19,345 research outputs found
Submodularity in Batch Active Learning and Survey Problems on Gaussian Random Fields
Many real-world datasets can be represented in the form of a graph whose edge
weights designate similarities between instances. A discrete Gaussian random
field (GRF) model is a finite-dimensional Gaussian process (GP) whose prior
covariance is the inverse of a graph Laplacian. Minimizing the trace of the
predictive covariance Sigma (V-optimality) on GRFs has proven successful in
batch active learning classification problems with budget constraints. However,
its worst-case bound has been missing. We show that the V-optimality on GRFs as
a function of the batch query set is submodular and hence its greedy selection
algorithm guarantees an (1-1/e) approximation ratio. Moreover, GRF models have
the absence-of-suppressor (AofS) condition. For active survey problems, we
propose a similar survey criterion which minimizes 1'(Sigma)1. In practice,
V-optimality criterion performs better than GPs with mutual information gain
criteria and allows nonuniform costs for different nodes
A bias in cosmic shear from galaxy selection: results from ray-tracing simulations
We identify and study a previously unknown systematic effect on cosmic shear
measurements, caused by the selection of galaxies used for shape measurement,
in particular the rejection of close (blended) galaxy pairs. We use ray-tracing
simulations based on the Millennium Simulation and a semi-analytical model of
galaxy formation to create realistic galaxy catalogues. From these, we quantify
the bias in the shear correlation functions by comparing measurements made from
galaxy catalogues with and without removal of close pairs. A likelihood
analysis is used to quantify the resulting shift in estimates of cosmological
parameters. The filtering of objects with close neighbours (a) changes the
redshift distribution of the galaxies used for correlation function
measurements, and (b) correlates the number density of sources in the
background with the density field in the foreground. This leads to a
scale-dependent bias of the correlation function of several percent,
translating into biases of cosmological parameters of similar amplitude. This
makes this new systematic effect potentially harmful for upcoming and planned
cosmic shear surveys. As a remedy, we propose and test a weighting scheme that
can significantly reduce the bias.Comment: 9 pages, 9 figures, version accepted for publication in Astronomy &
Astrophysic
Recoiling Supermassive Black Hole Escape Velocities from Dark Matter Halos
We simulate recoiling black hole trajectories from to in dark
matter halos, quantifying how parameter choices affect escape velocities. These
choices include the strength of dynamical friction, the presence of stars and
gas, the accelerating expansion of the universe (Hubble acceleration), host
halo accretion and motion, and seed black hole mass. CDM halo
accretion increases escape velocities by up to 0.6 dex and significantly
shortens return timescales compared to non-accreting cases. Other parameters
change orbit damping rates but have subdominant effects on escape velocities;
dynamical friction is weak at halo escape velocities, even for extreme
parameter values. We present formulae for black hole escape velocities as a
function of host halo mass and redshift. Finally, we discuss how these findings
affect black hole mass assembly as well as minimum stellar and halo masses
necessary to retain supermassive black holes.Comment: 10 pages, 17 figures. Updated to correct a typo (sign error) in fit
to escape velocity, for return by z=0 (eq. 19
Analysis of two-point statistics of cosmic shear: III. Covariances of shear measures made easy
In recent years cosmic shear, the weak gravitational lensing effect by the
large-scale structure of the Universe, has proven to be one of the
observational pillars on which the cosmological concordance model is founded.
Several cosmic shear statistics have been developed in order to analyze data
from surveys. For the covariances of the prevalent second-order measures we
present simple and handy formulae, valid under the assumptions of Gaussian
density fluctuations and a simple survey geometry. We also formulate these
results in the context of shear tomography, i.e. the inclusion of redshift
information, and generalize them to arbitrary data field geometries. We define
estimators for the E- and B-mode projected power spectra and show them to be
unbiased in the case of Gaussianity and a simple survey geometry. From the
covariance of these estimators we demonstrate how to derive covariances of
arbitrary combinations of second-order cosmic shear measures. We then
recalculate the power spectrum covariance for general survey geometries and
examine the bias thereby introduced on the estimators for exemplary
configurations. Our results for the covariances are considerably simpler than
and analytically shown to be equivalent to the real-space approach presented in
the first paper of this series. We find good agreement with other numerical
evaluations and confirm the general properties of the covariance matrices. The
studies of the specific survey configurations suggest that our simplified
covariances may be employed for realistic survey geometries to good
approximation.Comment: 15 pages, including 4 figures (Fig. 3 reduced in quality); minor
changes, Fig. 4 extended; published in A&
Cosmological constraints from the capture of non-Gaussianity in Weak Lensing data
Weak gravitational lensing has become a common tool to constrain the
cosmological model. The majority of the methods to derive constraints on
cosmological parameters use second-order statistics of the cosmic shear.
