arXiv subjects

Richard A. Battye

Publications and source records attributed to Richard A. Battye.

At least 37 records · Page 2Linked to original sources

arXiv2018

Cosmologically viable generalized Einstein-Aether theories

We investigate generalized Einstein-Aether theories that are compatible with the Planck Cosmic Microwave Background (CMB) temperature anisotropy, polarisation, and lensing data. For a given dark energy equation of state, $w_{\rm de}$, we formulate a designer approach and we investigate their impact on the CMB temperature anisotropy and matter power spectra. We use the Equation of State approach to parametrize the perturbations and find that this approach is particularly useful in identifying the most suitable and numerically efficient parameters to explore in a Markov chain Monte Carlo (MCMC) analysis. We find the data constrains models with $w_{\rm de} = -1$ to be compatible with $Λ$CDM. For $w_{\rm de} \not= -1$ models, which avoid the gravitational waves constraint through the entropy perturbation, we constrain $w_{\rm de}$ to be $w_{\rm de } = -1.06^{+0.08}_{-0.03}$ (CMB) and $w_{\rm de } =-1.04^{+0.05}_{-0.02}$ (CMB+Lensing) at $68\%$C.L., and find that these models can be different from $Λ$CDM and still be compatible with the data. We also find that these models can ameliorate some anomalies in $Λ$CDM when confronted with data, such as the low-$\ell$ and high-$k$ power in the CMB temperature anisotropy and matter power spectra respectively, but not simultaneously. We also investigate the anomalous lensing amplitude, quantified by $A_{\rm lens}$, and find that that for $w_{\rm de} = -1$ models, $A_{\rm lens} = 1.15^{+0.07}_{-0.08}$ (CMB) and $A_{\rm lens} = 1.12\pm0.05$ (CMB+Lensing) at $68\%$C.L. $\sim$ 2$σ$ larger than expected, similar to previous analyses of $Λ$CDM.

astro-ph.CO↗

arXiv2018

Cosmology with Phase 1 of the Square Kilometre Array; Red Book 2018: Technical specifications and performance forecasts

We present a detailed overview of the cosmological surveys that will be carried out with Phase 1 of the Square Kilometre Array (SKA1), and the science that they will enable. We highlight three main surveys: a medium-deep continuum weak lensing and low-redshift spectroscopic HI galaxy survey over 5,000 sqdeg; a wide and deep continuum galaxy and HI intensity mapping survey over 20,000 sqdeg from z = 0.35 - 3; and a deep, high-redshift HI intensity mapping survey over 100 sqdeg from z = 3 - 6. Taken together, these surveys will achieve an array of important scientific goals: measuring the equation of state of dark energy out to z ~ 3 with percent-level precision measurements of the cosmic expansion rate; constraining possible deviations from General Relativity on cosmological scales by measuring the growth rate of structure through multiple independent methods; mapping the structure of the Universe on the largest accessible scales, thus constraining fundamental properties such as isotropy, homogeneity, and non-Gaussianity; and measuring the HI density and bias out to z = 6. These surveys will also provide highly complementary clustering and weak lensing measurements that have independent systematic uncertainties to those of optical surveys like LSST and Euclid, leading to a multitude of synergies that can improve constraints significantly beyond what optical or radio surveys can achieve on their own. This document, the 2018 Red Book, provides reference technical specifications, cosmological parameter forecasts, and an overview of relevant systematic effects for the three key surveys, and will be regularly updated by the Cosmology Science Working Group in the run up to start of operations and the Key Science Programme of SKA1.

astro-ph.CO↗

arXiv2018

Measuring cosmic shear and birefringence using resolved radio sources

We develop a new method of extracting simultaneous measurements of weak lensing shear and a local rotation of the plane of polarization using observations of resolved radio sources. We show that the direction of polarization is statistically linked with that of the gradient of the total intensity field, and this provides the basis of our method. Using a number of sources spread over the sky, this method allows constraints to be placed on cosmic shear and birefringence, and it can be applied to any resolved radio sources for which such a correlation exists. Assuming that the rotation and shear are constant across the source, we use this relationship to construct a quadratic estimator and investigate its properties using simulated observations. We develop a calibration scheme using simulations based on the observed images to mitigate a bias which occurs in the presence of measurement errors and an astrophysical scatter on the polarization. The method is applied directly to archival data of radio galaxies where we measure a mean rotation signal of $\left<ω\right>=-2.03^{\circ}\pm0.75^{\circ}$ and an average shear compatible with zero using 30 reliable sources. This level of constraint on an overall rotation is comparable with current leading constraints from CMB experiments and is expected to increase by at least an order of magnitude with future high precision radio surveys, such as those performed by the SKA. We also measure the shear and rotation two-point correlation functions and estimate the number of sources required to detect shear and rotation correlations in future surveys.

