1,486 research outputs found
Monte Carlo Simulation of Ising Models with Dipole Interaction
Recently, a new memory effect was found in the metamagnetic domain structure
of the diluted Ising antiferromagnet by domain imaging
with Faraday contrast. Essential for this effect is the dipole interaction. We
simulate the low temperature behavior of diluted Ising-antiferromagnets by a
Monte Carlo simulation considering long range interaction. The metamagnetic
domain structure occurring due to the dipole interaction is investigated by
graphical representation. In the model considered the antiferromagnetic state
is stable for an external magnetic field smaller than a lower boundary
while for fields larger than an upper boundary the system is in the
saturated paramagnetic phase, where the spins are ferromagnetically polarized.
For magnetic fields in between these two boundaries a mixed phase occurs
consisting of ferromagnetic domains in an antiferromagnetic background. The
position of these ferromagnetic domains is stored in the system: after a cycle
in which the field is first removed and afterwards applied again the domains
reappear at their original positions. The reason for this effect can be found
in the frozen antiferromagnetic domain state which occurs after removing the
field.Comment: Latex, 10 pages; 3 postsript-figures, compressed tar-file, uuencoded,
report 10109
Back-reaction and effective acceleration in generic LTB dust models
We provide a thorough examination of the conditions for the existence of
back-reaction and an "effective" acceleration (in the context of Buchert's
averaging formalism) in regular generic spherically symmetric
Lemaitre-Tolman-Bondi (LTB) dust models. By considering arbitrary spherical
comoving domains, we verify rigorously the fulfillment of these conditions
expressed in terms of suitable scalar variables that are evaluated at the
boundary of every domain. Effective deceleration necessarily occurs in all
domains in: (a) the asymptotic radial range of models converging to a FLRW
background, (b) the asymptotic time range of non-vacuum hyperbolic models, (c)
LTB self-similar solutions and (d) near a simultaneous big bang. Accelerating
domains are proven to exist in the following scenarios: (i) central vacuum
regions, (ii) central (non-vacuum) density voids, (iii) the intermediate radial
range of models converging to a FLRW background, (iv) the asymptotic radial
range of models converging to a Minkowski vacuum and (v) domains near and/or
intersecting a non-simultaneous big bang. All these scenarios occur in
hyperbolic models with negative averaged and local spatial curvature, though
scenarios (iv) and (v) are also possible in low density regions of a class of
elliptic models in which local spatial curvature is negative but its average is
positive. Rough numerical estimates between -0.003 and -0.5 were found for the
effective deceleration parameter. While the existence of accelerating domains
cannot be ruled out in models converging to an Einstein de Sitter background
and in domains undergoing gravitational collapse, the conditions for this are
very restrictive. The results obtained may provide important theoretical clues
on the effects of back-reaction and averaging in more general non-spherical
models.Comment: Final version accepted for publication in Classical and Quantum
Gravity. 47 pages in IOP LaTeX macros, 12 pdf figure
Hubble flow variance and the cosmic rest frame
We characterize the radial and angular variance of the Hubble flow in the
COMPOSITE sample of 4534 galaxies, on scales in which much of the flow is in
the nonlinear regime. With no cosmological assumptions other than the existence
of a suitably averaged linear Hubble law, we find with decisive Bayesian
evidence (ln B >> 5) that the Hubble constant averaged in independent spherical
radial shells is closer to its asymptotic value when referred to the rest frame
of the Local Group, rather than the standard rest frame of the Cosmic Microwave
Background. An exception occurs for radial shells in the range 40/h-60/h Mpc.
