arXiv subjects

Ryan J. Turner

Publications and source records attributed to Ryan J. Turner.

10 recordsLinked to original sources

arXiv2026

The velocity coherence scale: a novel probe of cosmic homogeneity and a potential standard ruler

We introduce the velocity coherence scale $R_v$, the scale at which the spherical volume average of the trace of the velocity correlation tensor transitions from scaling faster than the sphere radius to scaling more slowly. This corresponds to the radius at which the average motion of galaxies along their separation vectors transitions from correlated to anti-correlated. We derive a theoretical estimator for $R_v$ by defining the bulk in spheres $B_R$, a velocity-field analogue of the mean scale counts used in density-field correlation analyses. We show that, for a statistically homogeneous matter distribution, the logarithmic derivative of $B_R$ and the correlation dimension $D_2$ share the same asymptotic behaviour and therefore can be used to estimate the scale of transition to statistical homogeneity. Furthermore, we show that in standard $Λ$CDM cosmologies the velocity coherence scale is tightly connected to the matter-radiation equality scale $k_{eq}$, and that its value in comoving coordinates is redshift-independent. These results highlight the potential of $R_v$ both as a standard ruler and as a physically motivated scale characterising the onset of cosmic homogeneity. We present a proof of concept using measurements of the PV correlation functions from SDSS. We show that the main challenge in determining $R_v$ is the limited precision of PV measurements compared to density ones, as they typically rely on smaller samples with larger uncertainties that scale roughly linearly with survey depth. Fitting our theoretical estimators for $R_v$, we obtain $R_v \approx 138^{+23}_{-54}\,\mathrm{Mpc}/h$. Finally, we show that more precise determinations should be achievable with current and upcoming peculiar velocity surveys.

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arXiv2025

The role of peculiar velocity uncertainties in standard siren cosmology

Local distance indicators such as standard sirens, in combination with spectroscopic redshift measurements of their host galaxies, allow us to estimate the present-day expansion rate of the Universe parameterised by Hubble's constant, H_0. However, these observed redshifts are systematically modified by the effect of galaxy peculiar velocities. Although these velocities may be estimated from the local density field by the process of velocity-field reconstruction, the intrinsic errors and covariance in these estimates contribute to the error in the H_0 determination. In this paper we demonstrate how the impact of peculiar velocities can be propagated into H_0 measurements from local distance indicators with observed redshifts, incorporating the full covariance of the velocity field induced by bulk flows. We apply our methods to cosmological simulations, testing the importance of this effect in the context of future analyses of gravitational wave sources with electromagnetic counterparts used as bright sirens. We conclude that H_0 errors may be increased by a factor of 2 in comparison with neglecting peculiar velocity covariance, for GW170817-like sirens located within 50 Mpc with 5% distance errors, with the highest impacts expected for sources at nearby distances or with small distance errors. Our analytical methods may also be applied to other local distance indicators, such as Type Ia supernovae.

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arXiv2025

Cosmology with Peculiar Velocity Surveys

Peculiar velocities are the motions of galaxies due to the gravitational influence of large-scale structure, and thus are an important cosmological probe of the underlying matter density field. In recent years the number of surveys designed to measure peculiar velocities has increased, to the point that it is plausible that we will have completely mapped out the peculiar velocity field in the local universe within the next decade. Such an abundance of data will enable us to place precise constraints on the growth rate of large-scale structure which in turn will inform us about the true nature of the laws of gravity and the standard cosmological model. In this chapter, the physics governing the generation of peculiar velocities, the methods of measuring them, and the statistical tools used to extract cosmological information from them are described. It will also cover a swathe of current and future surveys dedicated to collecting peculiar velocities, what their aims are, and what these datasets may mean for the future of cosmological analyses.

