24,821 research outputs found
Constraining Dark Energy with X-ray Galaxy Clusters, Supernovae and the Cosmic Microwave Background
We present new constraints on the evolution of dark energy from an analysis
of Cosmic Microwave Background, supernova and X-ray galaxy cluster data. Our
analysis employs a minimum of priors and exploits the complementary nature of
these data sets. We examine a series of dark energy models with up to three
free parameters: the current dark energy equation of state w_0, the early time
equation of state w_et and the scale factor at transition, a_t. From a combined
analysis of all three data sets, assuming a constant equation of state and that
the Universe is flat, we measure w_0=-1.05+0.10-0.12. Including w_et as a free
parameter and allowing a_t to vary over the range 0.5<a_t<0.95 where the data
sets have discriminating power, we measure w_0=-1.27+0.33-0.39 and
w_et=-0.66+0.44-0.62. We find no significant evidence for evolution in the dark
energy equation of state parameter with redshift. Marginal hints of evolution
in the supernovae data become less significant when the cluster constraints are
also included in the analysis. The complementary nature of the data sets leads
to a tight constraint on the mean matter density, Omega_m and alleviates a
number of other parameter degeneracies, including that between the scalar
spectral index n_s, the physical baryon density Omega_bh^2 and the optical
depth tau. This complementary nature also allows us to examine models in which
we drop the prior on the curvature. For non-flat models with a constant
equation of state, we measure w_0=-1.09+0.12-0.15 and Omega_de=0.70+-0.03. Our
analysis includes spatial perturbations in the dark energy fluid, assuming a
sound speed c_s^2 =1. For our most general dark energy model, not including
such perturbations would lead to spurious constraints on w_et which would be
tighter by approximately a factor two with the current data. (abridged)Comment: 11 pages, 13 figures, 2 tables. Accepted for publication in MNRAS.
Two new figures added: Fig.9 shows the effects of including dark energy
perturbations and Fig.10 compares X-ray cluster data with 2dF dat
A combined measurement of cosmic growth and expansion from clusters of galaxies, the CMB and galaxy clustering
Combining galaxy cluster data from the ROSAT All-Sky Survey and the Chandra
X-ray Observatory, cosmic microwave background data from the Wilkinson
Microwave Anisotropy Probe, and galaxy clustering data from the WiggleZ Dark
Energy Survey, the 6-degree Field Galaxy Survey and the Sloan Digital Sky
Survey III, we test for consistency the cosmic growth of structure predicted by
General Relativity (GR) and the cosmic expansion history predicted by the
cosmological constant plus cold dark matter paradigm (LCDM). The combination of
these three independent, well studied measurements of the evolution of the mean
energy density and its fluctuations is able to break strong degeneracies
between model parameters. We model the key properties of cosmic growth with the
normalization of the matter power spectrum, sigma_8, and the cosmic growth
index, gamma, and those of cosmic expansion with the mean matter density,
Omega_m, the Hubble constant, H_0, and a kinematical parameter equivalent to
that for the dark energy equation of state, w. For a spatially flat geometry,
w=-1, and allowing for systematic uncertainties, we obtain sigma_8=0.785+-0.019
and gamma=0.570+0.064-0.063 (at the 68.3 per cent confidence level). Allowing
both w and gamma to vary we find w=-0.950+0.069-0.070 and gamma=0.533+-0.080.
To further tighten the constraints on the expansion parameters, we also include
supernova, Cepheid variable and baryon acoustic oscillation data. For w=-1, we
have gamma=0.616+-0.061. For our most general model with a free w, we measure
Omega_m=0.278+0.012-0.011, H_0=70.0+-1.3 km s^-1 Mpc^-1 and
w=-0.987+0.054-0.053 for the expansion parameters, and sigma_8=0.789+-0.019 and
gamma=0.604+-0.078 for the growth parameters. These results are in excellent
agreement with GR+LCDM (gamma~0.55; w=-1) and represent the tightest and most
robust simultaneous constraint on cosmic growth and expansion to date.Comment: 14 pages, 5 figures, 1 table. Matches the accepted version for MNRAS.
