6,293 research outputs found
Detecting dark matter-dark energy coupling with the halo mass function
We use high-resolution simulations of large-scale structure formation to
analyze the effects of interacting dark matter and dark energy on the evolution
of the halo mass function. Using a chi-square likelihood analysis, we find
significant differences in the mass function between models of coupled dark
matter-dark energy and standard concordance cosmology Lambda-CDM out to
redshift z=1.5. We also find a preliminary indication that the Dark Energy
Survey should be able to distinguish these models from Lambda-CDM within its
mass and redshift contraints. While we can distinguish the effects of these
models from Lambda-CDM cosmologies with different fundamental parameters, DES
will require independent measurements of sigma-8 to confirm these effects.Comment: 5 pages, 3 figures, responded to referee comments, accepted by Ap
Universal Polar Codes for More Capable and Less Noisy Channels and Sources
We prove two results on the universality of polar codes for source coding and
channel communication. First, we show that for any polar code built for a
source there exists a slightly modified polar code - having the same
rate, the same encoding and decoding complexity and the same error rate - that
is universal for every source when using successive cancellation
decoding, at least when the channel is more capable than
and is such that it maximizes for the given channels
and . This result extends to channel coding for discrete
memoryless channels. Second, we prove that polar codes using successive
cancellation decoding are universal for less noisy discrete memoryless
channels.Comment: 10 pages, 3 figure
Cluster magnetic fields from active galactic nuclei
Active galactic nuclei (AGN) found at the centers of clusters of galaxies are
a possible source for weak cluster-wide magnetic fields. To evaluate this
scenario, we present 3D adaptive mesh refinement MHD simulations of a cool-core
cluster that include injection of kinetic, thermal, and magnetic energy via an
AGN-powered jet. Using the MHD solver in FLASH 2, we compare several
sub-resolution approaches that link the estimated accretion rate as measured on
the simulation mesh to the accretion rate onto the central black hole and the
resulting feedback. We examine the effects of magnetized outflows on the
accretion history of the black hole and discuss the ability of these models to
magnetize the cluster medium.Comment: 4 pages, 2 figures, submitted to conference proceedings "The
Monster's Fiery Breath: Feedback in Groups, Galaxies, and Clusters
The Influence of AGN Feedback on Galaxy Cluster Observables
Galaxy clusters are valuable cosmological probes. However, cluster mass
estimates rely on observable quantities that are affected by complicated
baryonic physics in the intracluster medium (ICM), including feedback from
active galactic nuclei (AGN). Cosmological simulations have started to include
AGN feedback using subgrid models. In order to make robust predictions, the
systematics of different implementations and parametrizations need to be
understood. We have developed an AGN subgrid model in FLASH that supports a few
different black hole accretion models and feedback models. We use this model to
study the effect of AGN on X-ray cluster observables and its dependence on
model variations.Comment: minor error corrected, to appear in proceedings of the conference
"The Monster's Fiery Breath: Feedback in Galaxies, Groups, and Clusters",
June 2009, Madison, Wisconsi
Using hybrid GPU/CPU kernel splitting to accelerate spherical convolutions
We present a general method for accelerating by more than an order of
magnitude the convolution of pixelated functions on the sphere with a
radially-symmetric kernel. Our method splits the kernel into a compact
real-space component and a compact spherical harmonic space component. These
components can then be convolved in parallel using an inexpensive commodity GPU
and a CPU. We provide models for the computational cost of both real-space and
Fourier space convolutions and an estimate for the approximation error. Using
these models we can determine the optimum split that minimizes the wall clock
time for the convolution while satisfying the desired error bounds. We apply
this technique to the problem of simulating a cosmic microwave background (CMB)
anisotropy sky map at the resolution typical of the high resolution maps
produced by the Planck mission. For the main Planck CMB science channels we
achieve a speedup of over a factor of ten, assuming an acceptable fractional
rms error of order 1.e-5 in the power spectrum of the output map.Comment: 9 pages, 11 figures, 1 table, accepted by Astronomy & Computing w/
minor revisions. arXiv admin note: substantial text overlap with
arXiv:1211.355
Universal Density Profile for Cosmic Voids
We present a simple empirical function for the average density profile of
cosmic voids, identified via the watershed technique in CDM N-body
simulations. This function is universal across void size and redshift,
accurately describing a large radial range of scales around void centers with
only two free parameters. In analogy to halo density profiles, these parameters
describe the scale radius and the central density of voids. While we initially
start with a more general four-parameter model, we find two of its parameters
to be redundant, as they follow linear trends with the scale radius in two
distinct regimes of the void sample, separated by its compensation scale.
Assuming linear theory, we derive an analytic formula for the velocity profile
of voids and find an excellent agreement with the numerical data as well. In
our companion paper [Sutter et al., Mon. Not. R. Astron. Soc. 442, 462 (2014)]
the presented density profile is shown to be universal even across tracer type,
properly describing voids defined in halo and galaxy distributions of varying
sparsity, allowing us to relate various void populations by simple rescalings.
This provides a powerful framework to match theory and simulations with
observational data, opening up promising perspectives to constrain competing
models of cosmology and gravity.Comment: 5 pages, 3 figures. Matches PRL published version after minor
correction
GRAIL – Grid Access and Instrumentation Tool
Since the release of Globus Toolkit 4 Web services enrich the world of Grid Computing. They provide methods to develop modular Grid applications which can be parallelized easily. The access to Web services is mostly solved by complex command line tools which need a good deal of knowledge of the underlaying Grid technologies. GRAIL is intended to fill the gap between existing Grid access methods and both the developer who wants to utilize the Grid for own developments and the user who wants to access the Grid without much additional knowledge. It simplifies the access and the testing of Web services for the Globus Grid middleware. GRAIL provides an easy to use graphical user interface for executing Web services and enables the user to construct complex relationships between services to realize parallel execution. The underlying framework allows an easy integration of any Web service or other arbitrary task without much additional effort for the developer. Existing technologies, shipped with the Globus Toolkit, are seamlessly integrated into GRAIL
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