arXiv subjects

Chris Clarkson

Publications and source records attributed to Chris Clarkson.

At least 19 recordsLinked to original sources

arXiv2026

Cosmology with Intensity Mapping via Statistics Beyond the Power Spectrum in the SKAO Era

The cosmological distribution of neutral hydrogen (HI) during the post-reionization era is highly non-Gaussian due to the underlying non-linear structure formation, complex galaxy biasing, and potential primordial non-Gaussianity. One needs higher-order (beyond two-point) statistics to maximally extract the non-Gaussian information out of the 21-cm intensity maps. This chapter summarizes the potential of several higher-order statistics, including voxel intensity distribution, emission line stacking, probability density functions, $\ell_1$-norm, bispectrum, and various marked statistics. Additionally, image-based morphological descriptors, such as the Largest Cluster Statistic, local dimensions, and Minkowski functionals, etc., can potentially characterize the morphology and geometry of the cosmic web encoded in the 21-cm intensity maps. This chapter presents forecasts of the detectability of these higher-order statistics in the context of the future SKAO observations. These forecasts incorporate instrumental noise, observational effects, and, in some cases, foreground removal in their analyses. With its unprecedented sensitivity, the future SKAO 21-cm observations will enable us to measure these higher-order statistics more precisely, possibly helping to break degeneracies between astrophysical and cosmological parameters, and maximizing the science outcome from these surveys.

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arXiv2026

Statistical Field Theory for Weak Gravitational Lensing

Standard weak-lensing calculations treat lensing as a linear remapping of the matter field along the line of sight. We instead formulate lensing as a stochastic field theory for the Sachs optical scalars, driven by random Ricci-focusing and Weyl-shearing fields. The resulting path integral generates a diagrammatic expansion for arbitrary $n$-point correlation functions of lensing observables, organised into linear response, nonlinear propagation, and driving-field cumulants. The conventional calculation emerges as the lowest-order, linear-propagation limit. Beyond it, nonlinear Sachs evolution couples to driving-field non-Gaussianity, mixing the matter cumulant hierarchy into the lensing hierarchy. A selection rule governs the couplings: an $n$-point observable receives a direct contribution from the $n$-point driving-field cumulant, and its leading hierarchy-mixing correction from the $(n+1)$-point cumulant via one nonlinear Sachs interaction, with higher cumulants entering only at higher order. The two-point function, for instance, is corrected by three-point cumulants of Ricci focusing and Weyl shearing, letting small-scale modes feed the lensing signal across scales and populating the $E$- and $B$-modes in comparable measure. Rather than a restrictive approximation scheme, the formalism is a paradigm shift: a unified framework naturally accommodating path corrections, higher-order matter statistics, stochasticity, and small-scale effects.

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arXiv2026

The observer power spectrum for lightcone statistics, integrated relativistic observables and wide angle effects

The statistics of large-scale structure are naturally described by power spectra in Fourier space. For fields on spatial hypersurfaces, translational invariance makes different Fourier modes uncorrelated and the power spectrum diagonal. Cosmological observables, however, are measured on our past lightcone, where wide-angle effects, radial evolution and integrated effects such as lensing break this symmetry: Fourier-space statistics become non-diagonal, with mode-mixing generated by the geometry of the lightcone itself. We define a more natural observer power spectrum by Fourier transforming over observer positions on a spatial hypersurface with fixed lightcone coordinates, rather than over source positions on a single lightcone. This forms a field on the observer hypersurface with a moveable light-ray leg. Statistical homogeneity of the observer hypersurface implies that this spectrum is diagonal for any observable, whether local or integrated and does not suffer from mode-mixing. We show how the various two-point statistics used in large-scale structure analysis are each recovered as projections of the observer spectrum. This extends directly to higher-order statistics. We illustrate it by constructing the relativistic kernel for the observed galaxy number count fluctuation, including density, redshift-space distortions, Doppler, lensing magnification, and integrated Sachs-Wolfe contributions.

