3,252 research outputs found
Gravitational Collapse of Dust with a Cosmological Constant
The recent analysis of Markovic and Shapiro on the effect of a cosmological
constant on the evolution of a spherically symmetric homogeneous dust ball is
extended to include the inhomogeneous and degenerate cases. The histories are
shown by way of effective potential and Penrose-Carter diagrams.Comment: 2 pages, 2 figures (png), revtex. To appear in Phys. Rev.
Magnetic correlations of the quasi-one-dimensional half-integer spin-chain antiferromagnets SrVO ( = Co, Mn)
Magnetic correlations of two iso-structural quasi-one-dimensional (1D)
antiferromagnetic spin-chain compounds SrVO ( = Co, Mn) have
been investigated by magnetization and powder neutron diffraction. Two
different collinear antiferromagnetic (AFM) structures, characterized by the
propagation vectors, = (0 0 1) and = (0 0 0), have been found below
5.2 K and 42.2 K for the Co- and Mn-compounds, respectively. For
the Mn-compound, AFM chains (along the axis) order ferromagnetically within
the plane, whereas, for the Co-compound, AFM chains order
ferro-/antiferromagnetically along the direction. The critical exponent
study confirms that the Co- and Mn-compounds belong to the Ising and Heisenberg
universality classes, respectively. For both compounds, short-range spin-spin
correlations are present over a wide temperature range above . The reduced
ordered moments at base temperature (1.5 K) indicate the presence of quantum
fluctuations in both compounds due to the quasi-1D magnetic interactions.Comment: 14 pages, 10 figures, 9 table
Junctions and thin shells in general relativity using computer algebra I: The Darmois-Israel Formalism
We present the GRjunction package which allows boundary surfaces and
thin-shells in general relativity to be studied with a computer algebra system.
Implementing the Darmois-Israel thin shell formalism requires a careful
selection of definitions and algorithms to ensure that results are generated in
a straight-forward way. We have used the package to correctly reproduce a wide
variety of examples from the literature. We present several of these
verifications as a means of demonstrating the packages capabilities. We then
use GRjunction to perform a new calculation - joining two Kerr solutions with
differing masses and angular momenta along a thin shell in the slow rotation
limit.Comment: Minor LaTeX error corrected. GRjunction for GRTensorII is available
from http://astro.queensu.ca/~grtensor/GRjunction.htm
Boundary Effects in Local Inflation and Spectrum of Density Perturbations
We observe that when a local patch in a radiation filled Robertson-Walker
universe inflates by some reason, outside perturbations can enter into the
inflating region. Generally, the physical wavelengths of these perturbations
become larger than the Hubble radius as they cross into the inflating space and
their amplitudes freeze out immediately. It turns out that the corresponding
power spectrum is not scale invariant. Although these perturbations cannot
reach out to a distance inner observer shielded by a de Sitter horizon, they
still indicate a curious boundary effect in local inflationary scenarios.Comment: 11 pages, 8 figures, revtex4, v4: minor typos corrected, twocolumn
versio
Tidal stirring and the origin of dwarf spheroidals in the Local Group
N-Body/SPH simulations are used to study the evolution of dwarf irregular
galaxies (dIrrs) entering the dark matter halo of the Milky Way or M31 on
plunging orbits. We propose a new dynamical mechanism driving the evolution of
gas rich, rotationally supported dIrrs, mostly found at the outskirts of the
Local Group (LG), into gas free, pressure supported dwarf spheroidals (dSphs)
or dwarf ellipticals (dEs), observed to cluster around the two giant spirals.
The initial model galaxies are exponential disks embedded in massive dark
matter halos and reproduce nearby dIrrs. Repeated tidal shocks at the
pericenter of their orbit partially strip their halo and disk and trigger
dynamical instabilities that dramatically reshape their stellar component.
After only 2-3 orbits low surface brightness (LSB) dIrrs are transformed into
dSphs, while high surface brightness (HSB) dIrrs evolve into dEs. This
evolutionary mechanism naturally leads to the morphology-density relation
observed for LG dwarfs. Dwarfs surrounded by very dense dark matter halos, like
the archetypical dIrr GR8, are turned into Draco or Ursa Minor, the faintest
and most dark matter dominated among LG dSphs. If disks include a gaseous
component, this is both tidally stripped and consumed in periodic bursts of
star formation. The resulting star formation histories are in good qualitative
agreement with those derived using HST color-magnitude diagrams for local
dSphs.Comment: 5 pages, 5 figures, to appear on ApJL. Simulation images and movies
can be found at the Local Group web page at
http://pcblu.uni.mi.astro.it/~lucio/LG/LG.htm
Surface Layers in General Relativity and Their Relation to Surface Tensions
For a thin shell, the intrinsic 3-pressure will be shown to be analogous to
-A, where A is the classical surface tension: First, interior and exterior
Schwarzschild solutions will be matched together such that the surface layer
generated at the common boundary has no gravitational mass; then its intrinsic
3-pressure represents a surface tension fulfilling Kelvin's relation between
mean curvature and pressure difference in the Newtonian limit. Second, after a
suitable definition of mean curvature, the general relativistic analogue to
Kelvin's relation will be proven to be contained in the equation of motion of
the surface layer.Comment: 12 pages, LaTeX, no figur
Measuring and predicting graded reader difficulty
This study used many-faceted Rasch measurement to investigate the difficulty of graded readers using a 3-item survey. Book difficulty was compared with Kyoto Level, Yomiyasusa Level, Lexile Level, book length, mean sentence length, and mean word frequency. Word frequency and Kyoto Level were found to be ineffective in predicting students' perceptions of book difficulty. Book length was found to be highly predictive of perceived book difficulty, with the Yomiyasusa Levels predicting 68% of variance, while the Lexile measure of mean sentence length was moderately predictive, with 40% of variance explained. These results show that current headword levelling of graded readers is ineffective and that publishers' book levels do not provide useful guidance in selection of books to read. It is therefore recommended that students use book length as their primary consideration in choosing books and that reading recommendations and purchasing decisions be based on Yomiyasusa Levels rather than publishers' levels
Acceleration of particles by rotating black holes: near-horizon geometry and kinematics
Nowadays, the effect of infinite energy in the centre of mass frame due to
near-horizon collisions attracts much attention.We show generality of the
effect combining two seemingly completely different approaches based on
properties of a particle with respect to its local light cone and calculating
its velocity in the locally nonrotaing frame directly. In doing so, we do not
assume that particles move along geodesics. Usually, a particle reaches a
horizon having the velocity equals that of light. However, there is also case
of "critical" particles for which this is not so. It is just the pair of usual
and critical particles that leads to the effect under discussion. The similar
analysis is carried out for massless particles. Then, critical particles are
distinguishable due to the finiteness of local frequency. Thus, both approach
based on geometrical and kinematic properties of particles moving near the
horizon, reveal the universal character of the effect.Comment: 8 page
Microscopic theory of quantum-transport phenomena in mesoscopic systems: A Monte Carlo approach
A theoretical investigation of quantum-transport phenomena in mesoscopic
systems is presented. In particular, a generalization to ``open systems'' of
the well-known semiconductor Bloch equations is proposed. The presence of
spatial boundary conditions manifest itself through self-energy corrections and
additional source terms in the kinetic equations, whose form is suitable for a
solution via a generalized Monte Carlo simulation. The proposed approach is
applied to the study of quantum-transport phenomena in double-barrier
structures as well as in superlattices, showing a strong interplay between
phase coherence and relaxation.Comment: to appear in Phys. Rev. Let
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