astro-ph.CO↗

arXiv2018

Gravitational wave constraints on dark sector models

We explore the constraints on dark sector models imposed by the recent observation of coincident gravitational waves and gamma rays from a binary neutron star merger, GW170817. Rather than focusing on specific models as has been considered by other authors, we explore this in the context of the equation of state approach of which the specific models are special cases. After confirming the strong constraints found by others for Horndeski, Einstein-Aether and massive gravity models, we discuss how it is possible to construct models which might evade the constraints from GW170817 but still leading to cosmologically interesting modifications to gravity. Possible examples are ``miracle cancellations" such as in $f(R)$ models, nonlocal models and higher-order derivatives. The latter two rely on the dimensionless ratio of the wave number of the observed gravitational waves to the Hubble expansion rate being very large ($\sim10^{19}$) which is used to suppress modifications to the speed of gravitational waves.

astro-ph.CO↗

arXiv2018

Do cosmological data rule out $f(\mathcal{R})$ with $w\neq-1$?

We review the Equation of State (EoS) approach to dark sector perturbations and apply it to $f(\mathcal{R})$ gravity models of dark energy. We show that the EoS approach is numerically stable and use it to set observational constraints on designer models. Within the EoS approach we build an analytical understanding of the dynamics of cosmological perturbations for the designer class of $f(\mathcal{R})$ gravity models, characterised by the parameter $B_0$ and the background equation of state of dark energy $w$. When we use the Planck Cosmic Microwave Background (CMB) temperature anisotropy, polarisation and lensing data as well as the Baryonic Acoustic Oscillation (BAO) data from SDSS and WiggleZ, we find $B_0<0.006$ (95\%CL) for the designer models with $w=-1$. Furthermore, we find $B_0<0.0045$ and $|w+1|<0.002$ (95\%CL) for the designer models with $w\neq -1$. Previous analyses found similar results for designer and Hu-Sawicki $f(\mathcal{R})$ gravity models using the Effective Field Theory (EFT) approach (Raveri et al. 2014, Hu et al. 2016), therefore this hints for the fact that generic $f(\mathcal{R})$ models with $w\neq-1$ can be tightly constrained by current cosmological data, complementary to solar system tests (Brax et al. 2008, Faulkner et al. 2007). When compared to a $w$CDM fluid with the same sound speed, we find that the equation of state for $f(\mathcal{R})$ models is better constrained to be close to -1 by about an order of magnitude, due to the strong dependence of the perturbations on $w$.

astro-ph.CO↗

arXiv2017

Cosmological perturbation theory in Generalized Einstein-Aether models

We investigate the evolution of cosmological perturbations in models of dark energy described by a time-like unit normalized vector field specified by a general function $\mathcal{F}(\mathcal{K})$, so-called Generalized Einstein-Aether models. First we study the background dynamics of such models via a designer approach in an attempt to model this theory as dark energy. We find that only one specific form of this designer approach matches $Λ$CDM at background order and we also obtain a differential equation which $\mathcal{F}(\mathcal{K})$ must satisfy for general $w$CDM cosmologies. We also present the equations of state for perturbations in Generalized Einstein-Aether models, which completely parametrize these models at the level of linear perturbations. A generic feature of modified gravity models is that they introduce new degrees of freedom. By fully eliminating these we are able to express the gauge invariant entropy perturbation and the scalar, vector, and tensor anisotropic stresses in terms of the perturbed fluid variables and metric perturbations only. These can then be used to study the evolution of perturbations in the scalar, vector, and tensor sectors and we use these to evolve the Newtonian gravitational potentials.

