Sarah Bridle
University of Manchester
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Physical Review D | 2002
Antony Lewis; Sarah Bridle
We present a fast Markov chain Monte Carlo exploration of cosmological parameter space. We perform a joint analysis of results from recent cosmic microwave background ~CMB! experiments and provide parameter constraints, including s 8, from the CMB independent of other data. We next combine data from the CMB, HST Key Project, 2dF galaxy redshift survey, supernovae type Ia and big-bang nucleosynthesis. The Monte Carlo method allows the rapid investigation of a large number of parameters, and we present results from 6 and 9 parameter analyses of flat models, and an 11 parameter analysis of non-flat models. Our results include constraints on the neutrino mass ( mn&0.3 eV), equation of state of the dark energy, and the tensor amplitude, as well as demonstrating the effect of additional parameters on the base parameter constraints. In a series of appendixes we describe the many uses of importance sampling, including computing results from new data and accuracy correction of results generated from an approximate method. We also discuss the different ways of converting parameter samples to parameter constraints, the effect of the prior, assess the goodness of fit and consistency, and describe the use of analytic marginalization over normalization parameters.
Monthly Notices of the Royal Astronomical Society | 2006
Catherine Heymans; Ludovic Van Waerbeke; David J. Bacon; Joel Bergé; G. M. Bernstein; Emmanuel Bertin; Sarah Bridle; Michael L. Brown; Douglas Clowe; Haakon Dahle; Thomas Erben; Meghan E. Gray; Marco Hetterscheidt; Henk Hoekstra; P. Hudelot; M. Jarvis; Konrad Kuijken; V. E. Margoniner; Richard Massey; Y. Mellier; Reiko Nakajima; Alexandre Refregier; Jason Rhodes; Tim Schrabback; David Michael Wittman
The Shear Testing Programme (STEP) is a collaborative project to improve the accuracy and reliability of all weak lensing measurements in preparation for the next generation of wide-field surveys. In this first STEP paper, we present the results of a blind analysis of simulated ground-based observations of relatively simple galaxy morphologies. The most successful methods are shown to achieve percent level accuracy. From the cosmic shear pipelines that have been used to constrain cosmology, we find weak lensing shear measured to an accuracy that is within the statistical errors of current weak lensing analyses, with shear measurements accurate to better than 7 per cent. The dominant source of measurement error is shown to arise from calibration uncertainties where the measured shear is over or underestimated by a constant multiplicative factor. This is of concern as calibration errors cannot be detected through standard diagnostic tests. The measured calibration errors appear to result from stellar contamination, false object detection, the shear measurement method itself, selection bias and/or the use of biased weights. Additive systematics (false detections of shear) resulting from residual point-spread function anisotropy are, in most cases, reduced to below an equivalent shear of 0.001, an order of magnitude below cosmic shear distortions on the scales probed by current surveys. Our results provide a snapshot view of the accuracy of current ground-based weak lensing methods and a benchmark upon which we can improve. To this end we provide descriptions of each method tested and include details of the eight different implementations of the commonly used Kaiser, Squires & Broadhurst method (KSB+) to aid the improvement of future KSB+ analyses.
