3,419 research outputs found
Tolman wormholes violate the strong energy condition
For an arbitrary Tolman wormhole, unconstrained by symmetry, we shall define
the bounce in terms of a three-dimensional edgeless achronal spacelike
hypersurface of minimal volume. (Zero trace for the extrinsic curvature plus a
"flare-out" condition.) This enables us to severely constrain the geometry of
spacetime at and near the bounce and to derive general theorems regarding
violations of the energy conditions--theorems that do not involve geodesic
averaging but nevertheless apply to situations much more general than the
highly symmetric FRW-based subclass of Tolman wormholes. [For example: even
under the mildest of hypotheses, the strong energy condition (SEC) must be
violated.] Alternatively, one can dispense with the minimal volume condition
and define a generic bounce entirely in terms of the motion of test particles
(future-pointing timelike geodesics), by looking at the expansion of their
timelike geodesic congruences. One re-confirms that the SEC must be violated at
or near the bounce. In contrast, it is easy to arrange for all the other
standard energy conditions to be satisfied.Comment: 8 pages, ReV-TeX 3.
Geometric structure of the generic static traversable wormhole throat
Traversable wormholes have traditionally been viewed as intrinsically
topological entities in some multiply connected spacetime. Here, we show that
topology is too limited a tool to accurately characterize a generic traversable
wormhole: in general one needs geometric information to detect the presence of
a wormhole, or more precisely to locate the wormhole throat. For an arbitrary
static spacetime we shall define the wormhole throat in terms of a
2-dimensional constant-time hypersurface of minimal area. (Zero trace for the
extrinsic curvature plus a "flare-out" condition.) This enables us to severely
constrain the geometry of spacetime at the wormhole throat and to derive
generalized theorems regarding violations of the energy conditions-theorems
that do not involve geodesic averaging but nevertheless apply to situations
much more general than the spherically symmetric Morris-Thorne traversable
wormhole. [For example: the null energy condition (NEC), when suitably weighted
and integrated over the wormhole throat, must be violated.] The major technical
limitation of the current approach is that we work in a static spacetime-this
is already a quite rich and complicated system.Comment: 25 pages; plain LaTeX; uses epsf.sty (four encapsulated postscript
figures
Wormhole Cosmology and the Horizon Problem
We construct an explicit class of dynamic lorentzian wormholes connecting
Friedmann-Robertson-Walker (FRW) spacetimes. These wormholes can allow two-way
transmission of signals between spatially separated regions of spacetime and
could permit such regions to come into thermal contact. The cosmology of a
network of early Universe wormholes is discussed.Comment: 13 pages, in RevTe
Gauge Field Back-reaction on a Black Hole
The order fluctuations of gauge fields in the vicinity of a blackhole
can create a repulsive antigravity region extending out beyond the renormalized
Schwarzschild horizon. If the strength of this repulsive force increases as
higher orders in the back-reaction are included, the formation of a
wormhole-like object could occur.Comment: 17 pages, three figures available on request, in RevTe
On a class of stable, traversable Lorentzian wormholes in classical general relativity
It is known that Lorentzian wormholes must be threaded by matter that
violates the null energy condition. We phenomenologically characterize such
exotic matter by a general class of microscopic scalar field Lagrangians and
formulate the necessary conditions that the existence of Lorentzian wormholes
imposes on them. Under rather general assumptions, these conditions turn out to
be strongly restrictive. The most simple Lagrangian that satisfies all of them
describes a minimally coupled massless scalar field with a reversed sign
kinetic term. Exact, non-singular, spherically symmetric solutions of
Einstein's equations sourced by such a field indeed describe traversable
wormhole geometries. These wormholes are characterized by two parameters: their
mass and charge. Among them, the zero mass ones are particularly simple,
allowing us to analytically prove their stability under arbitrary space-time
dependent perturbations. We extend our arguments to non-zero mass solutions and
conclude that at least a non-zero measure set of these solutions is stable.Comment: 23 pages, 4 figures, uses RevTeX4. v2: Changes to accommodate added
references. Statement about masses of the wormhole correcte
Effective Potential of a Black Hole in Thermal Equilibrium with Quantum Fields
Expectation values of one-loop renormalized thermal equilibrium stress-energy
tensors of free conformal scalars, spin- fermions and U(1) gauge
fields on a Schwarzschild black hole background are used as sources in the
semi-classical Einstein equation. The back-reaction and new equilibrium metric
are solved for at for each spin field. The nature of the modified
black hole spacetime is revealed through calculations of the effective
potential for null and timelike orbits. Significant novel features affecting
the motions of both massive and massless test particles show up at lowest order
in , where is the renormalized black hole mass,
and is the Planck mass. Specifically, we find the tendency for
\underline{stable} circular photon orbits, an increase in the black hole
capture cross sections, and the existence of a gravitationally repulsive region
associated with the black hole which is generated from the U(1) back-reaction.
