139 research outputs found
A Smooth Lattice construction of the Oppenheimer-Snyder spacetime
We present test results for the smooth lattice method using an
Oppenheimer-Snyder spacetime. The results are in excellent agreement with
theory and numerical results from other authors.Comment: 60 pages, 28 figure
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
Vacuum Polarization of a Massless Scalar Field in the Background of a Global Monopole with Finite Core
In this paper we analyze the vacuum polarization effects of a massless scalar
field in the background of a global monopole considering a inner structure to
it. Specifically we investigate the effect of its structure on the vacuum
expectation value of the square of the field operator, ,
admitting a non-minimal coupling between the field with the geometry: $\xi
{\cal{R}}\hat{\Phi}^2$. Also we calculate the corrections on the vacuum
expectation value of the energy-momentum tensor, , due to
the inner structure of the monopole. In order to develop these analysis, we
calculate the Euclidean Green function associated with the system for points in
the region outside the core. As we shall see, for specific value of the
coupling parameter , the corrections caused by the inner structure of the
monopole can provide relevant contributions on these vacuum polarizations.Comment: Accepted for publication in Classical and Quantum Gravity. Added
references. 22 pages, 1 figur
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
Effective gravitational equations for f(R) braneworld models
The viability of achieving gravitational consistent braneworld models in the
framework of a f(R) theory of gravity is investigated. After a careful
generalization of the usual junction conditions encompassing the embedding of
the 3-brane into a f(R) bulk, we provide a prescription giving the necessary
constraints in order to implement the projected second order effective field
equations on the brane.Comment: 15 pages, no figures. Accepted for publication in the Physical Review
Magnetic Surfaces in Stationary Axisymmetric General Relativity
In this paper a new method is derived for constructing electromagnetic
surface sources for stationary axisymmetric electrovac spacetimes endowed with
non-smooth or even discontinuous
Ernst potentials. This can be viewed as a generalization of some classical
potential theory results, since lack of continuity of the potential is related
to dipole density and lack of smoothness, to monopole density. In particular
this approach is useful for constructing the dipole source for the magnetic
field. This formalism involves solving a linear elliptic differential equation
with boundary conditions at infinity. As an example, two different models of
surface densities for the Kerr-Newman electrovac spacetime are derived.Comment: 15 page
Self-gravitating branes of codimension 4 in Lovelock gravity
We construct a familly of exact solutions of Lovelock equations describing
codimension four branes with discrete symmetry in the transverse space. Unlike
what is known from pure Einstein gravity, where such brane solutions of higher
codimension are singular, the solutions we find, for the complete Lovelock
theory, only present removable singularities. The latter account for a
localised tension-like energy-momentum tensor on the brane, in analogy with the
case of a codimension two self-gravitating cosmic string in pure Einstein
gravity. However, the solutions we discuss present two main distinctive
features : the tension of the brane receives corrections from the induced
curvature of the brane's worldsheet and, in a given Lovelock theory, the
spectrum of possible values of the tension is discrete. These solutions provide
a new framework for the study of higher codimension braneworlds.Comment: 22 page
Gravitational collapse in asymptotically Anti-de Sitter/de Sitter backgrounds
We study here the gravitational collapse of a matter cloud with a
non-vanishing tangential pressure in the presence of a non-zero cosmological
term. Conditions for bounce and singularity formation are derived for the
model. It is also shown that when the tangential pressures vanish, the bounce
and singularity conditions reduce to that of the dust case studied earlier. The
collapsing interior is matched with an exterior which is asymptotically de
Sitter or anti de Sitter, depending on the sign of cosmological constant. The
junction conditions for matching the cloud to exterior are specified. The
effect of the cosmological term on apparent horizons is studied in some detail,
and the nature of central singularity is analyzed. We also discuss here the
visibility of the singularity and implications for the cosmic censorship
conjecture.Comment: 11 pages, 1 figure, Revtex
On the entropy production of time series with unidirectional linearity
There are non-Gaussian time series that admit a causal linear autoregressive
moving average (ARMA) model when regressing the future on the past, but not
when regressing the past on the future. The reason is that, in the latter case,
the regression residuals are only uncorrelated but not statistically
independent of the future. In previous work, we have experimentally verified
that many empirical time series indeed show such a time inversion asymmetry.
For various physical systems, it is known that time-inversion asymmetries are
linked to the thermodynamic entropy production in non-equilibrium states. Here
we show that such a link also exists for the above unidirectional linearity.
We study the dynamical evolution of a physical toy system with linear
coupling to an infinite environment and show that the linearity of the dynamics
is inherited to the forward-time conditional probabilities, but not to the
backward-time conditionals. The reason for this asymmetry between past and
future is that the environment permanently provides particles that are in a
product state before they interact with the system, but show statistical
dependencies afterwards. From a coarse-grained perspective, the interaction
thus generates entropy. We quantitatively relate the strength of the
non-linearity of the backward conditionals to the minimal amount of entropy
generation.Comment: 16 page
Rotating perfect fluid sources of the NUT metric
Locally rotationally symmetric perfect fluid solutions of Einstein's
gravitational equations are matched along the hypersurface of vanishing
pressure with the NUT metric. These rigidly rotating fluids are interpreted as
sources for the vacuum exterior which consists only of a stationary region of
the Taub-NUT space-time. The solution of the matching conditions leaves
generally three parameters in the global solution. Examples of perfect fluid
sources are discussed.Comment: 8 pages, late
- …