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arXiv2026

Cosmological peculiar velocities in general relativity

We reconsider the late-time evolution of galaxy peculiar velocities in the 1+3 covariant approach to cosmological perturbation theory. It has recently been claimed that this approach predicts substantially stronger growth of peculiar velocities than standard metric-based perturbation theory -- on the grounds that the covariant treatment is fully relativistic whereas standard treatments are effectively Newtonian. We show that this is not the case. When the covariant equations are applied consistently, the $1+3$ approach reproduces exactly the standard perturbative result for peculiar-velocity growth. The stronger growth laws claimed in recent work arise from an inconsistent treatment of the coupled covariant system, in which terms constrained by the field equations are treated as if they were independent sources. Further claims are made that the stronger bulk flows can mimic accelerated expansion in a dust universe. We argue that these claims rest on a confusion between the kinematics of an arbitrarily chosen observer congruence and the physical expansion of the matter congruence traced by galaxies. We conclude that the standard treatment of peculiar velocities is correct and fully relativistic~-- and does not lead to anomalous bulk flows or to apparent accelerated expansion.

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arXiv2026

Halving and Doubling: Boosting the Detection of Relativistic Effects in the Galaxy Bispectrum with Optimal Subsample Selection

On the scale of the cosmic horizon, signatures that are unique to general relativity are concealed within the statistics of the large scale distribution of galaxies. These were thought to be beyond the reach of all but the most ambitious galaxy surveys, as they are substantially suppressed relative to standard redshift-space distortions. We show that the detectability of these higher-order relativistic effects can be dramatically enhanced by a sampling strategy that splits a galaxy catalogue into faint and bright subsamples and then combines their auto-bispectra. For current surveys such as DESI, this implies that this new signal will be detectable for the first time using our new strategy.

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arXiv2026

Calibration-independent consistency test of BAO and SNIa data: update

In a recent paper, arXiv:2509.19899, we presented a new method to test the consistency between uncalibrated BAO and SNIa data through a common parameter, the Alcock-Paczynski variable. Using Gaussian Processes, we can determine if various datasets are consistent, independently of dark energy or modified gravity models, and of the sound horizon and SNIa peak magnitude. We found that the DES-Y5 SNIa data showed non-negligible tension with other datasets. However, the recent update DES-Dovekie removes this tension. We find that all uncalibrated data from DESI DR2 BAO and three SNIa datasets, Union3, Pantheon+, and DES-Dovekie, are consistent with each other within $\sim 1\sigma$.

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arXiv2026

Probing General Relativity on Cosmological Scales in the 2040s

General relativity is exquisitely tested in strong-field regimes, yet its validity on cosmological scales remains largely unexplored. Upcoming wide and deep large-scale structure surveys will access the ultra-large, linear scales where relativistic effects - Doppler terms, gravitational redshift, lensing magnification, and potential evolution - leave significant imprints in the clustering of galaxies. These signatures represent unique probes of spacetime that are inaccessible to standard Newtonian analyses but increasingly important as survey volumes grow. We outline the scientific potential of next-generation facilities, such as those envisioned within ESO's Expanding Horizons programme, to deliver the first robust measurements of relativistic effects in large-scale structure through multi-tracer power spectra and the single-tracer bispectrum of high-redshift Lyman-break galaxies. Detecting these contributions would open a new window on gravity, enabling precision tests of general relativity and its alternatives on cosmological scales in the 2040s.

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arXiv2025

Prospects for measuring the Doppler magnification dipole with LSST and DESI

We forecast the detectability of the Doppler magnification dipole with a joint analysis of galaxy spectroscopic redshifts and size measurements. The Doppler magnification arises from an apparent size variation caused by galaxies' peculiar velocities when mapping them from redshift space to real space. This phenomenon is the dominant contribution to the convergence at low redshifts ($\lesssim$ 0.5). A practical observational strategy is to cross-correlate a galaxy number count tracer, e.g. from the Dark Energy Spectroscopic Instrument (DESI) Bright Galaxy Survey, with the convergence field reconstructed from galaxy size measurements obtained by the Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST). To assess the achievable precision of galaxy size measurements, we simulate LSST Y1-quality galaxy images with \textsc{Galsim} and measure them with the \textsc{Galight} profile fitting package. Our investigations, based on galaxy populations from LSST's synthetic galaxy catalogue \textsc{cosmoDC2}, show that the variance due to intrinsic galaxy size variation dominates over size measurement errors as expected, but may be lower than previous studies have suggested. Under our analysis assumptions, the Doppler magnification dipole would be detectable with a signal-to-noise ratio $\geq 10$ in multiple redshift bins between $0.1 \leq z \leq 0.5$ with DESI spectroscopic redshifts and LSST imaging.