Monthly Notices of the Royal Astronomical Society | 2007
Richard Massey; Catherine Heymans; Joel Bergé; G. M. Bernstein; Sarah Bridle; Douglas Clowe; H. Dahle; Richard S. Ellis; Thomas Erben; Marco Hetterscheidt; F. William High; Christopher M. Hirata; Henk Hoekstra; P. Hudelot; M. Jarvis; David E. Johnston; Konrad Kuijken; V. E. Margoniner; Rachel Mandelbaum; Y. Mellier; Reiko Nakajima; Stephane Paulin-Henriksson; Molly S. Peeples; Chris Roat; Alexandre Refregier; Jason Rhodes; Tim Schrabback; Mischa Schirmer; Uros Seljak; Elisabetta Semboloni
The Shear Testing Programme (STEP) is a collaborative project to improve the accuracy and reliability of weak-lensing measurement, in preparation for the next generation of wide-field surveys. We review 16 current and emerging shear-measurement methods in a common language, and assess their performance by running them (blindly) on simulated images that contain a known shear signal. We determine the common features of algorithms that most successfully recover the input parameters. A desirable goal would be the combination of their best elements into one ultimate shear-measurement method. In this analysis, we achieve previously unattained discriminatory precision via a combination of more extensive simulations and pairs of galaxy images that have been rotated with respect to each other. That removes the otherwise overwhelming noise from their intrinsic ellipticities. Finally, the robustness of our simulation approach is confirmed by testing the relative calibration of methods on real data. Weak-lensing measurements have improved since the first STEP paper. Several methods now consistently achieve better than 2 per cent precision, and are still being developed. However, we can now distinguish all methods from perfect performance. Our main concern continues to be the potential for a multiplicative shear calibration bias: not least because this cannot be internally calibrated with real data. We determine which galaxy populations are responsible for bias and, by adjusting the simulated observing conditions, we also investigate the effects of instrumental and atmospheric parameters. The simulated point spread functions are not allowed to vary spatially, to avoid additional confusion from interpolation errors. We have isolated several previously unrecognized aspects of galaxy shape measurement, in which focused development could provide further progress towards the sub-per cent level of precision desired for future surveys. These areas include the suitable treatment of image pixellization and galaxy morphology evolution. Ignoring the former effect affects the measurement of shear in different directions, leading to an overall underestimation of shear and hence the amplitude of the matter power spectrum. Ignoring the second effect could affect the calibration of shear estimators as a function of galaxy redshift, and the evolution of the lensing signal, which will be vital to measure parameters including the dark energy equation of state.
New Journal of Physics | 2007
Sarah Bridle; Lindsay J. King
Cosmic shear constrains cosmology by exploiting the apparent alignments of pairs of galaxies due to gravitational lensing by intervening mass clumps. However, galaxies may become (intrinsically) aligned with each other, and with nearby mass clumps, during their formation. This effect needs to be disentangled from the cosmic shear signal to place constraints on cosmology. We use the linear intrinsic alignment model as a base and compare it to an alternative model and data. If intrinsic alignments are ignored then the dark energy equation of state is biased by ~50%. We examine how the number of tomographic redshift bins affects uncertainties on cosmological parameters and find that when intrinsic alignments are included two or more times as many bins are required to obtain 80% of the available information. We investigate how the degradation in the dark energy figure of merit depends on the photometric redshift scatter. Previous studies have shown that lensing does not place stringent requirements on the photometric redshift uncertainty, so long as the uncertainty is well known. However, if intrinsic alignments are included the requirements become a factor of three tighter. These results are quite insensitive to the fraction of catastrophic outliers, assuming that this fraction is well known. We show the effect of uncertainties in photometric redshift bias and scatter. Finally, we quantify how priors on the intrinsic alignment model would improve dark energy constraints.
