arXiv subjects

Natalie B. Hogg

Publications and source records attributed to Natalie B. Hogg.

At least 19 recordsLinked to original sources

arXiv2026

Cosmology with the line-of-sight shear of strong gravitational lenses

Stage-IV photometric galaxy surveys are designed to measure the position and shapes of billions of galaxies. Their aim is to characterise the large-scale distribution of matter in the Universe using galaxy clustering and weak gravitational lensing. As a byproduct, stage-IV surveys are expected to detect more than a hundred thousand strong gravitational lenses. In this article, we propose the use of weak-lensing perturbations to strong lenses, specifically their line-of-sight (LOS) shear, as a cosmological probe. This new observable allows us to define three new correlation functions: the LOS shear with itself, with galaxy positions, and with galaxy shapes, thereby promoting the standard $3\times 2$pt correlation method to a $6\times 2$pt scheme. We design estimators for these new correlation functions and determine their expectation values as a function of the matter power spectrum. We then derive the analytical expression for the full covariance matrix of the $6\times 2$pt correlation scheme. Considering various scenarios for the stage-IV strong-lensing samples, we demonstrate that the cosmological information carried by the LOS shear of strong lenses will be detectable with a very high signal-to-noise ratio, even in the most pessimistic of cases. Strong lenses are thus extremely promising cosmological probes, whose synergy with galaxy positions and shapes should also contribute to mitigating systematics in stage-IV surveys.

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arXiv2026

Agentic research is oxymoronic

The use of agentic large language models obviates human interpretation of scientific results, and will lead to substantial distrust in the literature.

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arXiv2026

Line-of-sight shear in SLACS strong lenses II: validation tests with an extended sample

Strong gravitational lensing images are subject to shape distortions due to inhomogeneities along the line of sight. The leading order shape distortion is shear, which, if measurable, will be a complementary cosmological probe to traditional cosmic shear. In Hogg et al. (2025a), we modelled 23 of the SLACS strong lenses, studying the line-of-sight (LOS) shear under a variety of shear and mass model parametrisations. In this work, we successfully model 22 of an additional 27 lenses, extending our sample of LOS shear constraints to 45 in total. We find a mean shear magnitude of $0.11\pm 0.024$, showing that a significant fraction of the lenses modelled in this work possess LOS shears with unexpectedly large magnitudes, $|γ_{\rm LOS}| > 0.1$, even when an octupolar distortion is included in the lens mass. We further investigate if factors such as lens and source redshift, filter and PSF, or flux and signal-to-noise ratio in the lensed arcs correlate with shear. We find that none of these features play a statistically significant role in the production of unusually large shear magnitudes.

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arXiv2026

Line-of-sight shear in SLACS strong lenses I: shear and mass model parametrisations

Inhomogeneities along the line of sight in strong gravitational lensing distort the images produced, in an effect called shear. If measurable, this shear may provide independent constraints on cosmological parameters, complementary to traditional cosmic shear. We model 23 strong gravitational lenses from the Sloan Lens ACS (SLACS) catalogue with the aim of measuring the line-of-sight (LOS) shear for the first time. We use the 'minimal model' for the LOS shear, which has been shown to be theoretically safe from degeneracies with lens model parameters, a finding which has been confirmed using mock data. We use the dolphin automated modelling pipeline, which uses the lenstronomy software as a modelling engine, to model our selected lenses. Across the 23 lenses, we measure the LOS shear with a mean magnitude of $0.056 \pm 0.013$. Neglecting the post-Born correction to the potential of the main deflector due to foreground shear leads to a propagation of degeneracies in the LOS shear measurement with other lens model parameters, and the inclusion of an octupole moment in the lens mass profile does not lead to shear measurements that are in better agreement with the expectations from weak lensing.