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arXiv2025

Unbiased analysis of primordial non-Gaussianity: the multipoles of the full relativistic power spectrum

A major goal of ongoing and future cosmological surveys of the large-scale structure is to measure local type primordial non-Gaussianity in the galaxy power spectrum through the scale-dependent bias. General relativistic effects have been shown to be degenerate with this measurement, therefore requiring a non-Newtonian approach. In this work, we develop a consistent framework to compute integrated effects, including lensing convergence, time delay, and integrated Sachs--Wolfe, along with the local relativistic projection and wide-separation corrections in the multipoles of the power spectrum. We show that, for a \textit{Euclid}-like H$\alpha$-line galaxy survey and a MegaMapper-like Lyman-break galaxy survey, ignoring these effects leads to a bias on the best fit measurement of the amplitude of primordial non-Gaussianity, $f_{\rm NL}$, of around $ 3\,\sigma$ and $ 20 \, \sigma$ respectively. When we include these corrections, the uncertainty in our knowledge of the luminosity function leads to further uncertainty in our measurement of $f_{\rm NL}$. In this work, we show that this degeneracy can be partly mitigated by using a bright-faint multi-tracer analysis, where the observed galaxy sample is subdivided into two separate populations based on luminosity, which provides a $15$--$20\%$ improvement on the forecasted constraints of local type $f_{\rm NL}$. In addition, we present a novel calculation of the full multi-tracer covariance with the inclusion of wide-separation corrections~-- all of these results are implemented in the \textit{Python} code \textsc{CosmoWAP}.

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arXiv2025

Transverse Velocities in Real-Time Cosmology: Position Drift in Relativistic N-Body Simulations

The era of real-time cosmology has begun. It is now possible to directly measure the apparent drift of high-redshift astronomical sources across the sky $\textit{in real time}$. This so-called $\textit{position drift}$ provides a valuable probe of the peculiar velocity field and cosmic structure formation by giving direct access to the transverse velocity, which is notoriously difficult to measure and is typically inferred statistically from the density field in a model-dependent way. To fully exploit this new window into the Universe, it is essential to understand how cosmological structures affect position drift measurements. Here we present the first position drift study based on the general relativistic N-body simulation code $\texttt{gevolution}$. We calculate the position drift directly from the past light cone for ten different observers and compare the results to predictions from linear perturbation theory. At linear order, the position drift is directly proportional to the transverse velocity on the sky. This linear approximation reproduces our non-linear simulation results to within about 5%. We calculate power spectra for the position drift, splitting the signal into an E- and B-mode and compare the former to linear expectations, finding good agreement. The B-mode is suppressed on linear scales, but has similar amplitude as the E-mode on non-linear scales. We further demonstrate that light-cone inhomogeneities induce biases in the dipole of the drift, introducing redshift dependence of both the amplitude and direction. Although our analysis is not yet sufficient for a firm conclusion, our results suggest that these effects alone cannot explain the possible redshift-dependent dipole in Gaia DR3 data reported in the literature.

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arXiv2025

Covariant cosmography in the presence of local structures: comparing exact solutions and perturbation theory

Recent observational evidence of axially symmetric anisotropies in the local cosmic expansion rate motivates an investigation of whether they can be accounted for within the Lema\^itre-Tolman-Bondi (LTB) framework with an off-center observer. Within this setting, we compute the exact relativistic luminosity distance via the Sachs equation and compare it with the approximate expression obtained from the covariant cosmographic approach (including Hubble, deceleration, jerk and curvature parameters). This comparison allows us to identify the regimes in which the covariant cosmographic method remains reliable. In addition, we compare the LTB relativistic distance for small inhomogeneities with the corresponding result derived from linear perturbation theory (LPT) in the standard cosmological model. This analysis establishes a precise correspondence between the LTB and LPT approaches, offering a consistent dictionary for the interpretation of the observed anisotropies of the large-scale gravitational field. This analysis will be instrumental in interpreting expansion-rate anisotropies, facilitating investigations of the local Universe beyond the FLRW framework with a fully non-perturbative metric approach.

