5,705 research outputs found
Wormhole cosmic strings
We construct regular multi-wormhole solutions to a gravitating model
in three space-time dimensions, and extend these solutions to cylindrical
traversable wormholes in four and five dimensions. We then discuss the
possibility of identifying wormhole mouths in pairs to give rise to Wheeler
wormholes. Such an identification is consistent with the original field
equations only in the absence of the -model source, but with possible
naked cosmic string sources. The resulting Wheeler wormhole space-times are
flat outside the sources and may be asymptotically Minkowskian.Comment: 17 pages, LaTeX, 4 figures (hard copy available on request
Dynamic and instability of submarine avalanches
We perform a laboratory-scale experiment of submarine avalanches on a rough
inclined plane. A sediment layer is prepared and thereafter tilted up to an
angle lower than the spontaneous avalanche angle. The sediment is scrapped
until an avalanche is triggered. Based on the stability diagram of the sediment
layer, we investigate different structures for the avalanche front dynamics.
First we see a straight front descending the slope, and then a transverse
instability occurs. Eventually, a fingering instability shows up similar to
rivulets appearing for a viscous fluid flowing down an incline. The mechanisms
leading to this new instability and the wavelength selection are discussed.Comment: 4 pages, 6 figures, to appear in the proceedings of Powders and
Grains 200
Unsupervised Monocular Depth Estimation with Left-Right Consistency
Learning based methods have shown very promising results for the task of
depth estimation in single images. However, most existing approaches treat
depth prediction as a supervised regression problem and as a result, require
vast quantities of corresponding ground truth depth data for training. Just
recording quality depth data in a range of environments is a challenging
problem. In this paper, we innovate beyond existing approaches, replacing the
use of explicit depth data during training with easier-to-obtain binocular
stereo footage.
We propose a novel training objective that enables our convolutional neural
network to learn to perform single image depth estimation, despite the absence
of ground truth depth data. Exploiting epipolar geometry constraints, we
generate disparity images by training our network with an image reconstruction
loss. We show that solving for image reconstruction alone results in poor
quality depth images. To overcome this problem, we propose a novel training
loss that enforces consistency between the disparities produced relative to
both the left and right images, leading to improved performance and robustness
compared to existing approaches. Our method produces state of the art results
for monocular depth estimation on the KITTI driving dataset, even outperforming
supervised methods that have been trained with ground truth depth.Comment: CVPR 2017 ora
Black branes on the linear dilaton background
We show that the complete static black p-brane supergravity solution with a
single charge contains two and only two branches with respect to behavior at
infinity in the transverse space. One branch is the standard family of
asymptotically flat black branes, and another is the family of black branes
which asymptotically approach the linear dilaton background with antisymmetric
form flux (LDB). Such configurations were previously obtained in the
near-horizon near-extreme limit of the dilatonic asymptotically flat p-branes,
and used to describe the thermal phase of field theories involved in the DW/QFT
dualities and the thermodynamics of little string theory in the case of the
NS5-brane. Here we show by direct integration of the Einstein equations that
the asymptotically LDB p-branes are indeed exact supergravity solutions, and we
prove a new uniqueness theorem for static p-brane solutions satisfying cosmic
censorship. In the non-dilatonic case, our general non-asymptotically flat
p-branes are uncharged black branes on the background supported by the form flux. We develop the general formalism of
quasilocal quantities for non-asymptotically flat supergravity solutions with
antisymmetric form fields, and show that our solutions satisfy the first law of
theormodynamics. We also suggest a constructive procedure to derive rotating
asymptotically LDB brane solutions.Comment: 16 pages, revtex4, v2 - references added, "authors" metatag correcte
Topologically massive gravito-electrodynamics: exact solutions
We construct two classes of exact solutions to the field equations of
topologically massive electrodynamics coupled to topologically massive gravity
in 2 + 1 dimensions. The self-dual stationary solutions of the first class are
horizonless, asymptotic to the extreme BTZ black-hole metric, and regular for a
suitable parameter domain. The diagonal solutions of the second class, which
exist if the two Chern-Simons coupling constants exactly balance, include
anisotropic cosmologies and static solutions with a pointlike horizon.Comment: 15 pages, LaTeX, no figure
On the Papaloizou-Pringle instability in tidal disruption events
We demonstrate that the compact, thick disc formed in a tidal disruption
event may be unstable to non-axisymmetric perturbations in the form of the
Papaloizou-Pringle instability. We show this can lead to rapid redistribution
of angular momentum that can be parameterised in terms of an effective
Shakura-Sunyaev parameter. For remnants that have initially weak
magnetic fields, this may be responsible for driving mass accretion prior to
the onset of the magneto-rotational instability. For tidal disruptions around a
M black hole, the measured accretion rate is super-Eddington
but is not sustainable over many orbits. We thus identify a method by which the
torus formed in tidal disruption event may be significantly accreted before the
magneto-rotational instability is established.Comment: 9 pages, 10 figures, accepted for publication in MNRAS. Movies of
simulations available at https://youtu.be/kBLAjY8n9vI and
https://youtu.be/F8F0tmLbX3
Gravitating Chern-Simons vortices
The construction of self-dual vortex solutions to the Chern-Simons-Higgs
model (with a suitable eighth-order potential) coupled to Einstein gravity in
(2 + 1) dimensions is reconsidered. We show that the self-duality condition may
be derived from the sole assumption . Next, we derive a family of
exact, doubly self-dual vortex solutions, which interpolate between the
symmetrical and asymmetrical vacua. The corresponding spacetimes have two
regions at spatial infinity. The eighth-order Higgs potential is positive
definite, and closed timelike curves are absent, if the gravitational constant
is chosen to be negative.Comment: 11 pages, LaTe
Black hole mass and angular momentum in 2+1 gravity
We propose a new definition for the mass and angular momentum of neutral or
electrically charged black holes in 2+1 gravity with two Killing vectors. These
finite conserved quantities, associated with the SL(2,R) invariance of the
reduced mechanical system, are shown to be identical to the quasilocal
conserved quantities for an improved gravitational action corresponding to
mixed boundary conditions. They obey a general Smarr-like formula and, in all
cases investigated, are consistent with the first law of black hole
thermodynamics. Our framework is applied to the computation of the mass and
angular momentum of black hole solutions to several field-theoretical models.Comment: 23 pages, 3 references added, to be published in Physical Review
Multi-Black-Holes in Three Dimensions
We construct time-dependent multi-centre solutions to three-dimensional
general relativity with zero or negative cosmological constant. These solutions
correspond to dynamical systems of freely falling black holes and conical
singularities, with a multiply connected spacetime topology. Stationary
multi-black-hole solutions are possible only in the extreme black hole case.Comment: 8 pages, \LaTex, 4 figures (available on request), GCR 94/02/0
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