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arXiv2024

Forecasting the accuracy of velocity-field reconstruction

Joint analyses of the large-scale distribution of galaxies, and their motions under the gravitational influence of this density field, allow powerful tests of the cosmological model, including measurement of the growth rate of cosmic structure. In this paper we perform a statistical comparison between two important classes of method for performing these tests. In the first method, which we refer to as the "power-spectrum method", we measure the 2-point power spectra between the velocity and density tracers, and jointly fit these statistics using theoretical models. In the second method, which we refer to as the "reconstruction-and-scaling method", we use the density tracers to reconstruct a model velocity field through space, which we compare with the measured galaxy velocities on a point-by-point basis. By generating an ensemble of numerical simulations in a simplified test scenario, we show that the error in the growth rate inferred by the reconstruction-and-scaling method may be under-estimated, unless the full covariances of the underlying and reconstructed velocity fields are included in the analysis. In this case the inferred growth rate errors agree with both the power-spectrum method and a Fisher matrix forecast. We provide a roadmap for evaluating these covariances, considering reconstruction performed using both a Fourier basis within a cuboid, and a Spherical Fourier-Bessel basis within a curved-sky observational volume.

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arXiv2024

Constraining modified gravity scenarios with the 6dFGS and SDSS galaxy peculiar velocity datasets

The detailed nature of dark energy remains a mystery, leaving the possibility that its effects might be explained by changes to the laws of gravity on large scales. The peculiar velocities of galaxies directly trace the strength of gravity on cosmic scales and provide a means to further constrain such models. We generate constraints on different scenarios of gravitational physics by measuring peculiar velocity and galaxy clustering two-point correlations, using redshifts and distances from the 6-degree Field Galaxy Survey and the Sloan Digital Sky Survey Peculiar Velocity samples, and fitting them against models characteristic of different cosmologies. Our best-fitting results are all found to be in statistical agreement with General Relativity, in which context we measure the low-redshift growth of structure to be $fσ_8 = 0.329^{+0.081}_{-0.083}$, consistent with the prediction of the standard $Λ$CDM model. We also fit the modified gravity scenarios of Dvali-Gabadadze-Porrati (nDGP) and a Hu-Sawicki model of $f(R)$ gravity, finding the $2σ$ limit of their characteristic parameters to be $r_cH_0/c>6.987$ and $-\log_{10}(|f_{R0}|)>4.703$, respectively. These constraints are comparable to other literature values, though it should be noted that they are significantly affected by the prior adopted for their characteristic parameters. When applied to much larger upcoming peculiar velocity surveys such as DESI, this method will place rapidly-improving constraints on modified gravity models of cosmic expansion and growth.

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arXiv2023

Biases in velocity reconstruction: investigating the effects on growth rate and expansion measurements in the local universe

The local galaxy peculiar velocity field can be reconstructed from the surrounding distribution of large-scale structure and plays an important role in calibrating cosmic growth and expansion measurements. In this paper, we investigate the effect of the stochasticity of these velocity reconstructions on the statistical and systematic errors in cosmological inferences. By introducing a simple statistical model between the measured and theoretical velocities, whose terms we calibrate from linear theory, we derive the bias in the model velocity. We then use lognormal realisations to explore the potential impact of this bias when using a cosmic flow model to measure the growth rate of structure, and to sharpen expansion rate measurements from host galaxies for gravitational wave standard sirens with electromagnetic counterparts. Although our illustrative study does not contain fully realistic observational effects, we demonstrate that in some scenarios these corrections are significant and result in a measurable improvement in determinations of the Hubble constant compared to standard forecasts.

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arXiv2023

On the correlations of galaxy peculiar velocities and their covariance

Measurements of the peculiar velocities of large samples of galaxies enable new tests of the standard cosmological model, including determination of the growth rate of cosmic structure that encodes gravitational physics. With the size of such samples now approaching hundreds of thousands of galaxies, complex statistical analysis techniques and models are required to extract cosmological information. In this paper we summarise how correlation functions between galaxy velocities, and with the surrounding large-scale structure, may be utilised to test cosmological models. We present new determinations of the analytical covariance between such correlation functions, which may be useful for cosmological likelihood analyses. The statistical model we use to determine these covariances includes the sample selection functions, observational noise, curved-sky effects and redshift-space distortions. By comparing these covariance determinations with corresponding estimates from large suites of cosmological simulations, we demonstrate that these analytical models recover the key features of the covariance between different statistics and separations, and produce similar measurements of the growth rate of structure.