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arXiv2025

The anisotropic expansion rate of the local Universe and its covariant cosmographic interpretation

Without making any assumption on the underlying geometry and metric of the local Universe, we provide a measurement of the expansion rate fluctuation field using the Cosmicflows-4 and Pantheon+ samples in the redshift range $0.01 < z < 0.1$ ($30 \,h^{-1}\,\mathrm{Mpc} < R < 300\,h^{-1}\,\mathrm{Mpc}$). The amplitude of the anisotropic fluctuations is found to be of order a few percent relative to the monopole of the expansion rate. We further decompose the expansion rate fluctuation field into spherical harmonic components and analyze their evolution with redshift across the studied redshift range. At low redshift, the dipole is clearly dominant, with an amplitude of $\sim (2.2 \pm 0.15)\times 10^{-2}$, significantly larger than the higher--order modes. As redshift increases, the dipole amplitude steadily decreases, reaching roughly half its value in the highest redshift bin investigated. The quadrupole is also significant, at about half the dipole amplitude, and persists across all redshift bins, with no clear decreasing trend, although uncertainties grow at higher redshift. A nonzero octupole is also detected at low redshift. The dipole, quadrupole, and octupole components are found to be aligned, exhibiting axial symmetry around a common axis ($l = 295^\circ,\, b = 5^\circ$). We interpret the observed fluctuations in the expansion rate within the framework of covariant cosmography. Our results indicate that the multipoles of the expansion rate fluctuation field are primarily driven by a strong quadrupole in the covariant Hubble parameter, together with dipole and octupole contributions from the covariant deceleration parameter. These few parameters suffice to reconstruct the luminosity distance with high precision out to $z \sim 0.1$, in a manner that is model--independent, non--perturbative, and free from assumptions about peculiar velocities.

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arXiv2025

Calibration-independent consistency test of DESI DR2 BAO and SNIa

We investigate the consistency between DESI DR2 BAO and three SNIa datasets, Pantheon+, Union3, and DES-Y5. Our consistency test is {calibration}-independent since it is independent of cosmological nuisance parameters such as the absolute peak magnitude $M_B$ and the comoving sound horizon at the baryon drag epoch $r_d$. Importantly, the test is also model-agnostic, independent of any model of dark energy or modified gravity. We define a tension parameter to quantify tension across different datasets compared to DESI DR2 BAO. The Pantheon+ and Union3 data have tension $\lesssim\! 1\sigma$ across their redshift ranges, whereas the DES-Y5 tension is $\gtrsim3\sigma$ near $z=1$. This hints that DES-Y5 data has significant offset values for redshifts close to 1, compared to the other SNIa datasets. Since this consistency test is independent of cosmological nuisance parameters, the tension is minimal: other consistency tests involving differences in nuisance parameters may show greater tension.

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arXiv2025

Probing the Cosmological Principle with CMB lensing and cosmic shear

The standard cosmological model assumes the Cosmological Principle. However, recent observations hint at possible violations of isotropy on large scales, possibly through late-time anisotropic expansion. Here we investigate the potential of cross-correlations between CMB lensing convergence $\kappa$ and galaxy cosmic shear $B$-modes as a novel probe of such late-time anisotropies. Our signal-to-noise forecasts reveal that information from the $\kappa$-$B$ cross-correlation is primarily contained on large angular scales ($\ell \lesssim 200$). We find that this cross-correlation for a Euclid-like galaxy survey is sensitive to anisotropy at the percent level. Making use of tomography yields a modest improvement of $\sim 20\%$ in detection power. Incorporating the galaxy $E$-$B$ cross-correlations would further enhance these constraints.

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arXiv2025

Visualising relativistic effects in redshift space distortions of large scale structure

Observing large scale structure in redshift space gives rise to the well known redshift space distortions whereby a spherical distribution of galaxies is distorted into an ellipsoid along the line of sight of the observer. This effect is important on linear scales and so can be thought of as a Newtonian correction to the density perturbation even though their physical origin is in the Doppler effect. On larger scales subtler aspects of the Doppler and gravitational redshift effects give rise to further distortions in redshift space. These further contort objects beyond an ellipsoidal compression, into shapes with broken line-of-sight symmetry such as an egg- or bean-like shapes. In this paper, we aim to qualitatively picture how large over-dense regions, including clusters or superclusters, and under-dense regions, such as voids, undergoing infall or outflow respectively, become distorted in redshift space when higher-order relativistic effects are taken into account. This will contribute when analysing structure in real space such as stacking voids which are no longer radially symmetric when these effects are included.