New sections 3 and 6 added, containing 2 new figures. Table extended. The
results including BAO data have been slightly modified due to an updated BAO
analysis. Conclusions unchange
The Observed Growth of Massive Galaxy Clusters III: Testing General Relativity on Cosmological Scales
This is the third of a series of papers in which we derive simultaneous
constraints on cosmological parameters and X-ray scaling relations using
observations of the growth of massive, X-ray flux-selected galaxy clusters. Our
data set consists of 238 clusters drawn from the ROSAT All-Sky Survey, and
incorporates extensive follow-up observations using the Chandra X-ray
Observatory. Here we present improved constraints on departures from General
Relativity (GR) on cosmological scales, using the growth index, gamma, to
parameterize the linear growth rate of cosmic structure. Using the method of
Mantz et al. (2009a), we simultaneously and self-consistently model the growth
of X-ray luminous clusters and their observable-mass scaling relations,
accounting for survey biases, parameter degeneracies and systematic
uncertainties. We combine the cluster growth data with gas mass fraction, SNIa,
BAO and CMB data. This combination leads to a tight correlation between gamma
and sigma_8. Consistency with GR requires gamma~0.55. Under the assumption of
self-similar evolution and constant scatter in the scaling relations, and for a
flat LCDM model, we measure gamma(sigma_8/0.8)^6.8=0.55+0.13-0.10, with
0.79<sigma_8<0.89. Relaxing the assumptions on the scaling relations by
introducing two additional parameters to model possible evolution in the
normalization and scatter of the luminosity-mass relation, we obtain consistent
constraints on gamma that are only ~20% weaker than those above. Allowing the
dark energy equation of state, w, to take any constant value, we simultaneously
constrain the growth and expansion histories, and find no evidence for
departures from either GR or LCDM. Our results represent the most robust
consistency test of GR on cosmological scales to date. (Abridged)Comment: Accepted for publication in MNRAS. 11 pages, 5 figures, 1 table. New
figure added: Fig. 4 shows the tight constraints on gamma from the cluster
growth data alone compared with those from the other data sets combined
Constraining the Scatter in the Mass-Richness Relation of maxBCG Clusters With Weak Lensing and X-ray Data
We measure the logarithmic scatter in mass at fixed richness for clusters in
the maxBCG cluster catalog, an optically selected cluster sample drawn from
SDSS imaging data. Our measurement is achieved by demanding consistency between
available weak lensing and X-ray measurements of the maxBCG clusters, and the
X-ray luminosity--mass relation inferred from the 400d X-ray cluster survey, a
flux limited X-ray cluster survey. We find \sigma_{\ln
M|N_{200}}=0.45^{+0.20}_{-0.18} (95% CL) at N_{200} ~ 40, where N_{200} is the
number of red sequence galaxies in a cluster. As a byproduct of our analysis,
we also obtain a constraint on the correlation coefficient between \ln Lx and
\ln M at fixed richness, which is best expressed as a lower limit, r_{L,M|N} >=
0.85 (95% CL). This is the first observational constraint placed on a
correlation coefficient involving two different cluster mass tracers. We use
our results to produce a state of the art estimate of the halo mass function at
z=0.23 -- the median redshift of the maxBCG cluster sample -- and find that it
is consistent with the WMAP5 cosmology. Both the mass function data and its
covariance matrix are presented.Comment: 14 pages, 6 figures, submitted to Ap
Cosmological Constraints from the SDSS maxBCG Cluster Catalog
We use the abundance and weak lensing mass measurements of the SDSS maxBCG
cluster catalog to simultaneously constrain cosmology and the richness--mass
relation of the clusters. Assuming a flat \LambdaCDM cosmology, we find
\sigma_8(\Omega_m/0.25)^{0.41} = 0.832\pm 0.033 after marginalization over all
systematics. In common with previous studies, our error budget is dominated by
systematic uncertainties, the primary two being the absolute mass scale of the
weak lensing masses of the maxBCG clusters, and uncertainty in the scatter of
the richness--mass relation. Our constraints are fully consistent with the WMAP
five-year data, and in a joint analysis we find \sigma_8=0.807\pm 0.020 and
\Omega_m=0.265\pm 0.016, an improvement of nearly a factor of two relative to
WMAP5 alone. Our results are also in excellent agreement with and comparable in
precision to the latest cosmological constraints from X-ray cluster abundances.
