6,105 research outputs found
Gravitational Lenses With More Than Four Images: I. Classification of Caustics
We study the problem of gravitational lensing by an isothermal elliptical
density galaxy in the presence of a tidal perturbation. When the perturbation
is fairly strong and oriented near the galaxy's minor axis, the lens can
produce image configurations with six or even eight highly magnified images
lying approximately on a circle. We classify the caustic structures in the
model and identify the range of models that can produce such lenses. Sextuple
and octuple lenses are likely to be rare because they require special lens
configurations, but a full calculation of the likelihood will have to include
both the existence of lenses with multiple lens galaxies and the strong
magnification bias that affects sextuple and octuple lenses. At optical
wavelengths these lenses would probably appear as partial or complete Einstein
rings, but at radio wavelengths the individual images could probably be
resolved.Comment: 30 pages, including 12 postscript figures; accepted for publication
in Ap
The Post-Newtonian Limit of f(R)-gravity in the Harmonic Gauge
A general analytic procedure is developed for the post-Newtonian limit of
-gravity with metric approach in the Jordan frame by using the harmonic
gauge condition. In a pure perturbative framework and by using the Green
function method a general scheme of solutions up to order is shown.
Considering the Taylor expansion of a generic function it is possible to
parameterize the solutions by derivatives of . At Newtonian order,
, all more important topics about the Gauss and Birkhoff theorem are
discussed. The corrections to "standard" gravitational potential
(-component of metric tensor) generated by an extended uniform mass
ball-like source are calculated up to order. The corrections, Yukawa
and oscillating-like, are found inside and outside the mass distribution. At
last when the limit is considered the -gravity converges
in General Relativity at level of Lagrangian, field equations and their
solutions.Comment: 16 pages, 10 figure
Workers Compensation-Reported Injuries Among Security and Law Enforcement Personnel in the Private Versus Public Sectors
Background: Private and Public security and law enforcement (SLE) sectors perform multiple overlapping job duties.
Methods: Workers’ compensation (WC) SLE first reports of injury (FROI) data (2005–2015) were analyzed to describe injuries, identify differences in awarded WC benefits, and compare the probability of a FROI resulting in awarded benefits between Public and Private SLE. A Pearson’s chi-square test was utilized and reverse selection logistic regression was performed to estimate the odds ratio that a FROI would result in an awarded benefit for Private vs. Public SLE, while adjusting for relevant covariates. Results: Private SLE had higher FROI percentages for younger and for older workers, fall injuries, and back injuries, compared to Public SLE. The adjusted odds that a FROI resulted in an awarded benefit was 1.4 times higher for Private SLE compared to Public SLE; (95% confidence interval [CI] = 1.09,1.69). Middle-aged SLE employee adjusted odds of awarded benefits was 3.3 times (95% CI [1.96, 5.39]) higher compared to younger employees. Adjusted odds of awarded benefits was 3.8 times (95% CI [1.34, 10.61]) higher for gunshots and 1.7 times (95% CI [1.22, 2.39]) higher for fractures/dislocations compared to other nature of injuries. Motor vehicle injury, fall/slip, and strain related FROIs had elevated adjusted odds of awarded benefits compared to other injury causes.
Conclusions: Results highlight the importance of injury prevention education and worker safety training for Private and Public SLE sector workers on fall prevention (especially in Private SLE) and strain prevention (especially in Public SLE), as well as motor vehicle safety
Semiclassical and Quantum Black Holes and their Evaporation, de Sitter and Anti-de Sitter Regimes, Gravitational and String Phase Transitions
An effective string theory in physically relevant cosmological and black hole
space times is reviewed. Explicit computations of the quantum string entropy,
partition function and quantum string emission by black holes (Schwarzschild,
rotating, charged, asymptotically flat, de Sitter dS and AdS space times) in
the framework of effective string theory in curved backgrounds provide an
amount of new quantum gravity results as: (i) gravitational phase transitions
appear with a distinctive universal feature: a square root branch point
singularity in any space time dimensions. This is of the type of the de Vega -
Sanchez transition for the thermal self-gravitating gas of point particles.
(ii) There are no phase transitions in AdS alone. (iii) For background,
upper bounds of the Hubble constant H are found, dictated by the quantum string
phase transition.(iv) The Hawking temperature and the Hagedorn temperature are
the same concept but in different (semiclassical and quantum) gravity regimes
respectively. (v) The last stage of black hole evaporation is a microscopic
string state with a finite string critical temperature which decays as usual
quantum strings do in non-thermal pure quantum radiation (no information
loss).(vi) New lower string bounds are given for the Kerr-Newman black hole
angular momentum and charge, which are entirely different from the upper
classical bounds. (vii) Semiclassical gravity states undergo a phase transition
into quantum string states of the same system, these states are duals of each
other in the precise sense of the usual classical-quantum (wave-particle)
duality, which is universal irrespective of any symmetry or isommetry of the
space-time and of the number or the kind of space-time dimensions.Comment: review paper, no figures. to appear in Int Jour Mod Phys
Exotic Spaces in Quantum Gravity I: Euclidean Quantum Gravity in Seven Dimensions
It is well known that in four or more dimensions, there exist exotic
manifolds; manifolds that are homeomorphic but not diffeomorphic to each other.