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arXiv2025

Exploring future synergies for large-scale structure between gravitational waves and radio sources

Future third-generation gravitational wave detectors like the Einstein Telescope (ET) and Cosmic Explorer (CE) are expected to detect millions of binary black hole (BBH) mergers. Alongside these advances, upcoming radio surveys, such as the Square Kilometer Array Observatory (SKAO) will provide new sets of cosmological tracers. These include mapping the large-scale distribution of neutral hydrogen (\hi) using intensity mapping (IM) and \hi\ and radio continuum galaxies. In this work, we will investigate synergies between gravitational waves (GW) and radio tracers through a multi-tracer approach. We first forecast the precision on the clustering bias of GWs by cross-correlating data from an ET-like detector with an SKAO IM survey. Our results indicate that this approach can constrain the GW clustering bias to within $2\%$ up to $z = 2.5$. Additionally, we explore the potential of a triple cross-correlation using GWs, IM, and photometric galaxies from a survey like the Vera Rubin Observatory's Legacy Survey of Space and Time (LSST). This multi-tracer method enhances constraints on the magnification lensing effect, achieving percent-level precision, and allows for a measurement of the Doppler effect with approximately $15\%$ uncertainty. Furthermore we show for the first time that this method could achieve the precision required to measure subdominant gravitational potential contributions to the relativistic corrections, which had thought to be below cosmic variance. Our analysis highlights the potential of cross-correlations between GWs and radio tracers to improve constraints on astrophysical properties of BBHs, measure relativistic effects, and perform null tests of GR in cosmological scales.

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arXiv2025

Redshift Drift in Relativistic N-Body Simulations

The cosmological redshift drift promises to be the first observable directly measuring the evolution of the cosmic expansion rate and should be detectable with upcoming surveys by the Square Kilometre Array and the Extremely Large Telescope. To prepare for these upcoming measurements we study the redshift drift in detail using the relativistic N-body code $\texttt{gevolution}$, focusing on inhomogeneity-induced fluctuations. Using a ray-tracer, we calculate the redshift drift directly from the light cone at two different time steps. To investigate observer-dependent biases we consider 10 different observers. We find that inhomogeneity-induced fluctuations in the redshift drift can in extreme cases be of the same order as the cosmic signal for $z\lesssim0.15$. By comparing our results to first-order perturbation theory, we find that the extreme outliers are due to peculiar motion in over-densities and can be described by first-order perturbation theory to percent precision. We calculate angular power spectra that fit very well with our predictions based on perturbation theory at linear scales and show a surprisingly large non-linear signal. This shows that redshift drift not only has the power to measure the background expansion, but could also deliver information about the velocity and acceleration fields in clusters.

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arXiv2025

Improved null tests of $\Lambda$CDM and FLRW in light of DESI DR2

The DESI DR2 BAO data, in combination with CMB and different SNIa datasets, exclude the flat $\Lambda$CDM model at more than 2.5$\sigma$ when analyzed through the $w_0w_a$CDM parametrization for evolving dark energy. This simple parametrization may not accurately capture the behavior of the entire redshift range at late times, which may introduce bias in the results. We use null tests that probe for deviations from flat $\Lambda$CDM at late times, independent of any specific dark energy parametrization. We provide several diagnostics for null tests and discuss their advantages and disadvantages. In particular, we derive diagnostics that improve on previous ones, such as the popular $O_{\rm m}$ diagnostic. The diagnostics are derived from both background and perturbed quantities. Using the combination of DESI DR2 BAO and supernova data, with or without CMB data, we find that deviations from flat $\Lambda$CDM are at $\sim1\sigma$ confidence level in most of the redshift range (more than 1$\sigma$ for a few small redshift intervals in a few cases). When considering SDSS BAO data instead of DESI BAO data, in combination with PantheonPlus, with or without CMB data, we find even smaller deviations. Since spatial curvature can potentially modify the results, we also test for curvature in the general $\Lambda$CDM model and the general FLRW model. While there is slight evidence for nonzero cosmic curvature at lower redshifts in a general $\Lambda$CDM model, there is no statistically significant evidence in a general FLRW model.

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