4,832 research outputs found
Cluster Morphologies as a Test of Different Cosmological Models
We investigate how cluster morphology is affected by the cosmological
constant in low-density universes. Using high-resolution cosmological
N-body/SPH simulations of flat (\Omega_0 = 0.3, \lambda_0 = 0.7, \Lambda CDM)
and open (\Omega_0 = 0.3, \lambda_0 = 0, OCDM) cold dark matter universes, we
calculate statistical indicators to quantify the irregularity of the cluster
morphologies. We study axial ratios, center shifts, cluster clumpiness, and
multipole moment power ratios as indicators for the simulated clusters at z=0
and 0.5. Some of these indicators are calculated for both the X-ray surface
brightness and projected mass distributions. In \Lambda CDM all these
indicators tend to be larger than those in OCDM at z=0. This result is
consistent with the analytical prediction of Richstone, Loeb, & Turner, that
is, clusters in \Lambda CDM are formed later than in OCDM, and have more
substructure at z=0. We make a Kolmogorov-Smirnov test on each indicator for
these two models. We then find that the results for the multipole moment power
ratios and the center shifts for the X-ray surface brightness are under the
significance level (5%). We results also show that these two cosmological
models can be distinguished more clearly at z=0 than z = 0.5 by these
indicators.Comment: 30pages, 6figures, Accepted for publication in Ap
Graph cluster randomization: network exposure to multiple universes
A/B testing is a standard approach for evaluating the effect of online
experiments; the goal is to estimate the `average treatment effect' of a new
feature or condition by exposing a sample of the overall population to it. A
drawback with A/B testing is that it is poorly suited for experiments involving
social interference, when the treatment of individuals spills over to
neighboring individuals along an underlying social network. In this work, we
propose a novel methodology using graph clustering to analyze average treatment
effects under social interference. To begin, we characterize graph-theoretic
conditions under which individuals can be considered to be `network exposed' to
an experiment. We then show how graph cluster randomization admits an efficient
exact algorithm to compute the probabilities for each vertex being network
exposed under several of these exposure conditions. Using these probabilities
as inverse weights, a Horvitz-Thompson estimator can then provide an effect
estimate that is unbiased, provided that the exposure model has been properly
specified.
Given an estimator that is unbiased, we focus on minimizing the variance.
First, we develop simple sufficient conditions for the variance of the
estimator to be asymptotically small in n, the size of the graph. However, for
general randomization schemes, this variance can be lower bounded by an
exponential function of the degrees of a graph. In contrast, we show that if a
graph satisfies a restricted-growth condition on the growth rate of
neighborhoods, then there exists a natural clustering algorithm, based on
vertex neighborhoods, for which the variance of the estimator can be upper
bounded by a linear function of the degrees. Thus we show that proper cluster
randomization can lead to exponentially lower estimator variance when
experimentally measuring average treatment effects under interference.Comment: 9 pages, 2 figure
07181 Abstracts Collection -- Parallel Universes and Local Patterns
From 1 May 2007 to 4 May 2007 the Dagstuhl Seminar 07181 ``Parallel
Universes and Local Patterns\u27\u27
was held in the International Conference and Research Center (IBFI),
Schloss Dagstuhl. During the seminar, several participants
presented their current research, and ongoing work and open problems
were discussed. Abstracts of the presentations given during the
seminar as well as abstracts of seminar results and ideas are put
together in this paper. The first section describes the seminar
topics and goals in general. Links to extended abstracts or full
papers are provided, if available
The Local Group as a test of cosmological models
The dynamics of the Local Group and its environment provide a unique
challenge to cosmological models. The velocity field within 5h-1 Mpc of the
Local Group (LG) is extremely ``cold''. The deviation from a pure Hubble flow,
characterized by the observed radial peculiar velocity dispersion, is measured
to be about 60km/s. We compare the local velocity field with similarly defined
regions extracted from N-body simulations of Universes dominated by cold dark