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arXiv2026

JAXtronomy: A JAX port of lenstronomy

Gravitational lensing is a phenomenon where light bends around massive objects, resulting in distorted images seen by an observer. Studying gravitationally lensed systems provides insights into cosmology and astrophysics, including constraints of the expansion rate of the Universe and the distribution of dark matter. Thus, we introduce JAXtronomy, a re-implementation of the gravitational lensing software package lenstronomy (Birrer, 2021; Birrer & Amara, 2018) using JAX (Bradbury et al., 2018). JAX is a Python library that uses an accelerated linear algebra (XLA) compiler to improve the performance of computing software. Our core design principle of JAXtronomy is to maintain an identical API to that of lenstronomy. The main JAX features utilized in JAXtronomy are just-in-time compilation, which can lead to significant reductions in execution time, and automatic differentiation, which allows for the implementation of gradient-based algorithms that were previously impossible. Additionally, JAX allows code to be run on GPUs or parallelized across CPU cores, further boosting the performance of JAXtronomy.

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arXiv2026

An ultra-high-resolution map of (dark) matter

Ordinary matter-including particles such as protons and neutrons-accounts for only about one sixth of all matter in the Universe. The rest is dark matter, which does not emit or absorb light but plays a fundamental role in galaxy and structure evolution. Because it interacts only through gravity, one of the most direct probes is weak gravitational lensing: the deflection of light from distant galaxies by intervening mass. Here we present an extremely detailed, wide-area weak-lensing mass map, covering 0.77 deg x 0.70 deg, using high-resolution imaging from the James Webb Space Telescope (JWST) as part of the COSMOS-Web survey. By measuring the shapes of 129 galaxies per square arcminute-many independently in the F115W and F150W bands-we achieve an angular resolution of 1.00 +/- 0.01 arcmin. Our map has more than twice the resolution of earlier Hubble Space Telescope maps, revealing how dark and luminous matter co-evolve across filaments, clusters, and under-densities. It traces mass features out to z ~ 2, including the most distant structure at z ~ 1.1. The sensitivity to high-redshift lensing constrains galaxy environments at the peak of cosmic star formation and sets a high-resolution benchmark for testing theories about the nature of dark matter and the formation of large-scale cosmic structure

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arXiv2025

The COSMOS-Web Lens Survey (COWLS) III: forecasts versus data

We compare forecasts for the abundance and properties of strong gravitational lenses in the COSMOS-Web survey, a $0.54$ deg$^2$ survey of the COSMOS field using the NIRCam and MIRI instruments aboard JWST, with the first catalogue of strong lens candidates identified in the observed NIRCam data, COWLS. We modify the lenspop package to produce a forecast for strong lensing in COSMOS-Web. We add a new mock galaxy catalogue to use as the source population, as well as the COSMOS-Web survey specifications, including the transmission data for the four NIRCam filters used. We forecast 107 strong lenses can be detected in COSMOS-Web across all bands, assuming complete subtraction of the lens galaxy light. The majority of the lenses are forecast to have small Einstein radii ($θ_{\rm E} < 1$ arcsecond) and lie at redshifts between $0 < z <2$, whilst the source redshift distribution peaks at $z\sim 3$ and has a long tail extending up to $z \sim 11$, unambiguously showing that strong lensing in JWST can probe the entirety of the epoch of reionisation. We compare our forecast with the distributions of Einstein radii, lens photometric redshifts, and lens and source magnitudes in the observed lenses, finding that whilst the forecast and observed Einstein radii distributions match, the redshifts and magnitudes do not. The observed lens redshift distribution peaks at a slightly lower redshift than the forecast one, whilst the lens magnitudes are systematically brighter in the observed data than in the forecast.

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arXiv2025

The COSMOS-Web Lens Survey (COWLS) I: Discovery of >100 high redshift strong lenses in contiguous JWST imaging

We present the COSMOS-Web Lens Survey (COWLS), a sample of over 100 strong lens candidates from the $0.54$\,deg$^2$ COSMOS-Web survey, discovered using exquisite James Webb Space Telescope (JWST) imaging across four wavebands. Following two rounds of visual inspection, over 100 candidates were ranked as `high confidence' or `likely' by at least $50\%$ of inspectors. The COWLS sample has several notable properties: (i) magnified source galaxies spanning redshifts $z \sim 0.1$ to $z \sim 9$, which therefore extend into the epoch of reionisation; (ii) the highest-redshift lens galaxies known, pushing galaxy density profile evolution studies beyond $z \sim 2$; (iii) all lenses are distributed within a contiguous $0.54$\,deg$^2$ region, allowing for joint strong and weak lensing analyses; and (iv) a subset exhibits lensed source emission ray-traced near the lens galaxy centers, enabling studies of supermassive black holes and dust absorption. A key innovation of our approach is the use of lens modelling to aid in identifying lenses that may otherwise be missed. This paper is accompanied by the first COWLS public release, providing JWST NIRCam imaging in four bands, lens models, pixelized source reconstructions and lens redshift estimates : https://github.com/Jammy2211/COWLS_COSMOS_Web_Lens_Survey