astro-ph.CO↗

arXiv2017

Planck confronts large scale structure: methods to quantify discordance

Discordance in the $Λ$CDM cosmological model can be seen by comparing parameters constrained by CMB measurements to those inferred by probes of large scale structure. Recent improvements in observations, including final data releases from both Planck and SDSS-III BOSS, as well as improved astrophysical uncertainty analysis of CFHTLenS, allows for an update in the quantification of any tension between large and small scales. This paper is intended, primarily, as a discussion on the quantifications of discordance when comparing the parameter constraints of a model when given two different data sets. We consider KL-divergence, comparison of Bayesian evidences and other statistics which are sensitive to the mean, variance and shape of the distributions. However, as a by-product, we present an update to the similar analysis in (Battye, Charnock and Moss; 2015) where we find that, considering new data and treatment of priors, the constraints from the CMB and from a combination of LSS probes are in greater agreement and any tension only persists to a minor degree. In particular, we find the parameter constraints from the combination of LSS probes which are most discrepant with the Planck2015+Pol+BAO parameter distributions can be quantified at a 2.55$σ$ tension using the method introduced in (Battye, Charnock and Moss; 2015). If instead we use the distributions constrained by the combination of LSS probes which are in greatest agreement with those from Planck2015+Pol+BAO this tension is only 0.76$σ$.

astro-ph.CO↗

arXiv2016

Approximation of the potential in scalar field dark energy models

We study the nature of potentials in scalar field based models for dark energy - with both canonical and noncanonical kinetic terms. We calculate numerically, and using an analytic approximation around $a\approx 1$, potentials for models with constant equation-of-state parameter, $w_ϕ$. We find that for a wide range of models with canonical and noncanonical kinetic terms there is a simple approximation for the potential that holds when the scale factor is in the range $0.6\lesssim a\lesssim 1.4$. We discuss how this form of the potential can also be used to represent models with non-constant $w_ϕ$ and, hence, how it could be used in reconstruction from cosmological data.

astro-ph.CO↗

arXiv2016

Optimal scan strategies for future CMB satellite experiments

The B-mode polarisation power spectrum in the Cosmic Microwave Background (CMB) is about four orders of magnitude fainter than the CMB temperature power spectrum. Any instrumental imperfections that couple temperature fluctuations to B-mode polarisation must therefore be carefully controlled and/or removed. We investigate the role that a scan strategy can have in mitigating certain common systematics by averaging systematic errors down with many crossing angles. We present approximate analytic forms for the error on the recovered B-mode power spectrum that would result from differential gain, differential pointing and differential ellipticity for the case where two detector pairs are used in a polarisation experiment. We use these analytic predictions to search the parameter space of common satellite scan strategies in order to identify those features of a scan strategy that have most impact in mitigating systematic effects. As an example we go on to identify a scan strategy suitable for the CMB satellite proposed for the ESA M5 call. considering the practical considerations of fuel requirement, data rate and the relative orientation of the telescope to the earth. Having chosen a scan strategy we then go on to investigate the suitability of the scan strategy.

astro-ph.CO↗

arXiv2016

The Equation of State Approach to Cosmological Perturbations in f(R) Gravity

The discovery of apparent cosmological acceleration has spawned a huge number of dark energy and modified gravity theories. The f(R) models of gravity are obtained when one replaces the Ricci scalar in the Einstein-Hilbert action by an arbitrary function of the Ricci scalar. In this work we provide expressions for the equations of state (EoS) for perturbations which completely characterize the linearized perturbations in f(R) gravity, including the scalar, vector, and tensor modes. The EoS formalism is a powerful and elegant parametrization where the modification to General Relativity are treated as a dark-energy-fluid. The perturbed dark-energy-fluid variables such as the anisotropic stress or the entropy perturbation are explicitly given in terms of parameters of the model of interest.