Angular averages reveal a dipole structure in the Hubble flow, whose amplitude
changes markedly over the range 32/h-62/h Mpc. Whereas the LG frame dipole is
initially constant and then decreases significantly, the CMB frame dipole
initially decreases but then increases. The map of angular Hubble flow
variation in the LG rest frame is found to coincide with that of the residual
CMB temperature dipole, with correlation coefficient -0.92. These results are
difficult to reconcile with the standard kinematic interpretation of the motion
of the Local Group in response to the clustering dipole, but are consistent
with a foreground non-kinematic anisotropy in the distance-redshift relation of
0.5% on scales up to 65/h Mpc. Effectively, the differential expansion of space
produced by nearby nonlinear structures of local voids and denser walls and
filaments cannot be reduced to a local boost. This hypothesis suggests a
reinterpretation of bulk flows, which may potentially impact on calibration of
supernovae distances, anomalies associated with large angles in the CMB
anisotropy spectrum, and the dark flow inferred from the kinematic
Sunyaev-Zel'dovich effect. It is consistent with recent studies that find
evidence for a non-kinematic dipole in the distribution of distant radio
sources.Comment: 37 pages, 9 tables, 13 figures; v2 adds extensive new analysis
(including additional subsections, tables, figures); v3 adds a Monte Carlo
analysis (with additional table, figure) which further tightens the
statistical robustness of the dipole results; v4 adds further clarifications,
small corrections, references and discussion of Planck satellite results; v5
typos fixed, matches published versio
Usefulness of high resolution coastal models for operational oil spill forecast: the "Full City" accident
Oil spill modeling is considered to be an important part of a decision support system (DeSS) for oil spill combatment and is useful for remedial action in case of accidents, as well as for designing the environmental monitoring system that is frequently set up after major accidents. Many accidents take place in coastal areas, implying that low resolution basin scale ocean models are of limited use for predicting the trajectories of an oil spill. In this study, we target the oil spill in connection with the "Full City" accident on the Norwegian south coast and compare operational simulations from three different oil spill models for the area. The result of the analysis is that all models do a satisfactory job. The "standard" operational model for the area is shown to have severe flaws, but by applying ocean forcing data of higher resolution (1.5 km resolution), the model system shows results that compare well with observations. The study also shows that an ensemble of results from the three different models is useful when predicting/analyzing oil spill in coastal areas
Do low surface brightness galaxies have dense disks?
The disk masses of four low surface brightness galaxies (LSB) were estimated
using marginal gravitational stability criterion and the stellar velocity
dispersion data which were taken from Pizzella et al., 2008 [1]. The
constructed mass models appear to be close to the models of maximal disk. The
results show that the disks of LSB galaxies may be significantly more massive
than it is usually accepted from their brightnesses. In this case their surface
densities and masses appear to be rather typical for normal spirals. Otherwise,
unlike the disks of many spiral galaxies, the LSB disks are dynamically
overheated.Comment: 14 pages, 10 figures, submitted to Astronomy Report
Relaxation of the field-cooled magnetization of an Ising spin glass
The time and temperature dependence of the field-cooled magnetization of a
three dimensional Ising spin glass, Fe_{0.5}Mn_{0.5}TiO_{3}, has been
investigated. The temperature and cooling rate dependence is found to exhibit
memory phenomena that can be related to the memory behavior of the low
frequency ac-susceptibility. The results add some further understanding on how
to model the three dimensional Ising spin glass in real space.Comment: 8 pages RevTEX, 5 figure
Weighed scalar averaging in LTB dust models, part I: statistical fluctuations and gravitational entropy
We introduce a weighed scalar average formalism ("q-average") for the study
of the theoretical properties and the dynamics of spherically symmetric
Lemaitre-Tolman-Bondi (LTB) dust models models. The "q-scalars" that emerge by
applying the q-averages to the density, Hubble expansion and spatial curvature
(which are common to FLRW models) are directly expressible in terms of
curvature and kinematic invariants and identically satisfy FLRW evolution laws
without the back-reaction terms that characterize Buchert's average. The local
and non-local fluctuations and perturbations with respect to the q-average
convey the effects of inhomogeneity through the ratio of curvature and
kinematic invariants and the magnitude of radial gradients. All curvature and
kinematic proper tensors that characterize the models are expressible as
irreducible algebraic expansions on the metric and 4-velocity, whose
coefficients are the q-scalars and their linear and quadratic local
fluctuations. All invariant contractions of these tensors are quadratic
fluctuations, whose q-averages are directly and exactly related to statistical
correlation moments of the density and Hubble expansion scalar. We explore the
application of this formalism to a definition of a gravitational entropy
functional proposed by Hosoya et al (2004 Phys. Rev. Lett. 92 141302). We show
that a positive entropy production follows from a negative correlation between
fluctuations of the density and Hubble scalar, providing a brief outline on its
fulfillment in various LTB models and regions. While the q-average formalism is
specially suited for LTB and Szekeres models, it may provide a valuable
theoretical insight on the properties of scalar averaging in inhomogeneous
spacetimes in general.Comment: 27 pages in IOP format, 1 figure. Matches version accepted for
publication in Classical and Quantum Gravit
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