The remarkable consistency among these results demonstrates that cluster
abundance constraints are not only tight but also robust, and highlight the
power of optically-selected cluster samples to produce precision constraints on
cosmological parameters.Comment: comments welcom
Improved constraints on dark energy from Chandra X-ray observations of the largest relaxed galaxy clusters
We present constraints on the mean matter density, Omega_m, dark energy
density, Omega_de, and the dark energy equation of state parameter, w, using
Chandra measurements of the X-ray gas mass fraction (fgas) in 42 hot (kT>5keV),
X-ray luminous, dynamically relaxed galaxy clusters spanning the redshift range
0.05<z<1.1. Using only the fgas data for the 6 lowest redshift clusters at
z<0.15, for which dark energy has a negligible effect on the measurements, we
measure Omega_m=0.28+-0.06 (68% confidence, using standard priors on the Hubble
Constant, H_0, and mean baryon density, Omega_bh^2). Analyzing the data for all
42 clusters, employing only weak priors on H_0 and Omega_bh^2, we obtain a
similar result on Omega_m and detect the effects of dark energy on the
distances to the clusters at ~99.99% confidence, with Omega_de=0.86+-0.21 for a
non-flat LCDM model. The detection of dark energy is comparable in significance
to recent SNIa studies and represents strong, independent evidence for cosmic
acceleration. Systematic scatter remains undetected in the fgas data, despite a
weighted mean statistical scatter in the distance measurements of only ~5%. For
a flat cosmology with constant w, we measure Omega_m=0.28+-0.06 and
w=-1.14+-0.31. Combining the fgas data with independent constraints from CMB
and SNIa studies removes the need for priors on Omega_bh^2 and H_0 and leads to
tighter constraints: Omega_m=0.253+-0.021 and w=-0.98+-0.07 for the same
constant-w model. More general analyses in which we relax the assumption of
flatness and/or allow evolution in w remain consistent with the cosmological
constant paradigm. Our analysis includes conservative allowances for systematic
uncertainties. The small systematic scatter and tight constraints bode well for
future dark energy studies using the fgas method. (Abridged)Comment: Published in MNRAS. 20 pages, 11 figures. The data and analysis code
(in the form of a patch to CosmoMC) are now available at
http://www.stanford.edu/~drapetti/fgas_module
On the Angular Correlation Function of SZ Clusters : Extracting cosmological information from a 2D catalog
We discuss the angular correlation function of Sunyaev-Zel'dovich
(SZ)-detected galaxy clusters as a cosmological probe. As a projection of the
real-space cluster correlation function, the angular function samples the
underlying SZ catalog redshift distribution. It offers a way to study cosmology
and cluster evolution directly with the two-dimensional catalog, even before
extensive follow-up observations, thereby facilitating the immediate scientific
return from SZ surveys. As a simple illustration of the information content of
the angular function, we examine its dependence on the parameter pair Om_m,
sigma_8 in flat cosmologies. We discuss sources of modeling uncertainty and
consider application to the future Planck SZ catalog, showing how these two
parameters and the normalization of the SZ flux-mass relation can be
simultaneously found when the local X-ray cluster abundance constraint is
included.Comment: 11 pages, 5 figures. A&A, 410, 767; corrected typo, published versio
Solution Path Clustering with Adaptive Concave Penalty
Fast accumulation of large amounts of complex data has created a need for
more sophisticated statistical methodologies to discover interesting patterns
and better extract information from these data. The large scale of the data
often results in challenging high-dimensional estimation problems where only a
minority of the data shows specific grouping patterns. To address these
emerging challenges, we develop a new clustering methodology that introduces
the idea of a regularization path into unsupervised learning. A regularization
path for a clustering problem is created by varying the degree of sparsity
constraint that is imposed on the differences between objects via the minimax
concave penalty with adaptive tuning parameters. Instead of providing a single
solution represented by a cluster assignment for each object, the method
produces a short sequence of solutions that determines not only the cluster
assignment but also a corresponding number of clusters for each solution. The
optimization of the penalized loss function is carried out through an MM
algorithm with block coordinate descent. The advantages of this clustering
algorithm compared to other existing methods are as follows: it does not
require the input of the number of clusters; it is capable of simultaneously
separating irrelevant or noisy observations that show no grouping pattern,
which can greatly improve data interpretation; it is a general methodology that
can be applied to many clustering problems. We test this method on various
simulated datasets and on gene expression data, where it shows better or
competitive performance compared against several clustering methods.Comment: 36 page
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