28,897 research outputs found
Gravity-induced birefringence within the framework of Poincare gauge theory
Gauge theories of gravity provide an elegant and promising extension of
general relativity. In this paper we show that the Poincar\'e gauge theory
exhibits gravity-induced birefringence under the assumption of a specific gauge
invariant nonminimal coupling between torsion and Maxwell's field. Furthermore
we give for the first time an explicit expression for the induced phaseshift
between two orthogonal polarization modes within the Poincar\'e framework.
Since such a phaseshift can lead to a depolarization of light emitted from an
extended source this effect is, in principle, observable. We use white dwarf
polarimetric data to constrain the essential coupling constant responsible for
this effect.Comment: 12 pages, accepted for publication by Physical Review
Transverse oscillations of two coronal loops
We study transverse fast magnetohydrodynamic waves in a system of two coronal
loops modeled as smoothed, dense plasma cylinders in a uniform magnetic field.
The collective oscillatory properties of the system due to the interaction
between the individual loops are investigated from two points of view. Firstly,
the frequency and spatial structure of the normal modes are studied. The system
supports four trapped normal modes in which the loops move rigidly in the
transverse direction. The direction of the motions is either parallel or
perpendicular to the plane containing the axes of the loops. Two of these modes
correspond to oscillations of the loops in phase, while in the other two they
move in antiphase. Thus, these solutions are the generalization of the kink
mode of a single cylinder to the double cylinder case. Secondly, we analyze the
time-dependent problem of the excitation of the pair of tubes. We find that
depending on the shape and location of the initial disturbance, different
normal modes can be excited. The frequencies of normal modes are accurately
recovered from the numerical simulations. In some cases, because of the
simultaneous excitation of several eigenmodes, the system shows beating and the
phase lag between the loops is .Comment: Accepted for publication in The Astrophysical Journa
'It's a Form of Freedom': The experiences of people with disabilities within equestrian sport
This paper explores the embodied, gendered experiences of disabled horseâriders. Drawing on data from five inâdepth interviews with paradressage riders, the ways in which their involvement in elite disability sport impacts upon their sense of identity and confidence are explored, as well as the considerable health and social benefits that this involvement brings. Social models of disability are employed and the shortcomings of such models, when applied to disability sport, are highlighted. The data presented here demonstrates the necessity of seeing disability sport as an embodied experience and acknowledging the importance of impairment to the experiences of disabled athletes. Living within an impaired body is also a gendered experience and the implications of this when applied to elite disability sport are considered
The Polyakov Loop and its Relation to Static Quark Potentials and Free Energies
It appears well accepted in the literature that the correlator of Polyakov
loops in a finite temperature system decays with the "average" free energy of
the static quark-antiquark system, and can be decomposed into singlet and
adjoint (or octet for QCD) contributions. By fixing a gauge respecting the
transfer matrix, attempts have been made to extract those contributions
separately. In this paper we point out that the "average" and "adjoint"
channels of Polyakov loop correlators are misconceptions. We show analytically
that all channels receive contributions from singlet states only, and give a
corrected definition of the singlet free energy. We verify this finding by
simulations of the 3d SU(2) pure gauge theory in the zero temperature limit,
which allows to cleanly extract the ground state exponents and the non-trivial
matrix elements. The latter account for the difference between the channels
observed in previous simulations.Comment: 14 pages, 3 figures, 1 table; note and reference adde
Cannabinoid Receptor Involvement in Stress-Induced Cocaine Reinstatement: Potential Interaction with Noradrenergic Pathways
This study examined the role of endocannabinoid signaling in stress-induced reinstatement of cocaine seeking and explored the interaction between noradrenergic and endocannabinergic systems in the process. A well-validated preclinical model for human relapse, the rodent conditioned place preference assay, was used. Cocaine-induced place preference was established in C57BL/6 mice using injections of 15 mg/kg cocaine. Following extinction of preference for the cocaine-paired environment, reinstatement of place preference was determined following 6 min of swim stress or cocaine injection (15 mg/kg, i.p.). The role of endocannabinoid signaling was studied using the cannabinoid antagonist AM-251 (3 mg/kg, i.p.). Another cohort of mice was tested for reinstatement following administration of the cannabinoid agonist CP 55,940 (10, 20, or 40 ÎŒg/kg, i.p.). The alpha-2 adrenergic antagonist BRL-44408 (5 mg/kg, i.p.) with or without CP 55,940 (20 ÎŒg/kg) was administered to a third group of mice. We found that: (1) AM-251 blocked forced swim-induced, but not cocaine-induced, reinstatement of cocaine-seeking behavior; (2) the cannabinoid agonist CP 55,940 did not reinstate cocaine-seeking behavior when administered alone but did synergize with a non-reinstating dose of the alpha-2 adrenergic antagonist BRL-44408 to cause reinstatement. These results are consistent with the hypothesis that stress exposure triggers the endogenous activation of CB1 receptors and that activation of the endocannabinoid system is required for the stress-induced relapse of the mice to cocaine seeking. Further, the data suggest that the endocannabinoid system interacts with noradrenergic mechanisms to influence stress-induced reinstatement of cocaine-seeking behavior
Expanding Semiflows on Branched Surfaces and One-Parameter Semigroups of Operators
We consider expanding semiflows on branched surfaces. The family of transfer
operators associated to the semiflow is a one-parameter semigroup of operators.
The transfer operators may also be viewed as an operator-valued function of
time and so, in the appropriate norm, we may consider the vector-valued Laplace
transform of this function. We obtain a spectral result on these operators and
relate this to the spectrum of the generator of this semigroup. Issues of
strong continuity of the semigroup are avoided. The main result is the
improvement to the machinery associated with studying semiflows as
one-parameter semigroups of operators and the study of the smoothness
properties of semiflows defined on branched manifolds, without encoding as a
suspension semiflow
The resonant damping of fast magnetohydrodynamic oscillations in a system of two coronal slabs
Observations of transversal coronal loop oscillations very often show the
excitation and damping of oscillations in groups of coronal loops rather than
in individual and isolated structures. We present results on the oscillatory
properties (periods, damping rates, and spatial distribution of perturbations)
for resonantly damped oscillations in a system of two inhomogeneous coronal
slabs and compare them to the properties found in single slab loop models. A
system of two identical coronal loops is modeled, in Cartesian geometry, as
being composed by two density enhancements. The linear magnetohydrodynamic
(MHD) wave equations for oblique propagation of waves are solved and the
damping of the different solutions, due to the transversal inhomogeneity of the
density profile, is computed. The physics of the obtained results is analyzed
by an examination of the perturbed physical variables. We find that, due to the
interaction between the loops, the normal modes of oscillation present in a
single slab split into symmetric and antisymmetric oscillations when a system
of two identical slabs is considered. The frequencies of these solutions may
differ from the single slab results when the distance between the loops is of
the order of a few slab widths. Oblique propagation of waves weakens this
interaction, since solutions become more confined to the edges of the slabs.
The damping is strong for surface-like oscillations, while sausage body-like
solutions are unaffected. For some solutions, and small slab separations, the
damping in a system of two loops differs substantially from the damping of a
single loop.Comment: 25 pages, 9 figure
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