9,344 research outputs found
Closed timelike curves and causality violation
The conceptual definition and understanding of time, both quantitatively and
qualitatively is of the utmost difficulty and importance. As time is
incorporated into the proper structure of the fabric of spacetime, it is
interesting to note that General Relativity is contaminated with non-trivial
geometries which generate closed timelike curves. A closed timelike curve (CTC)
allows time travel, in the sense that an observer that travels on a trajectory
in spacetime along this curve, may return to an event before his departure.
This fact apparently violates causality, therefore time travel and it's
associated paradoxes have to be treated with great caution. The paradoxes fall
into two broad groups, namely the consistency paradoxes and the causal loops. A
great variety of solutions to the Einstein field equations containing CTCs
exist and it seems that two particularly notorious features stand out.
Solutions with a tipping over of the light cones due to a rotation about a
cylindrically symmetric axis and solutions that violate the energy conditions.
All these aspects are analyzed in this review paper.Comment: 19 pages, 6 figures. Invited chapter to appear in an edited
collection 'Classical and Quantum Gravity: Theory, Analysis and Applications
Stability of phantom wormholes
It has recently been shown that traversable wormholes may be supported by
phantom energy. In this work phantom wormhole geometries are modelled by
matching an interior traversable wormhole solution, governed by the equation of
state with , to an exterior vacuum spacetime at a
finite junction interface. The stability analysis of these phantom wormholes to
linearized spherically symmetric perturbations about static equilibrium
solutions is carried out. A master equation dictating the stability regions is
deduced, and by separating the cases of a positive and a negative surface
energy density, it is found that the respective stable equilibrium
configurations may be increased by strategically varying the wormhole throat
radius. The first model considered, in the absence of a thin shell, is that of
an asymptotically flat phantom wormhole spacetime. The second model constructed
is that of an isotropic pressure phantom wormhole, which is of particular
interest, as the notion of phantom energy is that of a spatially homogeneous
cosmic fluid, although it may be extended to inhomogeneous spherically
symmetric spacetimes.Comment: 9 pages, 9 figures, Revtex4. V2: five references adde
From the Flamm-Einstein-Rosen bridge to the modern renaissance of traversable wormholes
We consider the possibility of multiply-connected spacetimes, ranging from
the Flamm-Einstein-Rosen bridge, geons, and the modern renaissance of
traversable wormholes. A fundamental property in wormhole physics is the
flaring-out condition of the throat, which through the Einstein field equation
entails the violation of the null energy condition. In the context of modified
theories of gravity, it has also been shown that the normal matter can be
imposed to satisfy the energy conditions, and it is the higher order curvature
terms, interpreted as a gravitational fluid, that sustain these non-standard
wormhole geometries, fundamentally different from their counterparts in general
relativity. We explore interesting features of these geometries, in particular,
the physical properties and characteristics of these `exotic spacetimes'.Comment: 20 pages. MG14 rapporteur article based on the AT3 parallel session.
Includes a brief review of wormhole physics and of the contributions to the
AT3 sessio
Traversable wormholes supported by cosmic accelerated expanding equations of state
We explore the possibility that traversable wormholes be supported by
specific equations of state responsible for the present accelerated expansion
of the Universe, namely, phantom energy, the generalized Chaplygin gas, and the
van der Waals quintessence equation of state.Comment: 3 pages, contribution to the proceedings of MG11, Berlin, 23-29 July,
2006; based on an invited talk in the parallel session GT5, Wormholes, Energy
Conditions and Time Machine
Stable dark energy stars
The gravastar picture is an alternative model to the concept of a black hole,
where there is an effective phase transition at or near where the event horizon
is expected to form, and the interior is replaced by a de Sitter condensate. In
this work, a generalization of the gravastar picture is explored, by
considering a matching of an interior solution governed by the dark energy
equation of state, , to an exterior Schwarzschild
vacuum solution at a junction interface. The motivation for implementing this
generalization arises from the fact that recent observations have confirmed an
accelerated cosmic expansion, for which dark energy is a possible candidate.
