307 research outputs found
CosmoHammer: Cosmological parameter estimation with the MCMC Hammer
We study the benefits and limits of parallelised Markov chain Monte Carlo
(MCMC) sampling in cosmology. MCMC methods are widely used for the estimation
of cosmological parameters from a given set of observations and are typically
based on the Metropolis-Hastings algorithm. Some of the required calculations
can however be computationally intensive, meaning that a single long chain can
take several hours or days to calculate. In practice, this can be limiting,
since the MCMC process needs to be performed many times to test the impact of
possible systematics and to understand the robustness of the measurements being
made. To achieve greater speed through parallelisation, MCMC algorithms need to
have short auto-correlation times and minimal overheads caused by tuning and
burn-in. The resulting scalability is hence influenced by two factors, the MCMC
overheads and the parallelisation costs. In order to efficiently distribute the
MCMC sampling over thousands of cores on modern cloud computing infrastructure,
we developed a Python framework called CosmoHammer which embeds emcee, an
implementation by Foreman-Mackey et al. (2012) of the affine invariant ensemble
sampler by Goodman and Weare (2010). We test the performance of CosmoHammer for
cosmological parameter estimation from cosmic microwave background data. While
Metropolis-Hastings is dominated by overheads, CosmoHammer is able to
accelerate the sampling process from a wall time of 30 hours on a dual core
notebook to 16 minutes by scaling out to 2048 cores. Such short wall times for
complex data sets opens possibilities for extensive model testing and control
of systematics.Comment: Published version. 17 pages, 6 figures. The code is available at
http://www.astro.ethz.ch/refregier/research/Software/cosmohamme
Simulating the Large-Scale Structure of HI Intensity Maps
Intensity mapping of neutral hydrogen (HI) is a promising observational probe
of cosmology and large-scale structure. We present wide field simulations of HI
intensity maps based on N-body simulations of a box with
particles (particle mass ).
Using a conditional mass function to populate the simulated dark matter density
field with halos below the mass resolution of the simulation (), we assign HI to
those halos according to a phenomenological halo to HI mass relation. The
simulations span a redshift range of 0.35 < z < 0.9 in redshift bins of width
and cover a quarter of the sky at an angular resolution
of about 7'. We use the simulated intensity maps to study the impact of
non-linear effects and redshift space distortions on the angular clustering of
HI. Focusing on the autocorrelations of the maps, we apply and compare several
estimators for the angular power spectrum and its covariance. We verify that
these estimators agree with analytic predictions on large scales and study the
validity of approximations based on Gaussian random fields, particularly in the
context of the covariance. We discuss how our results and the simulated maps
can be useful for planning and interpreting future HI intensity mapping
surveys.Comment: 35 pages, 19 Figures. Accepted for publication in JCA
An Integrated System at the Bleien Observatory for Mapping the Galaxy
We describe the design and performance of the hardware system at the Bleien
Observatory. The system is designed to deliver a map of the Galaxy for studying
the foreground contamination of low-redshift (z=0.13--0.43) H
intensity mapping experiments as well as other astronomical Galactic studies.
