136 research outputs found
Moment-Based Ellipticity Measurement as a Statistical Parameter Estimation Problem
We show that galaxy ellipticity estimation for weak gravitational lensing
with unweighted image moments reduces to the problem of measuring a combination
of the means of three independent normal random variables. Under very general
assumptions, the intrinsic image moments of sources can be recovered from
observations including effects such as the point-spread function and
pixellation. Gaussian pixel noise turns these into three jointly normal random
variables, the means of which are algebraically related to the ellipticity. We
show that the random variables are approximately independent with known
variances, and provide an algorithm for making them exactly independent. Once
the framework is developed, we derive general properties of the ellipticity
estimation problem, such as the signal-to-noise ratio, a generic form of an
ellipticity estimator, and Cram\'er-Rao lower bounds for an unbiased estimator.
We then derive the unbiased ellipticity estimator using unweighted image
moments. We find that this unbiased estimator has a poorly behaved distribution
and does not converge in practical applications, but demonstrates how to derive
and understand the behaviour of new moment-based ellipticity estimators.Comment: 11 pages, 7 figures; v2 matches accepted version with minor change
Cosmology with photometric redshift surveys
We explore the utility of future photometric redshift imaging surveys for
delineating the large-scale structure of the Universe, and assess the resulting
constraints on the cosmological model. We perform two complementary types of
analysis: (1) We quantify the statistical confidence and the accuracy with
which such surveys will be able to detect and measure characteristic features
in the clustering power spectrum such as the acoustic oscillations and the
turnover, in a 'model-independent' fashion. We show for example that a 10,000
deg^2 imaging survey with depth r = 22.5 and photometric redshift accuracy
dz/(1+z) = 0.03 will detect the acoustic oscillations with 99.9% confidence,
measuring the associated preferred cosmological scale with 2% precision. Such a
survey will also detect the turnover with 95% confidence, determining the
corresponding scale with 20% accuracy. (2) By assuming a Lambda-CDM model power
spectrum we calculate the confidence with which a non-zero baryon fraction can
be deduced from such future galaxy surveys. We quantify 'wiggle detection' by
calculating the number of standard deviations by which the baryon fraction is
measured, after marginalizing over the shape parameter. This is typically a
factor of four more significant (in terms of number of standard deviations)
than the 'model-independent' result. We conclude that the precision with which
the clustering pattern may be inferred from future photometric redshift surveys
will be competitive with contemporaneous spectroscopic redshift surveys,
assuming that systematic effects can be controlled. We also note that an
analysis of Luminous Red Galaxies in the Sloan Digital Sky Survey may yield a
marginal detection of acoustic oscillations in the imaging survey, in addition
to that recently reported for the spectroscopic component.Comment: 23 pages, 22 figures, version accepted by MNRA
Cosmological baryonic and matter densities from 600,000 SDSS Luminous Red Galaxies with photometric redshifts
We analyze MegaZ-LRG, a photometric-redshift catalogue of Luminous Red
Galaxies (LRGs) based on the imaging data of the Sloan Digital Sky Survey
(SDSS) 4th Data Release. MegaZ-LRG, presented in a companion paper, contains
10^6 photometric redshifts derived with ANNz, an Artificial Neural Network
method, constrained by a spectroscopic sub-sample of 13,000 galaxies obtained
by the 2dF-SDSS LRG and Quasar (2SLAQ) survey. The catalogue spans the redshift
range 0.4 < z < 0.7 with an r.m.s. redshift error ~ 0.03(1+z), covering 5,914
deg^2 to map out a total cosmic volume 2.5 h^-3 Gpc^3. In this study we use the
most reliable 600,000 photometric redshifts to present the first cosmological
parameter fits to galaxy angular power spectra from a photometric redshift
survey. Combining the redshift slices with appropriate covariances, we
determine best-fitting values for the matter and baryon densities of Omega_m h
= 0.195 +/- 0.023 and Omega_b/Omega_m = 0.16 +/- 0.036 (with the Hubble
parameter h = 0.75 and scalar index of primordial fluctuations n = 1 held
fixed). These results are in agreement with and independent of the latest
studies of the Cosmic Microwave Background radiation, and their precision is
comparable to analyses of contemporary spectroscopic-redshift surveys. We
perform an extensive series of tests which conclude that our power spectrum
measurements are robust against potential systematic photometric errors in the
catalogue. We conclude that photometric-redshift surveys are competitive with
spectroscopic surveys for measuring cosmological parameters in the simplest
vanilla models. Future deep imaging surveys have great potential for further
improvement, provided that systematic errors can be controlled.Comment: 24 pages, 23 figures, MNRAS accepte
Cosmology with the Square Kilometre Array
We argue that the Square Kilometre Array has the potential to make both
redshift (HI) surveys and radio continuum surveys that will revolutionize
cosmological studies, provided that it has sufficient instantaneous
field-of-view that these surveys can cover a hemisphere in a timescale ~1 yr.
