740 research outputs found
More examples of structure formation in the Lemaitre-Tolman model
In continuing our earlier research, we find the formulae needed to determine
the arbitrary functions in the Lemaitre-Tolman model when the evolution
proceeds from a given initial velocity distribution to a final state that is
determined either by a density distribution or by a velocity distribution. In
each case the initial and final distributions uniquely determine the L-T model
that evolves between them, and the sign of the energy-function is determined by
a simple inequality. We also show how the final density profile can be more
accurately fitted to observational data than was done in our previous paper. We
work out new numerical examples of the evolution: the creation of a galaxy
cluster out of different velocity distributions, reflecting the current data on
temperature anisotropies of CMB, the creation of the same out of different
density distributions, and the creation of a void. The void in its present
state is surrounded by a nonsingular wall of high density.Comment: LaTeX 2e with eps figures. 30 pages, 11 figures, 30 figure files.
Revision matches published versio
Outskirts of Nearby Disk Galaxies: Star Formation and Stellar Populations
The properties and star formation processes in the far-outer disks of nearby
spiral and dwarf irregular galaxies are reviewed. The origin and structure of
the generally exponential profiles in stellar disks is considered to result
from cosmological infall combined with a non-linear star formation law and a
history of stellar migration and scattering from spirals, bars, and random
collisions with interstellar clouds. In both spirals and dwarfs, the far-outer
disks tend to be older, redder and thicker than the inner disks, with the
overall radial profiles suggesting inside-out star formation plus stellar
scattering in spirals, and outside-in star formation with a possible
contribution from scattering in dwarfs. Dwarf irregulars and the far-outer
parts of spirals both tend to be gas dominated, and the gas radial profile is
often non-exponential although still decreasing with radius. The ratio of
H-alpha to far-UV flux tends to decrease with lower surface brightness in these
regions, suggesting either a change in the initial stellar mass function or the
sampling of that function, or a possible loss of H-alpha photons.Comment: 20 pages, 8 figures, Invited review, Book chapter in "Outskirts of
Galaxies", Eds. J. H. Knapen, J. C. Lee and A. Gil de Paz, Astrophysics and
Space Science Library, Springer, in pres
Outbreak of encephalitic listeriosis in red-legged partridges (Alectoris rufa)
An outbreak of neurological disease was investigated in red-legged partridges between 8 and 28 days of age. Clinical signs included torticollis, head tilt and incoordination and over an initial eight day period approximately 30–40 fatalities occurred per day. No significant gross post mortem findings were detected. Histopathological examination of the brain and bacterial cultures followed by partial sequencing confirmed a diagnosis of encephalitis due to Listeria monocytogenes. Further isolates were obtained from follow-up carcasses, environmental samples and pooled tissue samples of newly imported day-old chicks prior to placement on farm. These isolates had the same antibiotic resistance pattern as the isolate of the initial post mortem submission and belonged to the same fluorescent amplified fragment length polymorphism (fAFLP) subtype. This suggested that the isolates were very closely related or identical and that the pathogen had entered the farm with the imported day-old chicks, resulting in disease manifestation in partridges between 8 and 28 days of age. Reports of outbreaks of encephalitic listeriosis in avian species are rare and this is to the best of our knowledge the first reported outbreak in red-legged partridges
Structure formation in the Lemaitre-Tolman model
Structure formation within the Lemaitre-Tolman model is investigated in a
general manner. We seek models such that the initial density perturbation
within a homogeneous background has a smaller mass than the structure into
which it will develop, and the perturbation then accretes more mass during
evolution. This is a generalisation of the approach taken by Bonnor in 1956. It
is proved that any two spherically symmetric density profiles specified on any
two constant time slices can be joined by a Lemaitre-Tolman evolution, and
exact implicit formulae for the arbitrary functions that determine the
resulting L-T model are obtained. Examples of the process are investigated
numerically.Comment: LaTeX 2e plus 14 .eps & .ps figure files. 33 pages including figures.
Minor revisions of text and data make it more precise and consistent.