matter (CDM). This test is able to strongly discriminate between models that
have different mean mass densities. We find that neither the Omega=1 (SCDM) nor
Omega=0.3 (OCDM) cold dark matter models can produce a single candidate Local
Group that is embedded in a region with such small peculiar velocities. For
these models, we measure velocity dispersions between 500-700km/s and
150-300km/s respectively, more than twice the observed value. Although both CDM
models fail to produce environments similar to those of our Local Group on a
scale of a few Mpc, they can give rise to many binary systems that have similar
orbital properties as the Milky Way--Andromeda system. The local,
gravitationally induced bias of halos in the CDM ``Local Group'' environment,
if defined within a sphere of 10 Mpc around each Local Group is about 1.5,
independent of Omega. No biasing scheme could reconcile the measured velocity
dispersions around Local Groups with the observed one. Identification of binary
systems using a halo finder (named Skid
(http://www-hpcc.astro.washington.edu/tools/DENMAX for a public version)) based
on local density maxima instead of a simple linking algorithm, gives a much
more complete sample. We show that a standard ``friend of friends'' algorithm
would miss 40% of the LG candidates present in the simulations.Comment: Latex file (19 pages) + 13 figures. Submitted to New Astronomy. Two
MPEG movies were not included. Also available (this time with the movies) at
http://www-hpcc.astro.washington.edu/faculty/fabio/index.htm
Lensing Sunyaev-Zel'dovich Clusters
Full-sky microwave surveys like the upcoming Planck satellite mission will
detect of order 10^4 galaxy clusters through their thermal Sunyaev-Zel'dovich
effect. I investigate the properties of the gravitationally lensing subsample
of these clusters. The main results are: (1) The combined sample comprises
>~70% of the complete sample. (2) It is confined to redshifts 0.2+-0.1, and to
masses (5+-3) x 10^14 solar masses. (3) Using a particular measure for the weak
lensing effect, viz. the aperture mass, cluster masses can be determined with a
relative accuracy of ~20% if their density profile is known. Consequently, the
mass function of the combined sample can accurately be measured. (4) For
low-density universes, I predict a sharp peak in the measured (aperture) mass
function near 5 x 10^14 solar masses and explain its origin, showing that the
peak will be absent in high-density universes. (5) The location of the peak and
the exponential decrease of the mass function on its high-mass side will allow
the determination of the amplitude of the dark-matter power spectrum on the
cluster scale and the baryon fraction in clusters, and constrain the thermal
history of the intracluster gas.Comment: submitted to Astronomy & Astrophysic
Cosmology and Cluster Halo Scaling Relations
We explore the effects of dark matter and dark energy on the dynamical
scaling properties of galaxy clusters. We investigate the cluster Faber-Jackson
(FJ), Kormendy and Fundamental Plane (FP) relations between the mass, radius
and velocity dispersion of cluster size halos in cosmological -body
simulations. The simulations span a wide range of cosmological parameters,
representing open, flat and closed Universes. Independently of the cosmology,
we find that the simulated clusters are close to a perfect virial state and do
indeed define a Fundamental Plane. The fitted parameters of the FJ, Kormendy
and FP relationships do not show any significant dependence on
and/or . The one outstanding effect is the influence of
on the thickness of the Fundamental Plane. Following the time
evolution of our models, we find slight changes of FJ and Kormendy parameters
in high universe, along with a slight decrease of FP fitting
parameters. We also see an initial increase of the FP thickness followed by a
convergence to a nearly constant value. The epoch of convergence is later for
higher values of while the thickness remains constant in the low
-models. We also find a continuous increase of the FP
thickness in the Standard CDM (SCDM) cosmology. There is no evidence that these
differences are due to the different power spectrum slope at cluster scales.
From the point of view of the FP, there is little difference between clusters
that quietly accreted their mass and those that underwent massive mergers. The
principal effect of strong mergers is to change significantly the ratio of the
half-mass radius to the harmonic mean radius .Comment: 24 pages, 17 figures, submitted to MNRA
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