arXiv subjects

Thomas Gessey-Jones

Publications and source records attributed to Thomas Gessey-Jones.

10 recordsLinked to original sources

arXiv2026

Optimisation of calibration sources for global 21-cm experiments: the REACH case

The spin-flip 21-cm signal from the Cosmic Dawn and the Epoch of Reionization is an essential probe of the conditions that led to the formation of the first luminous objects in the early Universe. However, its detection remains a major challenge owing to its low strength compared to the bright foregrounds and the requirement of precise calibration of the instrument to prevent systematics that could hinder a detection or lead to false inferences. REACH (Radio Experiment for the Analysis of Cosmic Hydrogen) is a radiometer experiment designed to detect this sky-averaged signal in the frequency range of 50--130~MHz. Using a wide-beam antenna, REACH calibration relies on internal reference sources, covering a broad range of temperatures and reflection coefficients. The choice of type and number of calibrators used significantly influences the quality of the calibration. This work investigates these effects and introduces a novel method for selecting an optimal set of calibration sources. With an optimised set, we aim to reduce calibration time, thereby increasing sky integration time while preserving calibration accuracy. We explore two optimisation strategies: one applied across the full receiver band and another performed on a frequency-by-frequency basis. Finally, we demonstrate that, with a total calibration time comparable to the conventional full-calibrator set, an optimised set with fewer calibrators achieves approximately a $15~\%$ reduction in calibrated temperature noise and improved absolute calibration of the instrument. This has implications for better calibration strategies in similar radiometer experiments.

astro-ph.IM↗

arXiv2026

Circuit Modeling for In Situ 21 cm Radiometer Calibration

Recent experiments in cosmology, particularly those aimed at detecting the faint, redshifted, global 21 cm hydrogen line (depth < ~200 mK, z > 7.5), have imposed stringent new requirements on radiometer calibration. In this work, we present a framework for circuit modeling and parameter inference to strengthen these calibration pipelines. This new approach enables in situ characterization of otherwise immeasurable systematics using physically motivated models. A combination of frequentist and Bayesian techniques are employed in a pipeline that supports iterative modeling, robust parameter estimation, and detailed uncertainty quantification. The framework is applied to the REACH telescope, where the precise correction of variations in the radio signal paths arising from component aging or environmental effects is critical. Circuit models of REACH's calibration sources are developed, with the goal of predicting source temperature corrections that are conventionally obtained from laboratory measurements. By fitting the models to measured data using a convolutional cost function, a strong agreement with RMS residuals no worse than -37 dB is obtained. However, Bayesian inference reveals that the resulting temperature corrections can have uncertainties on the order of 1 to 2 K, caused by reflection coefficient degeneracies, measurement noise, and errors in the models. To combat this, posteriors obtained from laboratory measurements are employed as updated priors, reducing correction uncertainties down to 75 mK. Ultimately, the framework provides a means of dynamically accounting for drift in system non-idealities over time, addressing the increasing precision demands of global 21 cm radio astronomy.

astro-ph.IM↗

arXiv2026

Calibrating Bayesian Tension Statistics using Neural Ratio Estimation

When fits of the same physical model to two different datasets disagree, we call this tension. Several apparent tensions in cosmology have occupied researchers in recent years, and a number of different metrics have been proposed to quantify tension. Many of these metrics suffer from limiting assumptions, and correctly calibrating these is essential if we want to successfully determine whether discrepancies are significant. A commonly used metric of tension is the evidence ratio R. The statistic has been widely adopted by the community as a Bayesian way of quantifying tensions, however, it has a non-trivial dependence on the prior that is not always accounted for properly. We show that this can be calibrated out effectively with Neural Ratio Estimation. We demonstrate our proposed calibration technique with an analytic example, a toy example inspired by 21-cm cosmology, and with observations of the Baryon Acoustic Oscillations from the Dark Energy Spectroscopic Instrument (DESI) and the Sloan Digital Sky Survey (SDSS). We find no significant tension between DESI and SDSS.

astro-ph.CO↗

arXiv2025

Narrowing the discovery space of the cosmological 21-cm signal using multi-wavelength constraints

