5,709 research outputs found
Thin-shell wormholes: Linearization stability
The class of spherically-symmetric thin-shell wormholes provides a
particularly elegant collection of exemplars for the study of traversable
Lorentzian wormholes. In the present paper we consider linearized (spherically
symmetric) perturbations around some assumed static solution of the Einstein
field equations. This permits us to relate stability issues to the (linearized)
equation of state of the exotic matter which is located at the wormhole throat.Comment: 4 pages; ReV_TeX 3.0; one postscript figur
Sudden future singularities in FLRW cosmologies
The standard energy conditions of classical general relativity are applied to
FLRW cosmologies containing sudden future singularities. Here we show, in a
model independent way, that although such cosmologies can satisfy the null,
weak and strong energy conditions, they always fail to satisfy the dominant
energy condition. They require a divergent spacelike energy flux in all but the
comoving frame.Comment: revtex4. Added references and a definition. To appear in CQ
Energy conditions in f(R) gravity and Brans-Dicke theories
The equivalence between f(R) gravity and scalar-tensor theories is invoked to
study the null, strong, weak and dominant energy conditions in Brans-Dicke
theory. We consider the validity of the energy conditions in Brans-Dicke theory
by invoking the energy conditions derived from a generic f(R) theory. The
parameters involved are shown to be consistent with an accelerated expanding
universe.Comment: 9 pages, 1 figure, to appear in IJMP
Cosmological milestones and energy conditions
Until recently, the physically relevant singularities occurring in FRW
cosmologies had traditionally been thought to be limited to the "big bang", and
possibly a "big crunch". However, over the last few years, the zoo of
cosmological singularities considered in the literature has become considerably
more extensive, with "big rips" and "sudden singularities" added to the mix, as
well as renewed interest in non-singular cosmological events such as "bounces"
and "turnarounds". In this talk, we present an extensive catalogue of such
cosmological milestones, both at the kinematical and dynamical level. First,
using generalized power series, purely kinematical definitions of these
cosmological events are provided in terms of the behaviour of the scale factor
a(t). The notion of a "scale-factor singularity" is defined, and its relation
to curvature singularities (polynomial and differential) is explored. Second,
dynamical information is extracted by using the Friedmann equations (without
assuming even the existence of any equation of state) to place constraints on
whether or not the classical energy conditions are satisfied at the
cosmological milestones. Since the classification is extremely general, and
modulo certain technical assumptions complete, the corresponding results are to
a high degree model-independent.Comment: 8 pages, 1 table, conference proceedings for NEB XII conference in
Nafplio, Greec
Truly naked spherically-symmetric and distorted black holes
We demonstrate the existence of spherically-symmetric truly naked black holes
(TNBH) for which the Kretschmann scalar is finite on the horizon but some
curvature components including those responsible for tidal forces as well as
the energy density measured by a free-falling observer are
infinite. We choose a rather generic power-like asymptotics for the metric
functions and analyze possible types of a horizon depending on the behavior of
curvature components in the free-falling frame. It is also shown in a general
case of distorted black holes that and tidal forces are either
both finite or both infinite. The general approach developed in the article
includes previously found examples and, in particular, TNBHs with an infinite
area of a horizon. The fact that the detection of singularity depends on a
frame may be relevant for a more accurate definition of the cosmic censorship
conjecture. TNBHs may be considered as a new example of so-called non-scalar
singularities for which the scalar curvature invariants are finite but some
components of the Riemann tensor may diverge in certain frames.Comment: 18 pages. 2 last sentences on quantum backreaction modified. Typos
corrected. To appear in PR
From wormhole to time machine: Comments on Hawking's Chronology Protection Conjecture
The recent interest in ``time machines'' has been largely fueled by the
apparent ease with which such systems may be formed in general relativity,
given relatively benign initial conditions such as the existence of traversable
wormholes or of infinite cosmic strings. This rather disturbing state of
affairs has led Hawking to formulate his Chronology Protection Conjecture,
whereby the formation of ``time machines'' is forbidden. This paper will use
several simple examples to argue that the universe appears to exhibit a
``defense in depth'' strategy in this regard. For appropriate parameter regimes
Casimir effects, wormhole disruption effects, and gravitational back reaction
effects all contribute to the fight against time travel. Particular attention
is paid to the role of the quantum gravity cutoff. For the class of model
problems considered it is shown that the gravitational back reaction becomes
large before the Planck scale quantum gravity cutoff is reached, thus
supporting Hawking's conjecture.Comment: 43 pages,ReV_TeX,major revision
Wormholes in String Theory
A wormhole is constructed by cutting and joining two spacetimes satisfying
the low energy string equations with a dilaton field. In spacetimes described
by the "string metric" the dilaton energy-momentum tensor need not satisfy the
weak or dominant energy conditions. In the cases considered here the dilaton
field violates these energy conditions and is the source of the exotic matter
required to maintain the wormhole. There is also a surface stress-energy, that
must be produced by additional matter, where the spacetimes are joined. It is
shown that wormholes can be constructed for which this additional matter
satisfies the weak and dominant energy conditions, so that it could be a form
of "normal" matter. Charged dilaton wormholes with a coupling between the
dilaton and the electromagnetic field that is more general than in string
theory are also briefly discussed.Comment: 9 pages, LaTex, submitted to Phys. Rev.
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
String Supported Wormhole Spacetimes and Causality Violations
We construct a static axisymmetric wormhole from the gravitational field of
two Schwarzschild particles which are kept in equilibrium by strings (ropes)
extending to infinity. The wormhole is obtained by matching two
three-dimensional timelike surfaces surrounding each of the particles and thus
spacetime becomes non-simply connected. Although the matching will not be exact
in general it is possible to make the error arbitrarily small by assuming that
the distance between the particles is much larger than the radius of the
wormhole mouths. Whenever the masses of the two wormhole mouths are different,
causality violating effects will occur.Comment: 12 pages, LaTeX, 1 figur
Gravitational vacuum polarization IV: Energy conditions in the Unruh vacuum
Building on a series of earlier papers [gr-qc/9604007, gr-qc/9604008,
gr-qc/9604009], I investigate the various point-wise and averaged energy
conditions in the Unruh vacuum. I consider the quantum stress-energy tensor
corresponding to a conformally coupled massless scalar field, work in the
test-field limit, restrict attention to the Schwarzschild geometry, and invoke
a mixture of analytical and numerical techniques. I construct a semi-analytic
model for the stress-energy tensor that globally reproduces all known numerical
results to within 0.8%, and satisfies all known analytic features of the
stress-energy tensor. I show that in the Unruh vacuum (1) all standard
point-wise energy conditions are violated throughout the exterior region--all
the way from spatial infinity down to the event horizon, and (2) the averaged
null energy condition is violated on all outgoing radial null geodesics. In a
pair of appendices I indicate general strategy for constructing semi-analytic
models for the stress-energy tensor in the Hartle-Hawking and Boulware states,
and show that the Page approximation is in a certain sense the minimal ansatz
compatible with general properties of the stress-energy in the Hartle-Hawking
state.Comment: 40 pages; plain LaTeX; uses epsf.sty (ten encapsulated postscript
figures); two tables (table and tabular environments). Should successfully
compile under both LaTeX 209 and the 209 compatibility mode of LaTeX2
- …