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arXiv2025

Implications of distance duality violation for the $H_0$ tension and evolving dark energy

We investigate whether a violation of the distance duality relation (DDR), $D_L(z) = (1+z)^2 D_A(z)$, connecting the angular diameter and luminosity distances, can explain the Hubble tension and alter the evidence for dynamical dark energy in recent cosmological observations. We constrain five phenomenological parameterisations of DDR violation using Baryon Acoustic Oscillation measurements from the DESI survey calibrated with the sound horizon derived from \textit{Planck} Cosmic Microwave Background data and the Pantheon+ Type Ia supernova (SNIa) catalogue calibrated with the supernova absolute magnitude from S$H_0$ES. We find that two toy models can resolve the tension: a constant offset in the DDR (equivalent to a shift in the calibration of the SNIa data), $D_L(z)/D_A(z)\simeq 0.925(1+z)^2$, which leaves the hint for evolving dark energy unaffected; or a change in the power-law redshift-dependence of the DDR, restricted to $z\lesssim 1$, $D_L(z)/D_A(z)\simeq(1+z)^{1.866}$, together with a {\it constant} phantom dark energy equation of state $w\sim -1.155$. The Bayesian evidence slightly favours the latter model. Our phenomenological approach motivates the investigation of physical models of DDR violation as a novel way to explain the Hubble tension.

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arXiv2025

The COSMOS-Web Lens Survey (COWLS) II: depth, resolution, and NIR coverage from JWST reveal 17 spectacular lenses

The COSMOS-Web Lens Survey (COWLS) presents the first systematic search for strong gravitational lenses in the COSMOS-Web field using data from the \textit{James Webb} Space Telescope (\textit{JWST}). Using high-resolution NIRCam imaging, we visually inspected over 42\,660 galaxies and identified over 400 lensing candidates. From this sample and based on \textit{JWST}/NIRCam imaging only, we report here the 17 most obvious and spectacular strong lensing systems. These lenses, characterised by large Einstein rings and arcs and their distinct lens and source colours, were found through only the visual inspection of the lens-light-subtracted image data and were immediately visible due to their spectacular appearance. We showcase how spectacular strong lenses are at the extremes of lens parameter space. Their exceptionally high signal-to-noise, multi-wavelength imaging enables unprecedented lensing analysis, including `\textit{HST}-dark' source galaxies that are also invisible in the deeper bluer \textit{JWST} wavebands, enabling clean deblending between the lens and the source. Sources may exhibit dramatic morphological changes across wavelengths, and dust absorption within lenses may be detectable by eye. No other instrument, including the \textit{Hubble} Space Telescope, can discover or image such lenses with comparable detail. We estimate that \textit{JWST} uncovers a new spectacular lens approximately every 10 to 12 NIRCam pointings, suggesting that over 40 such lenses remain undetected within its first three years of observations. All COWLS data is publicly available on GitHub.

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arXiv2024

Weak lensing of strong lensing: beyond the tidal regime

The analysis of strong lensing images usually involves an external convergence and shear, which are meant to model the effect of perturbations along the line of sight, on top of the main lens. Such a description of line-of-sight perturbations supposes that the corresponding gravitational fields can be treated in the tidal regime. Going one step further introduces additional effects, known as flexion, which have been hitherto neglected in strong lensing. In this work, we build a minimal model for the line-of-sight flexion, which adds four new complex parameters to the lens model. Contrary to convergence and shear, the line-of-sight flexion cannot be projected onto the main lens plane. For a $Λ$CDM cosmology, we predict the typical line-of-sight flexion to be on the order of $10^{-3} \mathrm{arcsec}^{-1}$ on galactic scales. Neglecting its effect in lens modelling is found to bias the recovery of other parameters; in particular, the line-of-sight shear can be biased up to $2σ$. Accounting for the line-of-sight flexion in our minimal framework restores accuracy, at the the cost of degrading precision. With current imaging capabilities, the line-of-sight flexion is unlikely to be measurable on individual strong lensing images; it must therefore be considered a nuisance parameter rather than an observable in its own right.