Adopting this assumption, we focus on two key experiments which will yield
fundamental new measurements in cosmology, characterizing the properties of the
mysterious dark energy which dominates the dynamics of today's Universe.
Experiment I will map out ~10^9 HI galaxies to redshift z~1.5, providing the
premier measurement of the clustering power spectrum of galaxies: accurately
delineating the acoustic oscillations and the `turnover'. Experiment II will
quantify the cosmic shear distortion of ~10^10 radio continuum sources,
determining a precise power spectrum of the dark matter, and its growth as a
function of cosmic epoch. We contrast the performance of the SKA in precision
cosmology with that of other facilities which will, probably or possibly, be
available on a similar timescale. We conclude that data from the SKA will yield
transformational science as the direct result of four key features: (i) the
immense cosmic volumes probed, exceeding future optical redshift surveys by
more than an order of magnitude; (ii) well-controlled systematic effects such
as the narrow `k-space window function' for Experiment I and the
accurately-known `point-spread function' for Experiment II; (iii) the ability
to measure with high precision large-scale modes in the clustering power
spectra, for which nuisance effects such as non-linear structure growth,
peculiar velocities and `galaxy bias' are minimised; and (iv) different
degeneracies between key parameters to those which are inherent in the CMB.Comment: 20 pages, 8 figures. To appear in "Science with the Square Kilometer
Array", eds. C.Carilli and S.Rawlings, New Astronomy Reviews (Elsevier:
Amsterdam
Testing for a Super-Acceleration Phase of the Universe
We propose a method to probe the phenomenological nature of dark energy which
makes no assumptions about the evolution of its energy density. We exemplify
this method with a test for a super-acceleration phase of the universe i.e., a
phase when the dark energy density grows as the universe expands. We show how
such a phase can be detected by combining SNIa (SNAP-like) and CMB (Planck)
data without making any assumptions about the evolution of the dark energy
equation of state, or about the value of the matter density parameter.Comment: Matches version accepted for publication in PRD. Added discussion of
the effect of the calibration parameter on detecting super-acceleration. 8
pages and 4 figure
Precision Cosmology? Not Just Yet
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,
we argue 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.Comment: 2 pages, 2 figures. Online journal version
http://www.sciencemag.org/cgi/content/full/299/5612/153
Cosmic Discordance: Are Planck CMB and CFHTLenS weak lensing measurements out of tune?
We examine the level of agreement between low redshift weak lensing data and
the CMB using measurements from the CFHTLenS and Planck+WMAP polarization. We
perform an independent analysis of the CFHTLenS six bin tomography results of
Heymans et al. (2013). We extend their systematics treatment and find the
cosmological constraints to be relatively robust to the choice of non-linear
modeling, extension to the intrinsic alignment model and inclusion of baryons.
We find that the 90% confidence contours of CFHTLenS and Planck+WP do not
overlap even in the full 6-dimensional parameter space of CDM, so the
two datasets are discrepant. Allowing a massive active neutrino or tensor modes
does not significantly resolve the disagreement in the full n-dimensional
parameter space. Our results differ from some in the literature because we use
the full tomographic information in the weak lensing data and marginalize over
systematics. We note that adding a sterile neutrino to CDM does bring
the 8-dimensional 64% contours to overlap, mainly due to the extra effective
number of neutrino species, which we find to be 0.84 0.35 (68%) greater
than standard on combining the datasets. We discuss why this is not a
completely satisfactory resolution, leaving open the possibility of other new
physics or observational systematics as contributing factors. We provide
updated cosmology fitting functions for the CFHTLenS constraints and discuss
the differences from ones used in the literature.Comment: 12 pages, 8 figures. We compare our findings with studies that
include other low redshift probes of structure. An interactive figure is
available at http://bit.ly/1oZH0KQ. This version is that accepted by MNRAS,
and so includes changes based on the referee's comments, and updates to the
analysis cod
The Redshift Sensitivities of Dark Energy Surveys
Great uncertainty surrounds dark energy, both in terms of its physics, and
the choice of methods by which the problem should be addressed. Here we
quantify the redshift sensitivities offered by different techniques. We focus
on the three methods most adept at constraining w, namely supernovae, cosmic
shear, and baryon oscillations. For each we provide insight into the family of
w(z) models which are permitted for a particular constraint on either w=w0 or
w=w0+wa(1-a). Our results are in the form of "weight functions", which describe
the fitted model parameters as a weighted average over the true functional
form. For example, we find the recent best-fit from the Supernovae Legacy
Survey (w=-1.023) corresponds to the average value of w(z) over the range
0<z<0.4. Whilst there is a strong dependence on the choice of priors, each
cosmological probe displays distinctive characteristics in their redshift
sensitivities. In the case of proposed future surveys, a SNAP-like supernova
survey probes a mean redshift of z ~ 0.3, with baryon oscillations and cosmic
shear at z ~ 0.6. If we consider the evolution of w, sensitivities shift to
slightly higher redshift. Finally, we find that the weight functions may be
expressed as a weighted average of the popular "principal components".Comment: 11 pages, 10 figures, changes reflect published versio
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