The cosmic 21-cm signal is a promising probe of the early Universe, owing to its sensitivity to the thermal state of the neutral intergalactic medium (IGM) and properties of the first luminous sources. Here, we constrain the 21-cm signal and infer IGM properties using the Population II galaxy parameters derived in a previous study through multi-wavelength synergies. This includes high-redshift UV luminosity functions (UVLFs) from Hubble Space Telescope (HST) and James Webb Space Telescope (JWST), cosmic X-ray and radio backgrounds (CXB and CRB), the SARAS 3 global 21-cm signal non-detection, and HERA 21-cm power spectrum upper limits. From CXB and HERA data, we infer the IGM kinetic temperature to be $T_\text{K}(z=15)\lesssim 7.7~\text{K}$, $2.5~\text{K} \lesssim T_\text{K}(z=10) \lesssim 66~\text{K}$, and $20~\text{K}\lesssim T_\text{K}(z=6) \lesssim 2078~\text{K}$ at 95% credible interval (C.I.). Similarly, CRB and HERA data limit the radio emission efficiency of galaxies, giving $T_\text{rad}(z=15) \lesssim 47~\text{K}$, $T_\text{rad}(z=10)\lesssim 51~\text{K}$, and $T_\text{rad}(z=6)\lesssim 101~\text{K}$. These constraints, strengthened by UVLFs from HST and JWST, enable the first $\textit{lower bound}$ on the cosmic 21-cm signal. We infer an absorption trough of depth ${-201~\text{mK}\lesssim T_\text{21,min} \lesssim -68~\text{mK}}$ at $z_\text{min}\approx10-16$, and a power spectrum of $8.7~\text{mK}^2 < Δ_{21}^2(z=15) < 197~\text{mK}^2$ at $k=0.35~h\text{Mpc}^{-1}$. Our results highlight the power of multi-wavelength synergies in constraining the early Universe. While promising for upcoming 21-cm experiments, the results depend on our assumption of a redshift-independent X-ray and radio efficiency of galaxies, and the exclusion of a flexible model for Population III stars.

astro-ph.CO↗

arXiv2025

Exploiting synergies between JWST and cosmic 21-cm observations to uncover star formation in the early Universe

In the current era of JWST, we continue to uncover a wealth of information about the Universe deep into the Epoch of Reionization. In this work, we use a suite of simulations with 21cmSPACE, to explore the astrophysical properties of early galaxies and their imprint on high-redshift observables. Our analysis incorporates a range of multi-wavelength datasets including UV luminosity functions (UVLFs) from HST and JWST spanning $z=6-14.5$, the 21-cm global signal and power spectrum limits from SARAS 3 and HERA respectively, as well as present-day diffuse X-ray and radio backgrounds. We constrain a flexible halo-mass and redshift dependent model of star-formation efficiency (SFE), defined as the fraction of gas converted into stars, and find that it is best described by minimal redshift evolution at $z\approx 6-10$, followed by rapid evolution at $z\approx10-15$. Using Bayesian inference, we derive functional posteriors of the SFE, inferring that halos of mass $M_h=10^{10}\,\mathrm{M}_\odot$ have efficiencies of $\approx 1 - 2\%$ at $z\lesssim10$, $\approx8\%$ at $z=12$ and $\approx21\%$ at $z=15$. We also highlight the synergy between UVLFs and global 21-cm signal from SARAS 3 in constraining the minimum virial conditions required for star-formation in halos. In parallel, we find the X-ray and radio efficiencies of early galaxies to be $f_X = 0.8^{+9.7}_{-0.4}$ and $f_r \lesssim 16.9$ respectively, improving upon previous works that exclude UVLF data. Our results underscore the critical role of UVLFs in constraining early galaxy properties, and their synergy with 21-cm and other multi-wavelength observations.

astro-ph.GA↗

arXiv2025

Effects of chemically homogeneous evolution of the first stars on the 21-cm signal and reionization

The first generation of stars, known as Population III (Pop III), played a crucial role in the early Universe through their unique formation environment and metal-free composition. These stars can undergo chemically homogeneous evolution (CHE) due to fast rotation, becoming more compact and hotter/bluer than their (commonly assumed) non-rotating counterparts. In this study, we investigate the impact of Pop III CHE on the 21-cm signal and cosmic reionization under various assumptions on Pop III star formation, such as their formation efficiency, initial mass function, and transition to metal-enriched star formation. We combine stellar spectra computed by detailed atmosphere models with semi-numerical simulations of Cosmic Dawn and the Epoch of Reionization ($z\sim 6-30$). The key effect of CHE arises from the boosted ionizing power of Pop III stars, which reduces the Pop III stellar mass density required to reproduce the observed Thomson scattering optical depth by a factor of $\sim 2$. Meanwhile, the maximum 21-cm global absorption signal is shallower by up to $\sim 15$ mK (11%), partly due to the reduced Lyman-band emission from CHE, and the large-scale ($k\sim 0.2\ \rm cMpc^{-1}$) power drops by a factor of a few at $z\gtrsim 25$. In general, the effects of CHE can be comparable to those of Pop III star formation parameters, showing an interesting interplay with distinct features in different epochs. These results highlight the importance of metal-free/poor stellar evolution in understanding the early Universe and suggest that future studies should consider joint constraints on the physics of star/galaxy formation and stellar evolution.

astro-ph.GA↗

arXiv2024

Understanding spectral artefacts in SKA-LOW 21-cm cosmology experiments: the impact of cable reflections