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arXiv2024

The interacting vacuum and tensions: a comparison of theoretical models

We analyse three interacting vacuum dark energy models with the aim of exploring whether the $H_0$ and $σ_8$ tensions can be simultaneously resolved in such models. We present the first ever derivation of the covariant gauge-invariant perturbation formalism for the interacting vacuum scenario, and, for the sub-class of geodesic cold dark matter models, connect the evolution of perturbation variables in this approach to the familiar cosmological observables. We show how $H_0$ and $σ_8$ evolve in three interacting vacuum models: firstly, a simple linear coupling between the vacuum and cold dark matter; secondly, a coupling which mimics the behaviour of a Chaplygin gas; and finally a coupling which mimics the Shan--Chen fluid dark energy model. We identify, if any, the regions of parameter space which would correspond to a simultaneous resolution of both tensions in these models. When constraints from observational data are added, we show how all the models described are constrained to be close to their $Λ$CDM limits.

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arXiv2024

Constraints on dark energy from TDCOSMO & SLACS lenses

Problems with the cosmological constant model of dark energy motivate the investigation of alternative scenarios. I make the first measurement of the dark energy equation of state using the hierarchical strong lensing time delay likelihood provided by TDCOSMO. I find that the combination of seven TDCOSMO lenses and 33 SLACS lenses is only able to provide a weak constraint on the dark energy equation of state, $w < -1.75$ at 68% confidence, which nevertheless implies the presence of a phantom dark energy component. When the strong lensing time delay data is combined with a collection of cosmic microwave background, baryon acoustic oscillation and Type Ia supernova data, I find that the equation of state is $w = -1.025\pm 0.029$.

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arXiv2023

Measuring line-of-sight shear with Einstein rings: a proof of concept

Line-of-sight effects in strong gravitational lensing have long been treated as a nuisance. However, it was recently proposed that the line-of-sight shear could be a cosmological observable in its own right, if it is not degenerate with lens model parameters. We firstly demonstrate that the line-of-sight shear can be accurately measured from a simple simulated strong lensing image with percent precision. We then extend our analysis to more complex simulated images and stress test the recovery of the line-of-sight shear when using deficient fitting models, finding that it escapes from degeneracies with lens model parameters, albeit at the expense of the precision. Lastly, we check the validity of the tidal approximation by simulating and fitting an image generated in the presence of many line-of-sight dark matter haloes, finding that an explicit violation of the tidal approximation does not necessarily prevent one from measuring the line-of-sight shear.

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arXiv2022

Dancing in the dark: detecting a population of distant primordial black holes

Primordial black holes (PBHs) are compact objects proposed to have formed in the early Universe from the collapse of small-scale over-densities. Their existence may be detected from the observation of gravitational waves (GWs) emitted by PBH mergers, if the signals can be distinguished from those produced by the merging of astrophysical black holes. In this work, we forecast the capability of the Einstein Telescope, a proposed third-generation GW observatory, to identify and measure the abundance of a subdominant population of distant PBHs, using the difference in the redshift evolution of the merger rate of the two populations as our discriminant. We carefully model the merger rates and generate realistic mock catalogues of the luminosity distances and errors that would be obtained from GW signals observed by the Einstein Telescope. We use two independent statistical methods to analyse the mock data, finding that, with our more powerful, likelihood-based method, PBH abundances as small as $f_\mathrm{PBH} \approx 7 \times 10^{-6}$ ($f_\mathrm{PBH} \approx 2\times10^{-6}$) would be distinguishable from $f_\mathrm{PBH} = 0$ at the level of $3σ$ with a one year (ten year) observing run of the Einstein Telescope. Our mock data generation code, darksirens, is fast, easily extendable and publicly available on GitLab.

