arXiv subjects

Jess Worsley

Publications and source records attributed to Jess Worsley.

4 recordsLinked to original sources

arXiv2026

CosmoVerse Data Challenge: How Tip of the Red Giant Branch Analysis Choices Shape the Inferred Hubble Constant

Tip of the red giant branch (TRGB)-based determinations of the Hubble constant ($H_0$), the current expansion rate of the Universe, depend on choices made in selecting and analyzing stellar photometry and in assembling the sample of calibrator galaxies hosting Type Ia supernovae (SNe Ia). The CosmoVerse Data Challenge invites participants to analyze public photometric catalogs within a shared distance-ladder framework, opening each step of the measurement to the community and testing how these choices propagate into $H_0$. Participants independently selected photometric catalogs, galaxies, and TRGB measurement settings, while using a common SN Ia compilation and fitting procedure to convert distances into $H_0$. Supported by public notebooks and a training school, the challenge received 26 submissions from 22 participants. All submissions used the same analysis notebook, many participants were trainees at the school, and the true galaxy distances are unknown; the results therefore characterize the sensitivity of the submissions to analysis and sample-selection choices and are not a combined determination of $H_0$ or an estimate of the systematic uncertainty in published TRGB measurements. The inferred $H_0$ values have a median of $77.10\,\mathrm{km\,s^{-1}\,Mpc^{-1}}$ and a standard deviation of $7.38\,\mathrm{km\,s^{-1}\,Mpc^{-1}}$ $(5.11\,\mathrm{km\,s^{-1}\,Mpc^{-1}}$ for the scaled median absolute deviation), $\sim 3.4$ $(\sim 2.4)$ times the typical quoted uncertainty of a submission. Differences in the measured tip of the anchor galaxy NGC 4258 add a coherent shift in the distance scale. The challenge shows how different choices within a common framework can produce substantially different values of $H_0$, and provides an open basis for tracing these differences and for making datasets and tools more transparent and accessible.

astro-ph.GA↗

arXiv2026

Beyond $j=1$: Observational Constraints on Almost-$Λ$CDM Cosmologies

The cosmographic condition $j(z)=1$ provides the kinematical signature of the spatially flat $Λ$CDM model independently of any specific dark-energy or modified-gravity theory. We investigate the extent to which current observations permit departures from this condition by considering three phenomenological ``almost-$Λ$CDM'' cosmographic closures, in which the cosmic jerk differs slightly from unity through a small deformation parameter $ε$. The models are constrained using Markov Chain Monte Carlo analyses of recent DESI baryon acoustic oscillation measurements together with compressed Planck cosmic microwave background likelihoods and the Union3, Pantheon+, and DESY5 Type Ia supernova compilations. Rather than assuming a parameterized dark-energy equation of state, our cosmographic framework reconstructs the expansion history directly from observations, with the effective dark-energy equation of state emerging as a derived quantity. We find that all three closures are tightly constrained to the vicinity of the $Λ$CDM cosmographic fixed point, with Planck data driving the preferred evolution toward $j_0\simeq1$ and $w_{\rm DE,0}\simeq-1$. Despite their distinct kinematical constructions, the reconstructed dark-energy evolution consistently exhibits smooth freezing behaviour close to $w=-1$, without crossing the phantom divide. Model comparison using the Akaike and Bayesian information criteria shows that the almost-$Λ$CDM models remain statistically competitive with standard dark-energy parameterizations while requiring fewer assumptions about the functional form of $w(z)$. These results demonstrate the power of model-independent cosmography for constraining the cosmic expansion history and provide a natural framework for future studies of cosmological perturbations and structure formation.

astro-ph.CO↗

arXiv2026

Constraining Scale-Dependent Growth in $f(R)$ Gravity with Future 21 cm Surveys

Recent observations, particularly from DESI, have provided intriguing hints of dynamical behaviour in late-time dark energy. Modified gravity theories offer a compelling framework for interpreting such phenomena, with $f(R)$ gravity emerging as one of the most extensively studied examples. A central challenge in these models, however, lies in determining the precise functional form of $f(R)$. Nevertheless, several viable models have been proposed that successfully reproduce the standard $Λ$CDM cosmology at high red shifts while generating late-time cosmic acceleration without an explicit dark energy component. Within this framework, the evolution of the linear matter density contrast becomes scale dependent, leading to a growth index that varies with both scale and redshift. In this work, we explore the capability of forthcoming 21 cm observations to constrain the growth index, as well as the combined neutral hydrogen (HI) bias and growth-rate parameter. Our results indicate that future 21 cm surveys can provide meaningful, though moderate, support for these modified gravity scenarios.

astro-ph.CO↗

arXiv2024

A model independent approach to the study of structure growth in $f(R)$ gravity

Over the last decade, much attention has been given to the study of modified gravity theories to find a more natural explanation for the late-time acceleration of the Universe. Particular attention has focused on the so-called $f(R)$ dark energy models. Instead of focusing on a particular f(R) model, we present a completely model-independent approach to study the background dynamics and the growth of matter density perturbations for those f(R) models that mimic the $Λ$CDM evolution at the background level. We do this by characterising the dynamics of the gravitational field using a set of dimensionless variables and using cosmography to determine the expansion history. We then illustrate the integrity of this method by fixing the cosmography to be the same as an exact $Λ$CDM model, allowing us to test the solution. We compare the exact evolution of the density contrast and growth index with what one obtains from various levels of the quasi-static approximation, without choosing the form of $f(R)$ dark energy.

gr-qc↗