gr-qc↗

arXiv2015

HI intensity mapping with FAST

We discuss the detectability of large-scale HI intensity fluctuations using the FAST telescope. We present forecasts for the accuracy of measuring the Baryonic Acoustic Oscillations and constraining the properties of dark energy. The FAST $19$-beam L-band receivers ($1.05$--$1.45$ GHz) can provide constraints on the matter power spectrum and dark energy equation of state parameters ($w_{0},w_{a}$) that are comparable to the BINGO and CHIME experiments. For one year of integration time we find that the optimal survey area is $6000\,{\rm deg}^2$. However, observing with larger frequency coverage at higher redshift ($0.95$--$1.35$ GHz) improves the projected errorbars on the HI power spectrum by more than $2~σ$ confidence level. The combined constraints from FAST, CHIME, BINGO and Planck CMB observations can provide reliable, stringent constraints on the dark energy equation of state.

astro-ph.CO↗

arXiv2015

A new map-making algorithm for CMB polarisation experiments

With the temperature power spectrum of the cosmic microwave background (CMB) at least four orders of magnitude larger than the B-mode polarisation power spectrum, any instrumental imperfections that couple temperature to polarisation must be carefully controlled and/or removed. Here we present two new map-making algorithms that can create polarisation maps that are clean of temperature-to-polarisation leakage systematics due to differential gain and pointing between a detector pair. Where a half wave plate is used, we show that the spin-2 systematic due to differential ellipticity can also by removed using our algorithms. The algorithms require no prior knowledge of the imperfections or temperature sky to remove the temperature leakage. Instead, they calculate the systematic and polarisation maps in one step directly from the time ordered data (TOD). The first algorithm is designed to work with scan strategies that have a good range of crossing angles for each map pixel and the second for scan strategies that have a limited range of crossing angles. The first algorithm can also be used to identify if systematic errors that have a particular spin are present in a TOD. We demonstrate the use of both algorithms and the ability to identify systematics with simulations of TOD with realistic scan strategies and instrumental noise.

astro-ph.CO↗

arXiv2015

Constraining dark sector perturbations I: cosmic shear and CMB lensing

We present current and future constraints on equations of state for dark sector perturbations. The equations of state considered are those corresponding to a generalized scalar field model and time-diffeomorphism invariant $L(g)$ theories that are equivalent to models of a relativistic elastic medium and also Lorentz violating massive gravity. We develop a theoretical understanding of the observable impact of these models. In order to constrain these models we use CMB temperature data from Planck, BAO measurements, CMB lensing data from Planck and the South Pole Telescope, and weak galaxy lensing data from CFHTLenS. We find non-trivial exclusions on the range of parameters, although the data remains compatible with $w=-1$. We gauge how future experiments will help to constrain the parameters. This is done via a likelihood analysis for CMB experiments such as CoRE and PRISM, and tomographic galaxy weak lensing surveys, focussing in on the potential discriminatory power of Euclid on mildly non-linear scales.

astro-ph.CO↗

arXiv2015

A demonstration of position angle-only weak lensing shear estimators on the GREAT3 simulations

We develop and apply the position angle-only shear estimator of Whittaker et al. (2014) to realistic galaxy images. This is done by demonstrating the method on the simulations of the GREAT3 challenge (Mandelbaum et al. 2014b), which include contributions from anisotropic PSFs. We measure the position angles of the galaxies using three distinct methods - the integrated light method, quadrupole moments of surface brightness, and using model-based ellipticity measurements provided by IM3SHAPE. A weighting scheme is adopted to address biases in the position angle measurements which arise in the presence of an anisotropic PSF. Biases on the shear estimates, due to measurement errors on the position angles and correlations between the measurement errors and the true position angles, are corrected for using simulated galaxy images and an iterative procedure. The properties of the simulations are estimated using the deep field images provided as part of the challenge. A method is developed to match the distributions of galaxy fluxes and half-light radii from the deep fields to the corresponding distributions in the field of interest. We recover angle-only shear estimates with a performance close to current well-established model and moments-based methods for all three angle measurement techniques. The Q-values for all three methods are found to be Q~400. The code is freely available online at http://www.jb.man.ac.uk/~mbrown/angle_only_shear/.

astro-ph.CO↗

arXiv2015

Removing beam asymmetry bias in precision CMB temperature and polarisation experiments