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arXiv2022

A local measurement of the growth rate from peculiar velocities and galaxy clustering correlations in the 6dF Galaxy Survey

Galaxy peculiar velocities provide an integral source of cosmological information that can be harnessed to measure the growth rate of large scale structure and constrain possible extensions to General Relativity. In this work, we present a method for extracting the information contained within galaxy peculiar velocities through an ensemble of direct peculiar velocity and galaxy clustering correlation statistics, including the effects of redshift space distortions, using data from the 6-degree Field Galaxy Survey. Our method compares the auto- and cross-correlation function multipoles of these observables, with respect to the local line of sight, with the predictions of cosmological models. We find that the uncertainty in our measurement is improved when combining these two sources of information in comparison to fitting to either peculiar velocity or clustering information separately. When combining velocity and density statistics in the range $27 < s < 123 \, h^{-1}$ Mpc we obtain a value for the local growth rate of $fσ_8 = 0.358 \pm 0.075$ and for the linear redshift distortion parameter $β= 0.298 \pm 0.065$, recovering both with $20.9$ per cent and $21.8$ per cent accuracy respectively. We conclude this work by comparing our measurement with other recent local measurements of the growth rate, spanning different datasets and methodologies. We find that our results are in broad agreement with those in the literature and are fully consistent with $Λ$CDM cosmology. Our methods can be readily scaled to analyse upcoming large galaxy surveys and achieve accurate tests of the cosmological model.

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arXiv2021

Improving estimates of the growth rate using galaxy-velocity correlations: a simulation study

We present an improved framework for estimating the growth rate of large-scale structure, using measurements of the galaxy-velocity cross-correlation in configuration space. We consider standard estimators of the velocity auto-correlation function, $ψ_1$ and $ψ_2$, the two-point galaxy correlation function, $ξ_{gg}$, and introduce a new estimator of the galaxy-velocity cross-correlation function, $ψ_3$. By including pair counts measured from random catalogues of velocities and positions sampled from distributions characteristic of the true data, we find that the variance in the galaxy-velocity cross-correlation function is significantly reduced. Applying a covariance analysis and $χ^2$ minimisation procedure to these statistics, we determine estimates and errors for the normalised growth rate $fσ_8$ and the parameter $β= f/b$, where $b$ is the galaxy bias factor. We test this framework on mock hemisphere datasets for redshift $z < 0.1$ with realistic velocity noise constructed from the L-PICOLA simulation code, and find that we are able to recover the fiducial value of $fσ_8$ from the joint combination of $ψ_1$ + $ψ_2$ + $ψ_3$ + $ξ_{gg}$, with 15\% accuracy from individual mocks. We also recover the fiducial $fσ_8$ to within 1$σ$ regardless of the combination of correlation statistics used. When we consider all four statistics together we find that the statistical uncertainty in our measurement of the growth rate is reduced by $59\%$ compared to the same analysis only considering $ψ_2$, by $53\%$ compared to the same analysis only considering $ψ_1$, and by $52\%$ compared to the same analysis jointly considering $ψ_1$ and $ψ_2$.

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arXiv2020

Galaxy And Mass Assembly (GAMA): Assimilation of KiDS into the GAMA database

The Galaxy And Mass Assembly Survey (GAMA) covers five fields with highly complete spectroscopic coverage ($>95$ per cent) to intermediate depths ($r<19.8$ or $i < 19.0$ mag), and collectively spans 250 square degrees of Equatorial or Southern sky. Four of the GAMA fields (G09, G12, G15 and G23) reside in the ESO VST KiDS and ESO VISTA VIKING survey footprints, which combined with our GALEX, WISE and Herschel data provide deep uniform imaging in the $FUV\,NUV\,ugriZYJHK_s\,W1\,W2\,W3\,W4\,P100\,P160\,S250\,S350\,S500$ bands. Following the release of KiDS DR4, we describe the process by which we ingest the KiDS data into GAMA (replacing the SDSS data previously used for G09, G12 and G15), and redefine our core optical and near-IR catalogues to provide a complete and homogeneous dataset. The source extraction and analysis is based on the new ProFound image analysis package, providing matched-segment photometry across all bands. The data are classified into stars, galaxies, artefacts, and ambiguous objects, and objects are linked to the GAMA spectroscopic target catalogue. Additionally, a new technique is employed utilising ProFound to extract photometry in the unresolved MIR-FIR regime. The catalogues including the full FUV-FIR photometry are described and will be fully available as part of GAMA DR4. They are intended for both standalone science, selection for targeted follow-up with 4MOST, as well as an accompaniment to the upcoming and ongoing radio arrays now studying the GAMA $23^h$ field.

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