More precisely, exotic manifolds are the same topological manifold but have
inequivalent differentiable structures. This situation is in contrast to the
uniqueness of the differentiable structure on topological manifolds in one, two
and three dimensions. As exotic manifolds are not diffeomorphic, one can argue
that quantum amplitudes for gravity formulated as functional integrals should
include a sum over not only physically distinct geometries and topologies but
also inequivalent differentiable structures. But can the inclusion of exotic
manifolds in such sums make a significant contribution to these quantum
amplitudes? This paper will demonstrate that it will. Simply connected exotic
Einstein manifolds with positive curvature exist in seven dimensions. Their
metrics are found numerically; they are shown to have volumes of the same order
of magnitude. Their contribution to the semiclassical evaluation of the
partition function for Euclidean quantum gravity in seven dimensions is
evaluated and found to be nontrivial. Consequently, inequivalent differentiable
structures should be included in the formulation of sums over histories for
quantum gravity.Comment: AmsTex, 23 pages 5 eps figures; replaced figures with ones which are
hopefully viewable in pdf forma
The Energy-Momentum Tensor in Fulling-Rindler Vacuum
The energy density in Fulling-Rindler vacuum, which is known to be negative
"everywhere" is shown to be positive and singular on the horizons in such a
fashion as to guarantee the positivity of the total energy. The mechanism of
compensation is displayed in detail.Comment: 9 pages, ULB-TH-15/9
Some Heuristic Semiclassical Derivations of the Planck Length, the Hawking Effect and the Unruh Effect
The formulae for Planck length, Hawking temperature and Unruh-Davies
temperature are derived by using only laws of classical physics together with
the Heisenberg principle. Besides, it is shown how the Hawking relation can be
deduced from the Unruh relation by means of the principle of equivalence; the
deep link between Hawking effect and Unruh effect is in this way clarified.Comment: LaTex file, 6 pages, no figure
The Heat Kernel Expansion on a Cone and Quantum Fields Near Cosmic Strings
An asymptotic expansion of the trace of the heat kernel on a cone where the
heat coefficients have a delta function behavior at the apex is obtained. It is
used to derive the renormalized effective action and total energy of a
self-interacting quantum scalar field on the cosmic string space-time. Analogy
is pointed out with quantum theory with boundaries. The surface infinities in
the effective action are shown to appear and are removed by renormalization of
the string tension. Besides, the total renormalized energy turns out to be
finite due to cancelation of the known non-integrable divergence in the energy
density of the field with a counterterm in the bare string tension.Comment: 20 pages, JINR preprint August, 1993, E2-93-291, LATEX fil
Localised and nonlocalised structures in nonlinear lattices with fermions
We discuss the quasiclassical approximation for the equations of motions of a
nonlinear chain of phonons and electrons having phonon mediated hopping.
Describing the phonons and electrons as even and odd grassmannian functions and
using the continuum limit we show that the equations of motions lead to a
Zakharov-like system for bosonic and fermionic fields. Localised and
nonlocalised solutions are discussed using the Hirota bilinear formalism.
Nonlocalised solutions turn out to appear naturally for any choice of wave
parameters. The bosonic localised solution has a fermionic dressing while the
fermionic one is an oscillatory localised field. They appear only if some
constraints on the dispersion are imposed. In this case the density of fermions
is a strongly localised travelling wave. Also it is shown that in the multiple
scales approach the emergent equation is linear. Only for the resonant case we
get a nonlinear fermionic Yajima-Oikawa system. Physical implications are
discussed.Comment: 7 pages, LaTeX, no figures. to appear in Europhysics Latter
Silicon nanoparticles and interstellar extinction
To examine a recently proposed hypothesis that silicon nanoparticles are the
source of extended red emission (ERE) in the interstellar medium, we performed
a detailed modeling of the mean Galactic extinction in the presence of silicon
nanoparticles. For this goal we used the appropriate optical constants of
nanosized Si, essentially different from those of bulk Si due to quantum
confinement. It was found that a dust mixture of silicon nanoparticles, bare
graphite grains, silicate core-organic refractory mantle grains and three-layer
silicate-water ice-organic refractory grains works well in explaining the
extinction and, in addition, results in the acceptable fractions of UV/visible
photons absorbed by silicon nanoparticles: 0.071-0.081. Since these fractions
barely agree with the fraction of UV/visible photons needed to excite the
observed ERE, we conclude that the intrinsic photon conversion efficiency of
the photoluminescence by silicon nanoparticles must be near 100%, if they are
the source of the ERE.Comment: Latex2e, uses emulateapj.sty (included), multicol.sty, epsf.sty, 6
pages, 3 figures (8 Postscript files), accepted for publication in ApJ
Letters, complete Postscript file is also available at
http://physics.technion.ac.il/~zubko/eb.html#SNP
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