Monthly Notices of the Royal Astronomical Society | 2002
Ofer Lahav; Sarah Bridle; Will J. Percival; J. A. Peacock; G. Efstathiou; Carlton M. Baugh; Joss Bland-Hawthorn; Terry J. Bridges; Russell D. Cannon; Shaun Cole; Matthew Colless; Chris A. Collins; Warrick J. Couch; Gavin B. Dalton; Roberto De Propris; Simon P. Driver; Richard S. Ellis; Carlos S. Frenk; Karl Glazebrook; C. A. Jackson; Ian Lewis; S. L. Lumsden; Stephen J. Maddox; Darren Madgwick; Stephen J. Moody; Peder Norberg; Bruce A. Peterson; W. Sutherland; Keith Taylor
We compare the amplitudes of fluctuations probed by the 2dF Galaxy Redshift Survey (2dFGRS) and by the latest measurements of the cosmic microwave background (CMB) anisotropies. By combining the 2dFGRS and CMB data, we find the linear-theory rms mass fluctuations in 8 h (-1) Mpc spheres to be sigma (8m) =0.73+/-0.05 (after marginalization over the matter density parameter Omega(m) and three other free parameters). This normalization is lower than the COBE normalization and previous estimates from cluster abundance, but it is in agreement with some revised cluster abundance determinations. We also estimate the scale-independent bias parameter of present-epoch L (s) =1.9L * APM-selected galaxies to be b (L (s) ,z =0)=1.10+/-0.08 on comoving scales of 0.02<k <0.15 h Mpc(-1). If luminosity segregation operates on these scales, L * galaxies would be almost unbiased, b (L (*) , z =0)approximate to0.96. These results are derived by assuming a flat LambdaCDM Universe, and by marginalizing over other free parameters and fixing the spectral index n =1 and the optical depth due to reionization tau =0. We also study the best-fitting pair (Omega(m) , b ), and the robustness of the results to varying n and tau . Various modelling corrections can each change the resulting b by 5-15 per cent. The results are compared with other independent measurements from the 2dFGRS itself, and from the Sloan Digital Sky Survey (SDSS), cluster abundance and cosmic shear.
Monthly Notices of the Royal Astronomical Society | 2010
Sarah Bridle; Sreekumar T. Balan; Matthias Bethge; Marc Gentile; Stefan Harmeling; Catherine Heymans; Michael Hirsch; Reshad Hosseini; M. Jarvis; D. Kirk; Thomas D. Kitching; Konrad Kuijken; Antony Lewis; Stephane Paulin-Henriksson; Bernhard Schölkopf; Malin Velander; Lisa Voigt; Dugan Witherick; Adam Amara; G. M. Bernstein; F. Courbin; M. S. S. Gill; Alan Heavens; Rachel Mandelbaum; Richard Massey; Baback Moghaddam; A. Rassat; Alexandre Refregier; Jason Rhodes; Tim Schrabback
We present the results of the Gravitational LEnsing Accuracy Testing 2008 (GREAT08) Challenge, a blind analysis challenge to infer weak gravitational lensing shear distortions from images. The primary goal was to stimulate new ideas by presenting the problem to researchers outside the shear measurement community. Six GREAT08 Team methods were presented at the launch of the Challenge and five additional groups submitted results during the 6-month competition. Participants analyzed 30 million simulated galaxies with a range in signal-to-noise ratio, point spread function ellipticity, galaxy size and galaxy type. The large quantity of simulations allowed shear measurement methods to be assessed at a level of accuracy suitable for currently planned future cosmic shear observations for the first time. Different methods perform well in different parts of simulation parameter space and come close to the target level of accuracy in several of these. A number of fresh ideas have emerged as a result of the Challenge including a re-examination of the process of combining information from different galaxies, which reduces the dependence on realistic galaxy modelling. The image simulations will become increasingly sophisticated in future GREAT Challenges, meanwhile the GREAT08 simulations remain as a benchmark for additional developments in shear measurement algorithms.
Science | 2003
Sarah Bridle; Ofer Lahav; Jeremiah P. Ostriker; Paul J. Steinhardt
The recent announcement from the Wilkinson Microwave Anisotropy Probe (WMAP) satellite experiment combined with other recent advances in observational cosmology verifies key components of the standard cosmological model. However, [ Bridle et al .][1] argue in their Perspective that there remain some significant open issues regarding the basic history and composition of the Universe and uncertainties in some of the most important parameters. [1]: http://www.sciencemag.org/cgi/content/full/299/5612/1532
Monthly Notices of the Royal Astronomical Society | 2003
Sarah Bridle; Antony Lewis; J. Weller; G. Efstathiou
We reconstruct the shape of the primordial power spectrum from the latest cosmic microwave background data, including the new results from the Wilkinson Microwave Anisotropy Probe (WMAP), and large scale structure data from the two degree field galaxy redshift survey (2dFGRS). We discuss two parameterizations taking into account the uncertainties in four cosmological parameters. First we parameterize the initial spectrum by a tilt and a running spectral index, finding marginal evidence for a running spectral index only if the first three WMAP multipoles (l = 2,3,4) are included in the analysis. Secondly, to investigate further the low CMB large scale power, we modify the conventional power-law spectrum by introducing a scale above which there is no power. We find a preferred position of the cut at k(c) similar to 3 X 10(-4) Mpc(-1) although k(c) = 0 (no cut) is not ruled out.