Despite their success, second-order statistics are not optimal and degeneracies
between some parameters remain. Tighter constraints can be obtained if
second-order statistics are combined with a statistic that is efficient to
capture non-Gaussianity. In this paper, we search for such a statistical tool
and we show that there is additional information to be extracted from
statistical analysis of the convergence maps beyond what can be obtained from
statistical analysis of the shear field. For this purpose, we have carried out
a large number of cosmological simulations along the {\sigma}8-{\Omega}m
degeneracy, and we have considered three different statistics commonly used for
non-Gaussian features characterization: skewness, kurtosis and peak count. To
be able to investigate non-Gaussianity directly in the shear field we have used
the aperture mass definition of these three statistics for different scales.
Then, the results have been compared with the results obtained with the same
statistics estimated in the convergence maps at the same scales. First, we show
that shear statistics give similar constraints to those given by convergence
statistics, if the same scale is considered. In addition, we find that the peak
count statistic is the best to capture non-Gaussianities in the weak lensing
field and to break the {\sigma}8-{\Omega}m degeneracy. We show that this
statistical analysis should be conducted in the convergence maps: first,
because there exist fast algorithms to compute the convergence map for
different scales, and secondly because it offers the opportunity to denoise the
reconstructed convergence map, which improves non-Gaussian features extraction.Comment: Accepted for publication in MNRAS (11 pages, 5 figures, 9 tables
Combined automotive safety and security pattern engineering approach
Automotive systems will exhibit increased levels of automation as well as ever tighter integration with other vehicles, traffic infrastructure, and cloud services. From safety perspective, this can be perceived as boon or bane - it greatly increases complexity and uncertainty, but at the same time opens up new opportunities for realizing innovative safety functions. Moreover, cybersecurity becomes important as additional concern because attacks are now much more likely and severe. However, there is a lack of experience with security concerns in context of safety engineering in general and in automotive safety departments in particular. To address this problem, we propose a systematic pattern-based approach that interlinks safety and security patterns and provides guidance with respect to selection and combination of both types of patterns in context of system engineering. A combined safety and security pattern engineering workflow is proposed to provide systematic guidance to support non-expert engineers based on best practices. The application of the approach is shown and demonstrated by an automotive case study and different use case scenarios.EC/H2020/692474/EU/Architecture-driven, Multi-concern and Seamless Assurance and Certification of Cyber-Physical Systems/AMASSEC/H2020/737422/EU/Secure COnnected Trustable Things/SCOTTEC/H2020/732242/EU/Dependability Engineering Innovation for CPS - DEIS/DEISBMBF, 01IS16043, Collaborative Embedded Systems (CrESt
A proposal on the galaxy intrinsic alignment self-calibration in weak lensing surveys
The galaxy intrinsic alignment causes the galaxy ellipticity-ellipticity
power spectrum between two photometric redshifts to decrease faster with
respect to the redshift separation , for fixed mean redshift. This
offers a valuable diagnosis on the intrinsic alignment. We show that the
distinctive dependences of the GG, II and GI correlations on over
the range |\Delta z^P|\la 0.2 can be understood robustly without strong
assumptions on the intrinsic alignment. This allows us to measure the intrinsic
alignment within each conventional photo-z bin of typical size \ga 0.2,
through lensing tomography of photo-z bin size . Both the
statistical and systematical errors in the lensing cosmology can be reduced by
this self-calibration technique.Comment: v2: minor revisions. 5 pages, 4 figures. MNRAS letters in pres
Comprehensive Two-Point Analyses of Weak Gravitational Lensing Surveys
We present a framework for analyzing weak gravitational lensing survey data,
including lensing and source-density observables, plus spectroscopic redshift
calibration data. All two-point observables are predicted in terms of
parameters of a perturbed Robertson-Walker metric, making the framework
independent of the models for gravity, dark energy, or galaxy properties. For
Gaussian fluctuations the 2-point model determines the survey likelihood
function and allows Fisher-matrix forecasting. The framework includes nuisance
terms for the major systematic errors: shear measurement errors, magnification
bias and redshift calibration errors, intrinsic galaxy alignments, and
inaccurate theoretical predictions. We propose flexible parameterizations of
the many nuisance parameters related to galaxy bias and intrinsic alignment.
For the first time we can integrate many different observables and systematic
errors into a single analysis. As a first application of this framework, we
demonstrate that: uncertainties in power-spectrum theory cause very minor
degradation to cosmological information content; nearly all useful information
(excepting baryon oscillations) is extracted with ~3 bins per decade of angular
scale; and the rate at which galaxy bias varies with redshift substantially
influences the strength of cosmological inference. The framework will permit
careful study of the interplay between numerous observables, systematic errors,
and spectroscopic calibration data for large weak-lensing surveys.Comment: submitted to Ap
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