1,528 research outputs found
Inertial effects of an accelerating black hole
We consider the static vacuum C metric that represents the gravitational
field of a black hole of mass undergoing uniform translational acceleration
such that . The influence of the inertial acceleration on
the exterior perturbations of this background are investigated. In particular,
we find no evidence for a direct spin-acceleration coupling.Comment: Proceedings of the XVI Conference of the Italian Society for General
Relativity and Gravitation (SIGRAV), Vietri (SA), September 13-16, 2004.
Prepared using revtex4 macro
Electrocardiogram of the Mixmaster Universe
The Mixmaster dynamics is revisited in a new light as revealing a series of
transitions in the complex scale invariant scalar invariant of the Weyl
curvature tensor best represented by the speciality index , which
gives a 4-dimensional measure of the evolution of the spacetime independent of
all the 3-dimensional gauge-dependent variables except for the time used to
parametrize it. Its graph versus time characterized by correlated isolated
pulses in its real and imaginary parts corresponding to curvature wall
collisions serves as a sort of electrocardiogram of the Mixmaster universe,
with each such pulse pair arising from a single circuit or ``complex pulse''
around the origin in the complex plane. These pulses in the speciality index
and their limiting points on the real axis seem to invariantly characterize
some of the so called spike solutions in inhomogeneous cosmology and should
play an important role as a gauge invariant lens through which to view current
investigations of inhomogeneous Mixmaster dynamics.Comment: version 3: 20 pages iopart style, 19 eps figure files for 8 latex
figures; added example of a transient true spike to contrast with the
permanent true spike example from the Lim family of true spike solutions;
remarks in introduction and conclusion adjusted and toned down; minor
adjustments to the remaining tex
Superradiance from BEC vortices: a numerical study
The scattering of sound wave perturbations from vortex excitations of
Bose-Einstein condensates(BEC) is investigated by numerical integration of the
associated Klein-Gordon equation. It is found that, at sufficiently high
angular speeds, sound wave-packets can extract a sizeable fraction of the
vortex energy through a mechanism of superradiant scattering. It is conjectured
that this superradiant regime may be detectable in BEC experiments.Comment: 4 pages, 4 figure
Massless field perturbations and gravitomagnetism in the Kerr-Taub-NUT spacetime
A single master equation is given describing spin test fields that are gauge- and tetrad-invariant perturbations of the Kerr-Taub-NUT spacetime representing a source with mass , gravitomagnetic monopole moment and gravitomagnetic dipole moment (angular momentum) per unit mass . This equation can be separated into its radial and angular parts. The behavior of the radial functions at infinity and near the horizon is studied and used to examine the influence of on the phenomenon of superradiance, while the angular equation leads to spin-weighted spheroidal harmonic solutions generalizing those of the Kerr spacetime. Finally the coupling between the spin of the perturbing field and the gravitomagnetic monopole moment is discussed
Time-Varying Gravitomagnetism
Time-varying gravitomagnetic fields are considered within the linear
post-Newtonian approach to general relativity. A simple model is developed in
which the gravitomagnetic field of a localized mass-energy current varies
linearly with time. The implications of this temporal variation of the source
for the precession of test gyroscopes and the motion of null rays are briefly
discussed.Comment: 10 pages; v2: slightly expanded version accepted for publication in
Class. Quantum Gra
Magnetic resonance findings and outcome in ten cats with traumatic spondylomyelopathy
In this retrospective study, the MR findings of ten cats with acute post-traumatic spondylomyelopathy were described and the most useful MR sequences were determined. Spinal cord injury (SCI), bone and muscle trauma were compared with the clinical outcome (recovery or euthanasia). The extension of spinal cord injury (SCI) was measured in vertebral body length (VBL). Of the ten cats, only five fully recovered. In the recovery group, no SCI (n=1) or SCI <1 VBL (n=4) were found. In the group of euthanized dogs, SCI > 2 VBLs (n=4) or spinal cord transection (n=1) were found. Lesions were best seen on T2WSE (spinal cord injury), STIR (soft tissue trauma) and T1WSE (bone injury). Low-field MR was therefore helpful to assess feline spinal trauma and may prove helpful to predict the clinical outcome, although a larger case series is needed. The authors suggest that protocols with low-field MR should include T1WSE, T2WSE and STIR sequences
Neutrino current in a gravitational plane wave collision background
The behaviour of a massless Dirac field on a general spacetime background
representing two colliding gravitational plane waves is discussed in the
Newman-Penrose formalism. The geometrical properties of the neutrino current
are analysed and explicit results are given for the special Ferrari-Ibanez
solution.Comment: 17 pages, 6 Postscript figures, accepted by International Journal of
Modern Physics
The Speciality Index as invariant indicator in the BKL Mixmaster Dynamics
The speciality index, which has been mainly used in Numerical Relativity for
studying gravitational waves phenomena as an indicator of the special or
non-special Petrov type character of a spacetime, is applied here in the
context of Mixmaster cosmology, using the Belinski-Khalatnikov-Lifshitz map.
Possible applications for the associated chaotic dynamics are discussed
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