138 research outputs found
Higher-Dimensional Bulk Wormholes and their Manifestations in Brane Worlds
There is nothing to prevent a higher-dimensional anti-de Sitter bulk
spacetime from containing various other branes in addition to hosting our
universe, presumed to be a positive-tension 3-brane. In particular, it could
contain closed, microscopic branes that form the boundary surfaces of void
bubbles and thus violate the null energy condition in the bulk. The possible
existence of such micro branes can be investigated by considering the
properties of the ground state of a pseudo-Wheeler-DeWitt equation describing
brane quantum dynamics in minisuperspace. If they exist, a concentration of
these micro branes could act as a fluid of exotic matter able to support
macroscopic wormholes connecting otherwise distant regions of the bulk. Were
the brane constituting our universe to expand into a region of the bulk
containing such higher-dimensional macroscopic wormholes, they would likely
manifest themselves in our brane as wormholes of normal dimensionality, whose
spontaneous appearance and general dynamics would seem inexplicably peculiar.
This encounter could also result in the formation of baby universes of a
particular type.Comment: 21 pages, 1 figur
Lovelock Thin-Shell Wormholes
We construct the asymptotically flat charged thin-shell wormholes of Lovelock
gravity in seven dimensions by cut-and-paste technique, and apply the
generalized junction conditions in order to calculate the energy-momentum
tensor of these wormholes on the shell. We find that for negative second order
and positive third order Lovelock coefficients, there are thin-shell wormholes
that respect the weak energy condition. In this case, the amount of normal
matter decreases as the third order Lovelock coefficient increases. For
positive second and third order Lovelock coefficients, the weak energy
condition is violated and the amount of exotic matter decreases as the charge
increases. Finally, we perform a linear stability analysis against a symmetry
preserving perturbation, and find that the wormholes are stable provided the
derivative of surface pressure density with respect to surface energy density
is negative and the throat radius is chosen suitable.Comment: 13 pages, 6 figure
Gravitational dynamics in s+1+1 dimensions II. Hamiltonian theory
We develop a Hamiltonian formalism of brane-world gravity, which singles out
two preferred, mutually orthogonal directions. One is a unit twist-free field
of spatial vectors with integral lines intersecting perpendicularly the brane.
The other is a temporal vector field with respect to which we perform the
Arnowitt-Deser-Misner decomposition of the Einstein-Hilbert Lagrangian. The
gravitational variables arise from the projections of the spatial metric and
their canonically conjugated momenta as tensorial, vectorial and scalar
quantities defined on the family of hypersurfaces containing the brane. They
represent the gravitons, a gravi-photon and a gravi-scalar, respectively. From
the action we derive the canonical evolution equations and the constraints for
these gravitational degrees of freedom both on the brane and outside it. By
integrating across the brane, the dynamics also generates the tensorial and
scalar projection of the Lanczos equation. The vectorial projection of the
Lanczos equation arises in a similar way from the diffeomorphism constraint.
Both the graviton and the gravi-scalar are continuous across the brane, however
the momentum of the gravi-vector has a jump, related to the energy transport
(heat flow) on the brane.Comment: 13 page
The Post-Quasistatic Approximation as a test bed for Numerical Relativity
It is shown that observers in the standard ADM 3+1 treatment of matter are
the same as the observers used in the matter treatment of Bondi: they are
comoving and local Minkowskian. Bondi's observers are the basis of the
post--quasitatic approximation (PQSA) to study a contracting distribution of
matter. This correspondence suggests the possibility of using the PQSA as a
test bed for Numerical Relativity. The treatment of matter by the PQSA and its
connection with the ADM 3+1 treatment are presented, for its practical use as a
calibration tool and as a test bed for numerical relativistic hydrodynamic
codes.Comment: 4 pages; to appear as a Brief Report in Physical Review
Inverse Ising inference using all the data
We show that a method based on logistic regression, using all the data,
solves the inverse Ising problem far better than mean-field calculations
relying only on sample pairwise correlation functions, while still
computationally feasible for hundreds of nodes. The largest improvement in
reconstruction occurs for strong interactions. Using two examples, a diluted
Sherrington-Kirkpatrick model and a two-dimensional lattice, we also show that
interaction topologies can be recovered from few samples with good accuracy and
that the use of -regularization is beneficial in this process, pushing
inference abilities further into low-temperature regimes.Comment: 5 pages, 2 figures. Accepted versio
Dynamical Casimir effect for gravitons in bouncing braneworlds
We consider a two-brane system in a five-dimensional anti-de Sitter
spacetime. We study particle creation due to the motion of the physical brane
which first approaches the second static brane (contraction) and then recedes
from it(expansion). The spectrum and the energy density of the generated
gravitons are calculated. We show that the massless gravitons have a blue
spectrum and that their energy density satisfies the nucleosynthesis bound with
very mild constraints on the parameters. We also show that the Kaluza-Klein
modes cannot provide the dark matter in an anti-de-Sitter braneworld. However,
for natural choices of parameters, backreaction from the Kaluza-Klein gravitons
may well become important. The main findings of this work have been published
in the form of a Letter [R. Durrer and M. Ruser, Phys. Rev. Lett. 99, 071601
(2007), arXiv:0704.0756].Comment: 40 pages, 34 figures, improved and extended version, matches
published versio
Effects of Electromagnetic Field on the Dynamical Instability of Cylindrical Collapse
The objective of this paper is to discuss the dynamical instability in the
context of Newtonian and post Newtonian regimes. For this purpose, we consider
non-viscous heat conducting charged isotropic fluid as a collapsing matter with
cylindrical symmetry. Darmois junction conditions are formulated. The
perturbation scheme is applied to investigate the influence of dissipation and
electromagnetic field on the dynamical instability. We conclude that the
adiabatic index has smaller value for such a fluid in cylindrically
symmetric than isotropic sphere
Physical causes of energy-density inhomogenization and stability of energy-density homogeneity in relativistic self--gravitating fluids
We identify the factors responsible for the appearance of energy-density
inhomogeneities in a self-gravitating fluid, and describe the evolution of
those factors from an initially homogeneous distribution. It is shown that a
specific combination of the Weyl tensor and/or local anisotropy of pressure
and/or dissipative fluxes entails the formation of energy-density
inhomogeneities. Different cases are analyzed in detail and in the particular
case of dissipative fluids, the role of relaxational processes as well as
non-local effects are brought out.Comment: 9 pages Revtex. Some changes in the Introduction to fit the version
published in Int. J. Mod. Phys. D. Some typos correcte
Shells around black holes: the effect of freely specifiable quantities in Einstein's constraint equations
We solve Einstein's constraint equations in the conformal thin-sandwich
decomposition to model thin shells of non-interacting particles in circular
orbit about a non-rotating black hole. We use these simple models to explore
the effects of some of the freely specifiable quantities in this decomposition
on the physical content of the solutions. Specifically, we adopt either maximal
slicing or Kerr-Schild slicing, and make different choices for the value of the
lapse on the black hole horizon. For one particular choice of these quantities
the resulting equations can be solved analytically; for all others we construct
numerical solutions. We find that these different choices have no effect on our
solutions when they are expressed in terms of gauge-invariant quantities.Comment: 9 pages, 4 figure
Gravitational collapse of homogeneous scalar fields
Conditions under which gravity coupled to self interacting scalar field
determines singularity formation are found and discussed. It is shown that,
under a suitable matching with an external space, the boundary, if collapses
completely, may give rise to a naked singularity. Issues related to the
strength of the singularity are discussed.Comment: LaTeX2e; revised versio
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