Currently scheduled for Phys Rev D vol 64, December 15 issu
You Can't Get Through Szekeres Wormholes - or - Regularity, Topology and Causality in Quasi-Spherical Szekeres Models
The spherically symmetric dust model of Lemaitre-Tolman can describe
wormholes, but the causal communication between the two asymptotic regions
through the neck is even less than in the vacuum
(Schwarzschild-Kruskal-Szekeres) case. We investigate the anisotropic
generalisation of the wormhole topology in the Szekeres model. The function
E(r, p, q) describes the deviation from spherical symmetry if \partial_r E \neq
0, but this requires the mass to be increasing with radius, \partial_r M > 0,
i.e. non-zero density. We investigate the geometrical relations between the
mass dipole and the locii of apparent horizon and of shell-crossings. We
present the various conditions that ensure physically reasonable
quasi-spherical models, including a regular origin, regular maxima and minima
in the spatial sections, and the absence of shell-crossings. We show that
physically reasonable values of \partial_r E \neq 0 cannot compensate for the
effects of \partial_r M > 0 in any direction, so that communication through the
neck is still worse than the vacuum.
We also show that a handle topology cannot be created by identifying
hypersufaces in the two asymptotic regions on either side of a wormhole, unless
a surface layer is allowed at the junction. This impossibility includes the
Schwarzschild-Kruskal-Szekeres case.Comment: zip file with LaTeX text + 6 figures (.eps & .ps). 47 pages. Second
replacement corrects some minor errors and typos. (First replacement prints
better on US letter size paper.
Heat-kernels and functional determinants on the generalized cone
We consider zeta functions and heat-kernel expansions on the bounded,
generalized cone in arbitrary dimensions using an improved calculational
technique. The specific case of a global monopole is analysed in detail and
some restrictions thereby placed on the coefficient. The computation
of functional determinants is also addressed. General formulas are given and
known results are incidentally, and rapidly, reproduced.Comment: 26p,LaTeX.(Cosmetic changes and eqns (9.8),(11.2) corrected.
D-Theory: Field Theory via Dimensional Reduction of Discrete Variables
A new non-perturbative approach to quantum field theory --- D-theory --- is
proposed, in which continuous classical fields are replaced by discrete
quantized variables which undergo dimensional reduction. The 2-d classical O(3)
model emerges from the (2+1)-d quantum Heisenberg model formulated in terms of
quantum spins. Dimensional reduction is demonstrated explicitly by simulating
correlation lengths up to 350,000 lattice spacings using a loop cluster
algorithm. In the framework of D-theory, gauge theories are formulated in terms
of quantum links --- the gauge analogs of quantum spins. Quantum links are
parallel transporter matrices whose elements are non-commuting operators. They
can be expressed as bilinears of anticommuting fermion constituents. In quantum
link models dimensional reduction to four dimensions occurs, due to the
presence of a 5-d Coulomb phase, whose existence is confirmed by detailed
simulations using standard lattice gauge theory. Using Shamir's variant of
Kaplan's fermion proposal, in quantum link QCD quarks appear as edge states of
a 5-d slab. This naturally protects their chiral symmetries without
fine-tuning. The first efficient cluster algorithm for a gauge theory with a
continuous gauge group is formulated for the U(1) quantum link model. Improved
estimators for Wilson loops are constructed, and dimensional reduction to
ordinary lattice QED is verified numerically.Comment: 15 pages, LaTeX, including 9 encapsulated postscript figures.
Contribution to Lattice 97 by 5 authors, to appear in Nuclear Physics B
(Proceeding Supplements). Requires psfig.tex and espcrc2.st
Heat Kernel Coefficients for Laplace Operators on the Spherical Suspension
In this paper we compute the coefficients of the heat kernel asymptotic
expansion for Laplace operators acting on scalar functions defined on the so
called spherical suspension (or Riemann cap) subjected to Dirichlet boundary
conditions. By utilizing a contour integral representation of the spectral zeta
function for the Laplacian on the spherical suspension we find its analytic
continuation in the complex plane and its associated meromorphic structure.
Thanks to the well known relation between the zeta function and the heat kernel
obtainable via Mellin transform we compute the coefficients of the asymptotic
expansion in arbitrary dimensions. The particular case of a -dimensional
sphere as the base manifold is studied as well and the first few heat kernel
coefficients are given explicitly.Comment: 26 Pages, 1 Figur
Resonant transmission through an open quantum dot
We have measured the low-temperature transport properties of a quantum dot
formed in a one-dimensional channel. In zero magnetic field this device shows
quantized ballistic conductance plateaus with resonant tunneling peaks in each
transition region between plateaus. Studies of this structure as a function of
applied perpendicular magnetic field and source-drain bias indicate that
resonant structure deriving from tightly bound states is split by Coulomb
charging at zero magnetic field.Comment: To be published in Phys. Rev. B (1997). 8 LaTex pages with 5 figure
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