9,879 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
Bound Modes in Dielectric Microcavities
We demonstrate how exactly bound cavity modes can be realized in dielectric
structures other than 3d photonic crystals. For a microcavity consisting of
crossed anisotropic layers, we derive the cavity resonance frequencies, and
spontaneous emission rates. For a dielectric structure with dissipative loss
and central layer with gain, the beta factor of direct spontaneous emission
into a cavity mode and the laser threshold is calculated.Comment: 5 pages, 3 figure
Static and symmetric wormholes respecting energy conditions in Einstein-Gauss-Bonnet gravity
Properties of -dimensional static wormhole solutions are
investigated in Einstein-Gauss-Bonnet gravity with or without a cosmological
constant . We assume that the spacetime has symmetries corresponding
to the isometries of an -dimensional maximally symmetric space with the
sectional curvature . It is also assumed that the metric is at
least and the -dimensional maximally symmetric subspace is
compact. Depending on the existence or absence of the general relativistic
limit , solutions are classified into general relativistic (GR)
and non-GR branches, respectively, where is the Gauss-Bonnet coupling
constant. We show that a wormhole throat respecting the dominant energy
condition coincides with a branch surface in the GR branch, otherwise the null
energy condition is violated there. In the non-GR branch, it is shown that
there is no wormhole solution for . For the matter field with
zero tangential pressure, it is also shown in the non-GR branch with
and that the dominant energy condition holds at the
wormhole throat if the radius of the throat satisfies some inequality. In the
vacuum case, a fine-tuning of the coupling constants is shown to be necessary
and the radius of a wormhole throat is fixed. Explicit wormhole solutions
respecting the energy conditions in the whole spacetime are obtained in the
vacuum and dust cases with and .Comment: 10 pages, 2 tables; v2, typos corrected, references added; v3,
interpretation of the solution for n=5 in section IV corrected; v4, a very
final version to appear in Physical Review
Cylindrically symmetric wormholes
This paper discusses traversable wormholes that differ slightly but
significantly from those of the Morris-Thorne type under the assumption of
cylindrical symmetry. The throat is a piecewise smooth cylindrical surface
resulting in a shape function that is not differentiable at some value. It is
proposed that the regular derivative be replaced by a one-sided derivative at
this value. The resulting wormhole geometry satisfies the weak energy
condition.Comment: Supplied missing figures; 15 pages AMSTe
Cylindrical thin-shell wormholes and energy conditions
We prove the impossibility of cylindrical thin-shell wormholes supported by
matter satisfying the energy conditions everywhere, under reasonable
assumptions about the asymptotic behaviour of the - in general different -
metrics at each side of the throat. In particular, we reproduce for singular
sources previous results corresponding to flat and conical asymptotics, and
extend them to a more general asymptotic behaviour. Besides, we establish
necessary conditions for the possibility of non exotic cylindrical thin-shell
wormholes.Comment: 9 pages; slightly improved version of the article accepted in Int. J.
Mod. Phys.
The quasi-classical model of the spherical configuration in general relativity
We consider the quasi-classical model of the spin-free configuration on the
basis of the self-gravitating spherical dust shell in General Relativity. For
determination of the energy spectrum of the stationary states on the basis of
quasi-classical quantization rules it is required to carry out some
regularization of the system. It is realized by an embedding of the initial
system in the extended system with rotation. Then, the stationary states of the
spherical shells are S-states of the system with the intrinsic momentum. The
quasi-classical treatment of a stability of the configuration is associated
with the Langer modification of a square of the quantum mechanical intrinsic
momentum. It gives value of critical bare mass of the shell determining
threshold of stability. For the shell with the bare mass smaller or equal to
the Planck's mass, the energy spectra of bound states are found. We obtain the
expression for tunneling probability of the shell and construct the
quasi-classical model of the pair creation and annihilation of the shells.Comment: 22 pages, sprocl.sty, 3 figure
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
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
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
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
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