We also consider the back-reaction arising from multiple fields, which will be
useful for treating a black hole in thermal equilibrium with field ensembles
belonging to gauge theories.Comment: 25 pages (not including seven figures), VAND-TH-93-6. Typed in Latex,
uses RevTex macro
Dilatonic wormholes: construction, operation, maintenance and collapse to black holes
The CGHS two-dimensional dilaton gravity model is generalized to include a
ghost Klein-Gordon field, i.e. with negative gravitational coupling. This
exotic radiation supports the existence of static traversible wormhole
solutions, analogous to Morris-Thorne wormholes. Since the field equations are
explicitly integrable, concrete examples can be given of various dynamic
wormhole processes, as follows. (i) Static wormholes are constructed by
irradiating an initially static black hole with the ghost field. (ii) The
operation of a wormhole to transport matter or radiation between the two
universes is described, including the back-reaction on the wormhole, which is
found to exhibit a type of neutral stability. (iii) It is shown how to maintain
an operating wormhole in a static state, or return it to its original state, by
turning up the ghost field. (iv) If the ghost field is turned off, either
instantaneously or gradually, the wormhole collapses into a black hole.Comment: 9 pages, 7 figure
Renormalization Group Analysis of a Quivering String Model of Posture Control
Scaling concepts and renormalization group (RG) methods are applied to a
simple linear model of human posture control consisting of a trembling or
quivering string subject to damping and restoring forces. The string is driven
by uncorrelated white Gaussian noise intended to model the corrections of the
physiological control system. We find that adding a weak quadratic nonlinearity
to the posture control model opens up a rich and complicated phase space
(representing the dynamics) with various non-trivial fixed points and basins of
attraction. The transition from diffusive to saturated regimes of the linear
model is understood as a crossover phenomenon, and the robustness of the linear
model with respect to weak non-linearities is confirmed. Correlations in
posture fluctuations are obtained in both the time and space domain. There is
an attractive fixed point identified with falling. The scaling of the
correlations in the front-back displacement, which can be measured in the
laboratory, is predicted for both the large-separation (along the string) and
long-time regimes of posture control.Comment: 20 pages, 13 figures, RevTeX, accepted for publication in PR
of the quantized fields in the Unruh state in the Schwarzschild spacetime
The renormalized expectation value of the stress energy tensor of the
conformally invariant massless fields in the Unruh state in the Schwarzschild
spacetime is constructed. It is achieved through solving the conservation
equation in conformal space and utilizing the regularity conditions in the
physical metric. The relations of obtained results to the existing
approximations are analysed.Comment: 17 pages, REVTE
Thin-shell wormholes in Einstein-Maxwell theory with a Gauss-Bonnet term
We study five dimensional thin-shell wormholes in Einstein-Maxwell theory
with a Gauss-Bonnet term. The linearized stability under radial perturbations
and the amount of exotic matter are analyzed as a function of the parameters of
the model. We find that the inclusion of the quadratic correction substantially
widens the range of possible stable configurations, and besides it allows for a
reduction of the exotic matter required to construct the wormholes.Comment: 13 pages, 6 figures; v2: minor changes and new references added.
Accepted for publication in General Relativity and Gravitatio
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