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arXiv2022

The resilience of the Etherington-Hubble relation

The Etherington reciprocity theorem, or distance duality relation (DDR), relates the mutual scaling of cosmic distances in any metric theory of gravity where photons are massless and propagate on null geodesics. In this paper, we make use of the DDR to build a consistency check based on its degeneracy with the Hubble constant, $H_0$. We parameterise the DDR using the form $η(z) = 1+ εz$, thus only allowing small deviations from its standard value. We use a combination of late time observational data to provide the first joint constraints on the Hubble parameter and $ε$ with percentage accuracy: $H_0 = 68.6 \pm 2.5$ kms$^{-1}$Mpc$^{-1}$ and $ε= 0.001^{+0.023}_{-0.026}$. We build our consistency check using these constraints and compare them with the results obtained in extended cosmological models using cosmic microwave background data. We find that extensions to $Λ$CDM involving massive neutrinos and/or additional dark radiation are in perfect agreement with the DDR, while models with non-zero spatial curvature show a preference for DDR violation, i.e., $ε\ne 0 $ at the level of $\sim 1.5 σ$. Most importantly, we find a mild 2$σ$ discrepancy between the validity of the DDR and the latest publicly available Cepheid-calibrated SNIa constraint on $H_0$. We discuss the potential consequences of this for both the Etherington reciprocity theorem and the $H_0$ tension.

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arXiv2022

Cosmology Intertwined: A Review of the Particle Physics, Astrophysics, and Cosmology Associated with the Cosmological Tensions and Anomalies

In this paper we will list a few important goals that need to be addressed in the next decade, also taking into account the current discordances between the different cosmological probes, such as the disagreement in the value of the Hubble constant $H_0$, the $σ_8$--$S_8$ tension, and other less statistically significant anomalies. While these discordances can still be in part the result of systematic errors, their persistence after several years of accurate analysis strongly hints at cracks in the standard cosmological scenario and the necessity for new physics or generalisations beyond the standard model. In this paper, we focus on the $5.0\,σ$ tension between the {\it Planck} CMB estimate of the Hubble constant $H_0$ and the SH0ES collaboration measurements. After showing the $H_0$ evaluations made from different teams using different methods and geometric calibrations, we list a few interesting new physics models that could alleviate this tension and discuss how the next decade's experiments will be crucial. Moreover, we focus on the tension of the {\it Planck} CMB data with weak lensing measurements and redshift surveys, about the value of the matter energy density $Ω_m$, and the amplitude or rate of the growth of structure ($σ_8,fσ_8$). We list a few interesting models proposed for alleviating this tension, and we discuss the importance of trying to fit a full array of data with a single model and not just one parameter at a time. Additionally, we present a wide range of other less discussed anomalies at a statistical significance level lower than the $H_0$--$S_8$ tensions which may also constitute hints towards new physics, and we discuss possible generic theoretical approaches that can collectively explain the non-standard nature of these signals.[Abridged]

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arXiv2022

Shan--Chen interacting vacuum cosmology

In this paper, we introduce a novel class of interacting vacuum models, based on recasting the equation of state originally developed in the context of lattice kinetic theory by Shan \& Chen as the coupling between the vacuum and cold dark matter (CDM). This coupling allows the vacuum to evolve and is nonlinear around a characteristic energy scale $ρ_*$, changing into a linear coupling with a typical power law evolution at scales much lower and much higher than $ρ_*$. Focusing on the simplest sub-class of models where the interaction consists only of an energy exchange and the CDM remains geodesic, we first illustrate the various possible models that can arise from the Shan--Chen coupling, with several different behaviours at both early and late times depending on the values of the model parameters selected. We then place the first observational constraints on this Shan--Chen interacting vacuum scenario, performing an MCMC analysis to find those values of the model and cosmological parameters which are favoured by observational data. We focus on models where the nonlinearity of the coupling is relevant at late times, choosing for the reference energy scale $ρ_*$, the critical energy density in $Λ$CDM. We show that the observational data we use are compatible with a wide range of models which result in different cosmologies. However, we also show that $Λ$CDM is preferred over all of the Shan--Chen interacting vacuum models that we study, and comment on the inability of these models to relax the $H_0$ and $σ_8$ tensions.

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