This hardware system is composed of a 7m parabolic dish, a dual-polarization
corrugated horn feed, a pseudo correlation receiver, a Fast Fourier Transform
spectrometer, and an integrated control system that controls and monitors the
progress of the data collection. The main innovative designs in the hardware
are (1) the pseudo correlation receiver and the cold reference source within
(2) the high dynamic range, high frequency resolution spectrometer and (3) the
phase-switch implementation of the system. This is the first time these
technologies are used together for a L-band radio telescope to achieve an
electronically stable system, which is an essential first step for wide-field
cosmological measurements. This work demonstrates the prospects and challenges
for future H intensity mapping experiments.Comment: 11 pages, 12 figures, 1 table, Submitted to MNRA
HOPE: A Python Just-In-Time compiler for astrophysical computations
The Python programming language is becoming increasingly popular for
scientific applications due to its simplicity, versatility, and the broad range
of its libraries. A drawback of this dynamic language, however, is its low
runtime performance which limits its applicability for large simulations and
for the analysis of large data sets, as is common in astrophysics and
cosmology. While various frameworks have been developed to address this
limitation, most focus on covering the complete language set, and either force
the user to alter the code or are not able to reach the full speed of an
optimised native compiled language. In order to combine the ease of Python and
the speed of C++, we developed HOPE, a specialised Python just-in-time (JIT)
compiler designed for numerical astrophysical applications. HOPE focuses on a
subset of the language and is able to translate Python code into C++ while
performing numerical optimisation on mathematical expressions at runtime. To
enable the JIT compilation, the user only needs to add a decorator to the
function definition. We assess the performance of HOPE by performing a series
of benchmarks and compare its execution speed with that of plain Python, C++
and the other existing frameworks. We find that HOPE improves the performance
compared to plain Python by a factor of 2 to 120, achieves speeds comparable to
that of C++, and often exceeds the speed of the existing solutions. We discuss
the differences between HOPE and the other frameworks, as well as future
extensions of its capabilities. The fully documented HOPE package is available
at http://hope.phys.ethz.ch and is published under the GPLv3 license on PyPI
and GitHub.Comment: Accepted for publication in Astronomy and Computing. 14 pages, 1
figure. The code is available at http://hope.phys.ethz.c
Verjährungsfristverkürzung und Haftungsbeschränkung
Zwischen Art. 210 Abs. 4 OR und Art. 199 OR besteht ein Wertungswiderspruch. Es wäre nämlich wertungsmässig widersprüchlich, wenn gemäss Art. 210 Abs. 4 OR im Konsumentenkaufrecht die Verjährung für neue Kaufsachen nicht unter zwei Jahre und bei gebrauchten Sachen nicht unter ein Jahr verkürzt werden darf, demgegenüber aber die Sachgewährleistungsansprüche des Käufers im Konsumentenkaufrecht über Art. 199 OR wegbedungen werden könnten. Deshalb wird in dieser Dissertation der zwischen Art. 210 Abs. 4 OR und Art. 199 OR bestehende Wertungswiderspruch im Rahmen der juristischen Methodenlehre ausgelegt. Es kann vorweggenommen werden, dass aufgrund der Auslegung dieses Wertungswiderspruchs den Sachgewährleistungsansprüchen im Konsumentenkaufrecht von Gesetzes wegen zwingende Natur zukommt. Dieses Auslegungsresultat führt zu von der Lehre und Rechtsprechung noch nicht abgehandelten Fragestellungen und eröffnet die Möglichkeit, verschiedene Rechtsfragen im Konsumentenkaufrecht anhand einer neuen Auslegungsoptik zu interpretieren
Investigation of metal flow in bridge die extrusion of Alloy 6063 and subsequent effect on surface quality and weld seam integrity
This paper describes a detailed study of tube extrusion by simulation using finite element method (FEM). The finite element model used one-sixth of symmetry. The extrusion load, emperature evolution and metal flow were predicted. Innovative methods, combining both grid and surface tools, were used to define in detail the flow of material. These showed clearly the inner and outer surface formation mechanisms of the tube extrusion. The seam weld, an important quality indicator, was also evaluated by selecting an appropriate criterion
Accelerated Modeling of Near and Far-Field Diffraction for Coronagraphic Optical Systems
Accurately predicting the performance of coronagraphs and tolerancing optical
surfaces for high-contrast imaging requires a detailed accounting of
diffraction effects. Unlike simple Fraunhofer diffraction modeling, near and
far-field diffraction effects, such as the Talbot effect, are captured by
plane-to-plane propagation using Fresnel and angular spectrum propagation. This
approach requires a sequence of computationally intensive Fourier transforms
and quadratic phase functions, which limit the design and aberration
sensitivity parameter space which can be explored at high-fidelity in the
course of coronagraph design. This study presents the results of optimizing the
multi-surface propagation module of the open source Physical Optics Propagation
in PYthon (POPPY) package. This optimization was performed by implementing and
benchmarking Fourier transforms and array operations on graphics processing
units, as well as optimizing multithreaded numerical calculations using the
NumExpr python library where appropriate, to speed the end-to-end simulation of
observatory and coronagraph optical systems. Using realistic systems, this
study demonstrates a greater than five-fold decrease in wall-clock runtime over
POPPY's previous implementation and describes opportunities for further
improvements in diffraction modeling performance.Comment: Presented at SPIE ASTI 2018, Austin Texas. 11 pages, 6 figure
Impact of Long-Term Antithrombotic and Statin Therapy on the Clinical Outcome in Patients with Cavernous Malformations of the Central Nervous System: A Single-Center Case Series of 428 Patients
INTRODUCTION
Literature regarding the safety and efficacy of antithrombotic (antiplatelet or anticoagulant) therapy and statins in patients with cavernous malformations (CMs) of the central nervous system is sparse, resulting in uncertainty about its use in clinical practice. The aim of this study was to analyze the impact of antithrombotic therapy and statins on the risk of hemorrhage and focal neurological deficit in patients with CMs.