Monthly Notices of the Royal Astronomical Society | 2012
Thomas D. Kitching; Sreekumar T. Balan; Sarah Bridle; N. Cantale; F. Courbin; T. F. Eifler; Marc Gentile; M. S. S. Gill; Stefan Harmeling; Catherine Heymans; Michael Hirsch; K. Honscheid; Tomasz Kacprzak; D. Kirkby; Daniel Margala; Richard Massey; P. Melchior; G. Nurbaeva; K. Patton; J. Rhodes; Barnaby Rowe; Andy Taylor; M. Tewes; Massimo Viola; Dugan Witherick; Lisa Voigt; J. Young; Joe Zuntz
We present the results from the first public blind point-spread function (PSF) reconstruction challenge, the GRavitational lEnsing Accuracy Testing 2010 (GREAT10) Star Challenge. Reconstruction of a spatially varying PSF, sparsely sampled by stars, at non-star positions is a critical part in the image analysis for weak lensing where inaccuracies in the modeled ellipticity e and size R^2 can impact the ability to measure the shapes of galaxies. This is of importance because weak lensing is a particularly sensitive probe of dark energy and can be used to map the mass distribution of large scale structure. Participants in the challenge were presented with 27,500 stars over 1300 images subdivided into 26 sets, where in each set a category change was made in the type or spatial variation of the PSF. Thirty submissions were made by nine teams. The best methods reconstructed the PSF with an accuracy of σ(e) ≈ 2.5 × 10^(–4) and σ(R^2)/R^2 ≈ 7.4 × 10^(–4). For a fixed pixel scale, narrower PSFs were found to be more difficult to model than larger PSFs, and the PSF reconstruction was severely degraded with the inclusion of an atmospheric turbulence model (although this result is likely to be a strong function of the amplitude of the turbulence power spectrum).
Astronomy and Astrophysics | 2010
Benjamin Joachimi; Sarah Bridle
Aims. Cosmic shear is a powerful method to constrain cosmology, provided that any systematic effects are under control. The intrinsic alignment of galaxies is expected to severely bias parameter estimates if not taken into account. We explore the potential of a joint analysis of tomographic galaxy ellipticity, galaxy number density, and ellipticity-number density cross-correlations to simultaneously constrain cosmology and self-calibrate unknown intrinsic alignment and galaxy bias contributions. Methods. We treat intrinsic alignments and galaxy biasing as free functions of scale and redshift and marginalise over the resulting parameter sets. Constraints on cosmology are calculated by combining the likelihoods from all two-point correlations between galaxy ellipticity and galaxy number density. The information required for these calculations is already available in a standard cosmic shear data set. We include contributions to these functions from cosmic shear, intrinsic alignments, galaxy clustering and magnification effects. Results. In a Fisher matrix analysis we compare our constraints with those from cosmic shear alone in the absence of intrinsic alignments. For a potential future large area survey, such as Euclid, the extra information from the additional correlation functions can make up for the additional free parameters in the intrinsic alignment and galaxy bias terms, depending on the flexibility in the models. For example, the dark energy task force figure of merit is recovered even when more than 100 free parameters are marginalised over. We find that the redshift quality requirements are similar to those calculated in the absence of intrinsic alignments.