3,864 research outputs found
Kaluza-Klein solitons reexamined
In (4 + 1) gravity the assumption that the five-dimensional metric is
independent of the fifth coordinate authorizes the extra dimension to be either
spacelike or timelike. As a consequence of this, the time coordinate and the
extra coordinate are interchangeable, which in turn allows the conception of
different scenarios in 4D from a single solution in 5D. In this paper, we make
a thorough investigation of all possible 4D scenarios, associated with this
interchange, for the well-known Kramer-Gross-Perry-Davidson-Owen set of
solutions. We show that there are {\it three} families of solutions with very
distinct geometrical and physical properties. They correspond to different sets
of values of the parameters which characterize the solutions in 5D. The
solutions of physical interest are identified on the basis of physical
requirements on the induced-matter in 4D. We find that only one family
satisfies these requirements; the other two violate the positivity of
mass-energy density. The "physical" solutions possess a lightlike singularity
which coincides with the horizon. The Schwarzschild black string solution as
well as the zero moment dipole solution of Gross and Perry are obtained in
different limits. These are analyzed in the context of Lake's geometrical
approach. We demonstrate that the parameters of the solutions in 5D are not
free, as previously considered. Instead, they are totally determined by
measurements in 4D. Namely, by the surface gravitational potential of the
astrophysical phenomena, like the Sun or other stars, modeled in Kaluza-Klein
theory. This is an important result which may help in observations for an
experimental/observational test of the theory.Comment: In V2 we include an Appendix, where we examine the conformal
approach. Minor changes at the beginning of section 2. In V3 more references
are added. Minor editorial changes in the Introduction and Conclusions
section
An exact self-similar solution for an expanding ball of radiation
We give an exact solution of the Einstein equations which in 4D can be
interpreted as a spherically symmetric dissipative distribution of matter, with
heat flux, whose effective density and pressure are nonstatic, nonuniform, and
satisfy the equation of state of radiation. The matter satisfies the usual
energy and thermodynamic conditions. The energy density and temperature are
related by the Stefan-Boltzmann law. The solution admits a homothetic Killing
vector in , which induces the existence of self-similar symmetry in 4D,
where the line element as well as the dimensionless matter quantities are
invariant under a simple "scaling" group.Comment: New version expanded and improved. To appear in Int. J. Mod. Phys.
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
Static Ricci-flat 5-manifolds admitting the 2-sphere
We examine, in a purely geometrical way, static Ricci-flat 5-manifolds
admitting the 2-sphere and an additional hypersurface-orthogonal Killing
vector. These are widely studied in the literature, from different physical
approaches, and known variously as the Kramer - Gross - Perry - Davidson - Owen
solutions. The 2-fold infinity of cases that result are studied by way of new
coordinates (which are in most cases global) and the cases likely to be of
interest in any physical approach are distinguished on the basis of the
nakedness and geometrical mass of their associated singularities. It is argued
that the entire class of solutions has to be considered unstable about the
exceptional solutions: the black string and soliton cases. Any physical theory
which admits the non-exceptional solutions as the external vacuua of a
collapsing object has to accept the possibility of collapse to zero volume
leaving behind the weakest possible, albeit naked, geometrical singularities at
the origin.Finally, it is pointed out that these types of solutions generalize,
in a straightforward way, to higher dimensions.Comment: Generalize, in a straightforward way, to higher dimension
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
Striped morphologies induced by magnetic impurities in d-wave superconductors
We study striped morphologies induced by magnetic impurities in d-wave
superconductors (DSCs) near optimal doping by self-consistently solving the
Bogoliubov-de Gennes equations based on the model. For the
single impurity case, it is found that the stable ground state is a modulated
checkerboard pattern. For the two-impurity case, the stripe-like structures in
order parameters are induced due to the impurity-pinning effect. The
modulations of DSC and charge orders share the same period of four lattice
constants (4), which is half the period of modulations in the coexisting
spin order. Interestingly, when three or more impurities are inserted, the
impurities could induce more complex striped morphologies due to quantum
interference. Further experiments of magnetic impurity substitution in DSCs are
expected to check these results.Comment: 8 pages, 4 figure
Dark Matter Disc Enhanced Neutrino Fluxes from the Sun and Earth
As disc galaxies form in a hierarchical cosmology, massive merging satellites
are preferentially dragged towards the disc plane. The material accreted from
these satellites forms a dark matter disc that contributes 0.25 - 1.5 times the
non-rotating halo density at the solar position. Here, we show the importance
of the dark disc for indirect dark matter detection in neutrino telescopes.
Previous predictions of the neutrino flux from WIMP annihilation in the Earth
and the Sun have assumed that Galactic dark matter is spherically distributed
with a Gaussian velocity distribution, the standard halo model. Although the
dark disc has a local density comparable to the dark halo, its higher phase
space density at low velocities greatly enhances capture rates in the Sun and
Earth. For typical dark disc properties, the resulting muon flux from the Earth
is increased by three orders of magnitude over the SHM, while for the Sun the
increase is an order of magnitude. This significantly increases the sensitivity
of neutrino telescopes to fix or constrain parameters in WIMP models. The flux
from the Earth is extremely sensitive to the detailed properties of the dark
disc, while the flux from the Sun is more robust. The enhancement of the muon
flux from the dark disc puts the search for WIMP annihilation in the Earth on
the same level as the Sun for WIMP masses < 100 GeV.Comment: 7 pages, 4 figures, added a short paragraph to the discussion
section, conclusions unchanged, published versio
DEVELOPMENT OF A TEST PROTOCOL TO IDENTIFY POTENTIAL RISK FACTORS FOR LOWER LIMB INJURIES IN BALLET DANCERS
The aim of the present study was to develop a test protocol to identify potential risk factors for lower limb injuries during ballet jump landings. Two ballet dancers, a beginner level and an advanced level dancer, participated in the study. The dancers performed multiple Sautés in first position, Grand Jeté, and Grand Pas de Chat jump landings on a plantar pressure mat on top of a force platform. The participants wore ballet slipper thongs, while a three-segment kinematic model of the foot was used to provide a more detailed understanding of foot posture during landings, and to investigate how the dancers adapted to the high foot/ankle loadings. Potential risk factors were identified as high free moment peaks, high impact velocities, and improper technique with missing turnouts during landings. Furthermore, there seems a potential to distinguish between dancers’ ability levels and associated injury risks
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