Asymmetric beams can create significant bias in estimates of the power spectra from CMB experiments. With the temperature power spectrum many orders of magnitude stronger than the B-mode power spectrum any systematic error that couples the two must be carefully controlled and/or removed. Here, we derive unbiased estimators for the CMB temperature and polarisation power spectra taking into account general beams and general scan strategies. A simple consequence of asymmetric beams is that, even with an ideal scan strategy where every sky pixel is seen at every orientation, there will be residual coupling from temperature power to B-mode power if the orientation of the beam asymmetry is not aligned with the orientation of the co-polarisation. We test our correction algorithm on simulations of two temperature-only experiments and demonstrate that it is unbiased. The simulated experiments use realistic scan strategies, noise levels and highly asymmetric beams. We also develop a map-making algorithm that is capable of removing beam asymmetry bias at the map level. We demonstrate its implementation using simulations and show that it is capable of accurately correcting both temperature and polarisation maps for all of the effects of beam asymmetry including the effects of temperature to polarisation leakage.

astro-ph.CO↗

arXiv2015

Separating weak lensing and intrinsic alignments using radio observations

We discuss methods for performing weak lensing using radio observations to recover information about the intrinsic structural properties of the source galaxies. Radio surveys provide unique information that can benefit weak lensing studies, such as HI emission, which may be used to construct galaxy velocity maps, and polarized synchrotron radiation; both of which provide information about the unlensed galaxy and can be used to reduce galaxy shape noise and the contribution of intrinsic alignments. Using a proxy for the intrinsic position angle of an observed galaxy, we develop techniques for cleanly separating weak gravitational lensing signals from intrinsic alignment contamination in forthcoming radio surveys. Random errors on the intrinsic orientation estimates introduce biases into the shear and intrinsic alignment estimates. However, we show that these biases can be corrected for if the error distribution is accurately known. We demonstrate our methods using simulations, where we reconstruct the shear and intrinsic alignment auto and cross-power spectra in three overlapping redshift bins. We find that the intrinsic position angle information can be used to successfully reconstruct both the lensing and intrinsic alignment power spectra with negligible residual bias.

astro-ph.CO↗

arXiv2015

Constraining dark sector perturbations II: ISW and CMB lensing tomography

Any Dark Energy (DE) or Modified Gravity (MG) model that deviates from a cosmological constant requires a consistent treatment of its perturbations, which can be described in terms of an effective entropy perturbation and an anisotropic stress. We have considered a recently proposed generic parameterisation of DE/MG perturbations and compared it to data from the Planck satellite and six galaxy catalogues, including temperature-galaxy (Tg), CMB lensing-galaxy and galaxy-galaxy (gg) correlations. Combining these observables of structure formation with tests of the background expansion allows us to investigate the properties of DE/MG both at the background and the perturbative level. Our constraints on DE/MG are mostly in agreement with the cosmological constant paradigm, while we also find that the constraint on the equation of state w (assumed to be constant) depends on the model assumed for the perturbation evolution. We obtain $w=-0.92^{+0.20}_{-0.16}$ (95% CL; CMB+gg+Tg) in the entropy perturbation scenario; in the anisotropic stress case the result is $w=-0.86^{+0.17}_{-0.16}$. Including the lensing correlations shifts the results towards higher values of w. If we include a prior on the expansion history from recent Baryon Acoustic Oscillations (BAO) measurements, we find that the constraints tighten closely around $w=-1$, making it impossible to measure any DE/MG perturbation evolution parameters. If, however, upcoming observations from surveys like DES, Euclid or LSST show indications for a deviation from a cosmological constant, our formalism will be a useful tool towards model selection in the dark sector.

astro-ph.CO↗

arXiv2015

Classically isospinning Skyrmion solutions

We investigate how isospin affects the geometrical shape and energy of classical soliton solutions of topological charges $B=1-4,8$ in the Skyrme model. The novel approach in our work is that we study classically isospinning Skyrmions beyond the rigid-body approximation; that is, we explicitly allow the soliton solutions to deform and to break the symmetries of the static configurations. Our fully three-dimensional relaxation calculations reveal that the symmetries of isospinning Skyrme solitons can differ significantly from the ones of the static configurations. In particular, isospinning Skyrmion solutions can break up into lower-charge Skyrmions, can deform into new solution types that do not exist at vanishing angular frequency $ω$ or energy degeneracy can be removed. These types of deformations have been largely ignored in previous work on modeling nuclei by quantized Skyrmion solutions.

hep-th↗