corrupt the bispectrum, possibly even rendering the bispectrum useless for parameter estimation. On larger scales, foreground removal using GMCA is found to recover the EoR bispectrum to a reasonable level of accuracy (even better than the power spectrum for certain configurations). Further studies are necessary to understand the error and/or bias associated with foreground removal before the 21cm bispectrum can be practically applied in analysis of future data.

Ian Hothi

Papers

Comparing foreground removal techniques for recovery of the LOFAR-EoR 21 cm power spectrum

The bispectrum and 21-cm foregrounds during the Epoch of Reionization