34 research outputs found
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
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
Probing dark energy with cluster counts and cosmic shear power spectra: including the full covariance
(Abridged) Combining cosmic shear power spectra and cluster counts is
powerful to improve cosmological parameter constraints and/or test inherent
systematics. However they probe the same cosmic mass density field, if the two
are drawn from the same survey region, and therefore the combination may be
less powerful than first thought. We investigate the cross-covariance between
the cosmic shear power spectra and the cluster counts based on the halo model
approach, where the cross-covariance arises from the three-point correlations
of the underlying mass density field. Fully taking into account the
cross-covariance as well as non-Gaussian errors on the lensing power spectrum
covariance, we find a significant cross-correlation between the lensing power
spectrum signals at multipoles l~10^3 and the cluster counts containing halos
with masses M>10^{14}Msun. Including the cross-covariance for the combined
measurement degrades and in some cases improves the total signal-to-noise
ratios up to plus or minus 20% relative to when the two are independent. For
cosmological parameter determination, the cross-covariance has a smaller effect
as a result of working in a multi-dimensional parameter space, implying that
the two observables can be considered independent to a good approximation. We
also discuss that cluster count experiments using lensing-selected mass peaks
could be more complementary to cosmic shear tomography than mass-selected
cluster counts of the corresponding mass threshold. Using lensing selected
clusters with a realistic usable detection threshold (S/N~6 for a ground-based
survey), the uncertainty on each dark energy parameter may be roughly halved by
the combined experiments, relative to using the power spectra alone.Comment: 32 pages, 15 figures. Revised version, invited original contribution
to gravitational lensing focus issue, New Journal of Physic
Dark energy constraints from cosmic shear power spectra: impact of intrinsic alignments on photometric redshift requirements
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 per cent. 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 per cent 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.Comment: 14 pages and 9 figures. Replaced with final version accepted in
"Gravitational Lensing" Focus Issue of the New Journal of Physics at
http://www.iop.org/EJ/abstract/1367-2630/9/12/E0
Bayesian analysis of weak gravitational lensing and Sunyaev-Zel'dovich data for six galaxy clusters
We present an analysis of observations made with the Arcminute Microkelvin
Imager (AMI) and the Canada-France-Hawaii Telescope (CFHT) of six galaxy
clusters in a redshift range of 0.16--0.41. The cluster gas is modelled using
the Sunyaev--Zel'dovich (SZ) data provided by AMI, while the total mass is
modelled using the lensing data from the CFHT. In this paper, we: i) find very
good agreement between SZ measurements (assuming large-scale virialisation and
a gas-fraction prior) and lensing measurements of the total cluster masses out
to r_200; ii) perform the first multiple-component weak-lensing analysis of
A115; iii) confirm the unusual separation between the gas and mass components
in A1914; iv) jointly analyse the SZ and lensing data for the relaxed cluster
A611, confirming our use of a simulation-derived mass-temperature relation for
parameterizing measurements of the SZ effect.Comment: 22 pages, 12 figures, 12 tables, published by MNRA
Cosmological parameters from CMB and other data: a Monte-Carlo approach
We present a fast Markov Chain Monte-Carlo exploration of cosmological
parameter space. We perform a joint analysis of results from recent CMB
experiments and provide parameter constraints, including sigma_8, from the CMB
independent of other data. We next combine data from the CMB, HST Key Project,
2dF galaxy redshift survey, supernovae 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 (m_nu < 0.3eV), 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
appendices 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.Comment: Quintessence results now include perturbations. Changes to match
version accepted by PRD. MCMC code and data are available at
http://cosmologist.info/cosmomc/ along with a B&W printer-friendly version of
the pape
The impact of camera optical alignments on weak lensing measures for the Dark Energy Survey
Telescope Point Spread Function (PSF) quality is critical for realising the
potential of cosmic weak lensing observations to constrain dark energy and test
General Relativity. In this paper we use quantitative weak gravitational
lensing measures to inform the precision of lens optical alignment, with
specific reference to the Dark Energy Survey (DES). We compute optics spot
diagrams and calculate the shear and flexion of the PSF as a function of
position on the focal plane. For perfect optical alignment we verify the high
quality of the DES optical design, finding a maximum PSF contribution to the
weak lensing shear of 0.04 near the edge of the focal plane. However this can
be increased by a factor of approximately three if the lenses are only just
aligned within their maximum specified tolerances. We calculate the E and
B-mode shear and flexion variance as a function of de-centre or tilt of each
lens in turn. We find tilt accuracy to be a few times more important than
de-centre, depending on the lens considered. Finally we consider the compound
effect of de-centre and tilt of multiple lenses simultaneously, by sampling
from a plausible range of values of each parameter. We find that the compound
effect can be around twice as detrimental as when considering any one lens
alone. Furthermore, this combined effect changes the conclusions about which
lens is most important to align accurately. For DES, the tilt of the first two
lenses is the most important.Comment: 10 pages, 8 figure