13,270 research outputs found
Network calculus for parallel processing
In this note, we present preliminary results on the use of "network calculus"
for parallel processing systems, specifically MapReduce
The wedding of modified dynamics and non-exotic dark matter in galaxy clusters
We summarize the status of Modified Newtonian Dynamics (MOND) in galaxy
clusters. The observed acceleration is typically larger than the acceleration
threshold of MOND in the central regions, implying that some dark matter is
necessary to explain the mass discrepancy there. A plausible resolution of this
issue is that the unseen mass in MOND is in the form of ordinary neutrinos with
masses just below the experimentally detectable limit. In particular, we show
that the lensing mass reconstructions of the clusters 1E0657-56 (the bullet
cluster) and Cl0024+17 (the ring) do not pose a new challenge to this scenario.
However, the mass discrepancy for cool X-ray emitting groups, in which
neutrinos cannot cluster, pose a more serious problem, meaning that dark
baryons could present a more satisfactory solution to the problem of unseen
mass in MOND clusters.Comment: to appear in World Scientific, proceedings of DARK 200
De-aliasing Undersampled Volume Images for Visualization
We present and illustrate a new technique, Image Correlation Supersampling (ICS), for resampling volume data that are undersampled in one dimension. The resulting data satisfies the sampling theorem, and, therefore, many visualization algorithms that assume the theorem is satisfied can be applied to the data. Without the supersampling the visualization algorithms create artifacts due to aliasing. The assumptions made in developing the algorithm are often satisfied by data that is undersampled temporally. Through this supersampling we can completely characterize phenomena with measurements at a coarser temporal sampling rate than would otherwise be necessary. This can save acquisition time and storage space, permit the study of faster phenomena, and allow their study without introducing aliasing artifacts. The resampling technique relies on a priori knowledge of the measured phenomenon, and applies, in particular, to scalar concentration measurements of fluid flow. Because of the characteristics of fluid flow, an image deformation that takes each slice image to the next can be used to calculate intermediate slice images at arbitrarily fine spacing. We determine the deformation with an automatic, multi-resolution algorithm
Diffusion in a multi-component Lattice Boltzmann Equation model
Diffusion phenomena in a multiple component lattice Boltzmann Equation (LBE)
model are discussed in detail. The mass fluxes associated with different
mechanical driving forces are obtained using a Chapman-Enskog analysis. This
model is found to have correct diffusion behavior and the multiple diffusion
coefficients are obtained analytically. The analytical results are further
confirmed by numerical simulations in a few solvable limiting cases. The LBE
model is established as a useful computational tool for the simulation of mass
transfer in fluid systems with external forces.Comment: To appear in Aug 1 issue of PR
Galilean invariance of lattice Boltzmann models
It is well-known that the original lattice Boltzmann (LB) equation deviates
from the Navier-Stokes equations due to an unphysical velocity dependent
viscosity. This unphysical dependency violates the Galilean invariance and
limits the validation domain of the LB method to near incompressible flows. As
previously shown, recovery of correct transport phenomena in kinetic equations
depends on the higher hydrodynamic moments. In this Letter, we give specific
criteria for recovery of various transport coefficients. The Galilean
invariance of a general class of LB models is demonstrated via numerical
experiments
Renormalization group approach to anisotropic superconductivity
The superconducting instability of the Fermi liquid state is investigated by
considering anisotropic electron-boson couplings. Both electron-electron
interactions and anisotropic electron-boson couplings are treated with a
renormalization-group method that takes into account retardation effects.
Considering a non-interacting circular Fermi surface, we find analytical
solutions for the flow equations and derive a set of generalized Eliashberg
equations. Electron-boson couplings with different momentum dependences are
studied, and we find superconducting instabilities of the metallic state with
competition between order parameters of different symmetries. Numerical
solutions for some couplings are given to illustrate the frequency dependence
of the vertices at different coupling regimes.Comment: 9 pages, 7 figures. Final version as published in Phys. Rev.
Is Gravitational Lensing by Intercluster Filaments Always Negligible?
Intercluster filaments negligibly contribute to the weak lensing signal in
general relativity (GR), . In the context of
relativistic modified Newtonian dynamics (MOND) introduced by Bekenstein,
however, a single filament inclined by from the line of
sight can cause substantial distortion of background sources pointing towards
the filament's axis (); this is rigorous
for infinitely long uniform filaments, but also qualitatively true for short
filaments (Mpc), and even in regions where the projected matter
density of the filament is equal to zero. Since galaxies and galaxy clusters
are generally embedded in filaments or are projected on such structures, this
contribution complicates the interpretation of the weak lensing shear map in
the context of MOND. While our analysis is of mainly theoretical interest
providing order-of-magnitude estimates only, it seems safe to conclude that
when modeling systems with anomalous weak lensing signals, e.g. the "bullet
cluster" of Clowe et al., the "cosmic train wreck" of Abell 520 from Mahdavi et
al., and the "dark clusters" of Erben et al., filamentary structures might
contribute in a significant and likely complex fashion. On the other hand, our
predictions of a (conceptual) difference in the weak lensing signal could, in
principle, be used to falsify MOND/TeVeS and its variations.Comment: 11 pages, 6 figures, published versio
Holographic Superconductor for a Lifshitz fixed point
We consider the gravity dual of strongly coupled system at a Lifshitz-fixed
point and finite temperature, which was constructed in a recent work
arXiv:0909.0263. We construct an Abelian Higgs model in that background and
calculate condensation and conductivity using holographic techniques. We find
that condensation happens and DC conductivity blows up when temperature turns
below a critical value.Comment: 14 pages, 4 figures, v4: improved version, references adde
A conjecture on the origin of dark energy
The physical origin of holographic dark energy (HDE) is investigated. The
main existing explanations, namely the UV/IR connection argument of Cohen et
al, Thomas' bulk holography argument, and Ng's spacetime foam argument, are
shown to be not satisfactory. A new explanation of the HDE model is then
proposed based on the ideas of Thomas and Ng. It is suggested that the dark
energy might originate from the quantum fluctuations of spacetime limited by
the event horizon of the universe. Several potential problems of the
explanation are also discussed.Comment: 11 pages, no figure
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