METHODS
The authors' institutional database was screened for all patients with CMs of the central nervous system treated at their institution between 2006 and 2018. Patients with radiological and/or histological diagnosis of CMs, clinical baseline characteristics, available patient's medication history, and follow-up data were included in this study. Time-to-event probability (hemorrhage or focal neurological deficit) as well as the number of events (hemorrhage or focal neurological deficit) during follow-up were assessed in patients who were categorized according to their medical treatment (antithrombotic therapy, statins, combined therapy, or no treatment).
RESULTS
Four hundred twenty-eight patients with CMs were eligible and included in the final analysis. Sixty-nine (16.1%) patients were on long-term antithrombotic therapy and 46 (10.6%) on long-term statins, of whom 31 patients were on a combination of both. The probability of experiencing first hemorrhage or focal neurological deficit was less likely in patients on antiplatelet therapy (HR 0.09, 95% CI 0.021-0.39, p = 0.001), anticoagulant therapy (HR 0.12, 95% CI 0.016-0.85, p = 0.034), or the combination thereof (HR 0.12, 95% CI 0.016-0.93, p = 0.043) compared to patients with no antithrombotic treatment. The number of hemorrhages and focal neurological deficits were significantly lower in patients on antithrombotic therapy compared to patients on no treatment during follow-up. In patients on statins alone, the time-to-event probability was comparable to that of patients on no treatment (HR 0.91, 95% CI 0.438-1.91, p = 0.812), and the number of events was similar to patients on no treatment.
CONCLUSION
The results of our study provide further evidence that antithrombotic therapy alone or in combination with statins in patients with CMs of the central nervous system does not increase the risk of hemorrhage or focal neurological deficit but, on the contrary, may have some benefit
Mortality in Patients with Brainstem Cavernous Malformations
OBJECTIVE
Brainstem cavernous malformations (BSCM)-associated mortality has been reported up to 20% in patients managed conservatively, whereas postoperative mortality rates range from 0 to 1.9%. Our aim was to analyze the actual risk and causes of BSCM-associated mortality in patients managed conservatively and surgically based on our own patient cohort and a systematic literature review.
METHODS
Observational, retrospective single-center study encompassing all patients with BSCM that presented to our institution between 2006 and 2018. In addition, a systematic review was performed on all studies encompassing patients with BSCM managed conservatively and surgically.
RESULTS
Of 118 patients, 54 were treated conservatively (961.0 person years follow-up in total). No BSCM-associated mortality was observed in our conservatively as well as surgically managed patient cohort. Our systematic literature review and analysis revealed an overall BSCM-associated mortality rate of 2.3% (95% CI: 1.6-3.3) in 22 studies comprising 1,251 patients managed conservatively and of 1.3% (95% CI: 0.9-1.7) in 99 studies comprising 3,275 patients with BSCM treated surgically.
CONCLUSION
The BSCM-associated mortality rate in patients managed conservatively is almost as low as in patients treated surgically and much lower than in frequently cited reports, most probably due to the good selection nowadays in regard to surgery
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