The Cosmic Dawn was marked by the formation of the first stars, and preceded the Epoch of Reionization (EoR), when the Universe underwent a fundamental transformation caused by the radiation from these first stars and galaxies. Interferometric 21-cm experiments aim to probe redshifted neutral hydrogen signals from these periods, constraining the conditions of the early Universe. The SKA-Low instrument of the Square Kilometre Array (SKA) is envisaged to be the largest and most sensitive radio telescope at metre and centimetre wavelengths. The latest Aperture Array Verification Systems feature 7-m coaxial transmission lines connecting the low noise amplifiers to optical transmitters at the front of the analogue-receiving chain. An impedance mismatch between these components results in a partially reflected electromagnetic signal, introducing chromatic aberrations into the instrument bandpass. This causes power from the foreground signals to appear at higher delays, potentially contaminating the 'EoR window', a region in which the 21-cm signal should be detectable. We present an end-to-end simulation pipeline for SKA-Low using a composite sky model combining radio foregrounds from the Galactic and Extragalactic All-Sky MWA (GLEAM) Survey, Haslam $408\,$MHz, and a 1.5-cGpc 21-cm brightness temperature cube generated with the 21cmSPACE simulator. We derive a model for the scattering parameters of a coaxial transmission line in terms of its specifications and bulk material properties. Assuming identical cables of length $\leq 15.0\,$m with impedance mismatch $\leq 10\, Ω$, the reflection is confined below the EoR window. However, we demonstrate that even a 0.1 per cent length tolerance introduces contamination with an absolute fractional difference of $\sim 10$ per cent across all accessible k-modes.

astro-ph.CO↗

arXiv2024

Piecewise Normalizing Flows

Normalizing flows are an established approach for modelling complex probability densities through invertible transformations from a base distribution. However, the accuracy with which the target distribution can be captured by the normalizing flow is strongly influenced by the topology of the base distribution. A mismatch between the topology of the target and the base can result in a poor performance, as is typically the case for multi-modal problems. A number of different works have attempted to modify the topology of the base distribution to better match the target, either through the use of Gaussian Mixture Models (Izmailov et al., 2020; Ardizzone et al., 2020; Hagemann & Neumayer, 2021) or learned accept/reject sampling (Stimper et al., 2022). We introduce piecewise normalizing flows which divide the target distribution into clusters, with topologies that better match the standard normal base distribution, and train a series of flows to model complex multi-modal targets. We demonstrate the performance of the piecewise flows using some standard benchmarks and compare the accuracy of the flows to the approach taken in Stimper et al. (2022) for modelling multi-modal distributions. We find that our approach consistently outperforms the approach in Stimper et al. (2022) with a higher emulation accuracy on the standard benchmarks.

stat.ML↗

arXiv2024

Cosmic mysteries and the hydrogen 21-cm line: bridging the gap with lunar observations

The hydrogen 21-cm signal is predicted to be the richest probe of the young Universe including eras known as the cosmic Dark Ages, the Cosmic Dawn when the first star and black hole formed, and the Epoch of Reionization. This signal holds the key to deciphering processes that take place at the early stages of cosmic history. In this opinion piece, we discuss the potential scientific merit of lunar observations of the 21-cm signal and their advantages over more affordable terrestrial efforts. The moon is a prime location for radio cosmology which will enable precision observations of the low-frequency radio sky. The uniqueness of such observations is that they will provide an unparalleled opportunity to test cosmology and the nature of dark matter using the Dark Ages 21-cm signal. No less enticing is the opportunity to obtain a much clearer picture of Cosmic Dawn than what is achievable from the ground, which will allow us to probe properties of the first stars and black holes.

astro-ph.CO↗

arXiv2020

Narrative structure of A Song of Ice and Fire creates a fictional world with realistic measures of social complexity

Network science and data analytics are used to quantify static and dynamic structures in George R.R. Martin's epic novels, A Song of Ice and Fire, works noted for their scale and complexity. By tracking the network of character interactions as the story unfolds, it is found that structural properties remain approximately stable and comparable to real-world social networks. Furthermore, the degrees of the most connected characters reflect a cognitive limit on the number of concurrent social connections that humans tend to maintain. We also analyse the distribution of time intervals between significant deaths measured with respect to the in-story timeline. These are consistent with power-law distributions commonly found in inter-event times for a range of non-violent human activities in the real world. We propose that structural features in the narrative that are reflected in our actual social world help readers to follow and to relate to the story, despite its sprawling extent. It is also found that the distribution of intervals between significant deaths in chapters is different to that for the in-story timeline; it is geometric rather than power law. Geometric distributions are memoryless in that the time since the last death does not inform as to the time to the next. This provides measurable support for the widely held view that significant deaths in A Song of Ice and Fire are unpredictable chapter-by-chapter.

physics.soc-ph↗