Several relativistic dark energy stellar configurations are analyzed by
imposing specific choices for the mass function. The first case considered is
that of a constant energy density, and the second choice, that of a monotonic
decreasing energy density in the star's interior. The dynamical stability of
the transition layer of these dark energy stars to linearized spherically
symmetric radial perturbations about static equilibrium solutions is also
explored. It is found that large stability regions exist that are sufficiently
close to where the event horizon is expected to form, so that it would be
difficult to distinguish the exterior geometry of the dark energy stars,
analyzed in this work, from an astrophysical black hole.Comment: 10 pages, 6 figures, Revtex4. V2: comments and references added, 11
pages. V3: Significant additions and clarifications, 12 page
Exotic solutions in General Relativity: Traversable wormholes and 'warp drive' spacetimes
The General Theory of Relativity has been an extremely successful theory,
with a well established experimental footing, at least for weak gravitational
fields. Its predictions range from the existence of black holes, gravitational
radiation to the cosmological models, predicting a primordial beginning, namely
the big-bang. All these solutions have been obtained by first considering a
plausible distribution of matter, and through the Einstein field equation, the
spacetime metric of the geometry is determined. However, one may solve the
Einstein field equation in the reverse direction, namely, one first considers
an interesting and exotic spacetime metric, then finds the matter source
responsible for the respective geometry. In this manner, it was found that some
of these solutions possess a peculiar property, namely 'exotic matter,'
involving a stress-energy tensor that violates the null energy condition. These
geometries also allow closed timelike curves, with the respective causality
violations. These solutions are primarily useful as 'gedanken-experiments' and
as a theoretician's probe of the foundations of general relativity, and include
traversable wormholes and superluminal 'warp drive' spacetimes. Thus, one may
be tempted to denote these geometries as 'exotic' solutions of the Einstein
field equation, as they violate the energy conditions and generate closed
timelike curves. In this article, in addition to extensively exploring
interesting features, in particular, the physical properties and
characteristics of these 'exotic spacetimes,' we also analyze other non-trivial
general relativistic geometries which generate closed timelike curves.Comment: 52 pages, 20 figures, RevTex4. Invited chapter to appear in an edited
collection 'Classical and Quantum Gravity Research Progress', Nova Science
Publisher
Traversable wormholes supported by dark gravity
A fundamental property in wormhole physics is the flaring-out condition of
the throat, which through the Einstein field equation entails the violation of
the null energy condition. In the context of modified theories of gravity, it
has also been shown that the normal matter can be imposed to satisfy the energy
conditions, and it is the higher order curvature terms, interpreted as a
gravitational fluid, that sustain these non-standard wormhole geometries,
fundamentally different from their counterparts in general relativity. We
review recent work in wormhole physics in the context of modified theories of
gravity.Comment: 3 pages; contribution to the proceedings of the Thirteenth Marcel
Grossmann Meeting, Stockholm University, Sweden, 1-7 July, 2012; based on a
talk in the AT3 "Gravitational Fields with Sources, Regular Black Holes,
Quasiblack Holes, and Analog Black Holes" parallel sessio
Stable dark energy stars: An alternative to black holes?
In this work, a generalization of the Mazur-Mottola gravastar model is
explored, by considering a matching of an interior solution governed by the
dark energy equation of state, , to an exterior
Schwarzschild vacuum solution at a junction interface, situated near to where
the event horizon is expected to form. The motivation for implementing this
generalization arises from the fact that recent observations have confirmed an
accelerated cosmic expansion, for which dark energy is a possible candidate.Comment: 3 pages, contribution to the proceedings of MG11, Berlin, 23-29 July,
2006; based on an invited talk in the parallel session BHT5, Alternative
Black Hole Model
Van der Waals quintessence stars
The van der Waals quintessence equation of state is an interesting scenario
for describing the late universe, and seems to provide a solution to the puzzle
of dark energy, without the presence of exotic fluids or modifications of the
Friedmann equations. In this work, the construction of inhomogeneous compact
spheres supported by a van der Waals equation of state is explored. These
relativistic stellar configurations shall be denoted as {\it van der Waals
quintessence stars}. Despite of the fact that, in a cosmological context, the
van der Waals fluid is considered homogeneous, inhomogeneities may arise
through gravitational instabilities. Thus, these solutions may possibly
originate from density fluctuations in the cosmological background. Two
specific classes of solutions, namely, gravastars and traversable wormholes are
analyzed. Exact solutions are found, and their respective characteristics and
physical properties are further explored.Comment: 8 pages, 1 figure, Revtex4. V2: clarifying comments and references
added, now 9 page
Nature of time and causality in Physics
The conceptual definition and understanding of the nature of time, both
qualitatively and quantitatively is of the utmost difficulty and importance,
and plays a fundamental role in physics. Physical systems seem to evolve in
paths of increasing entropy and of complexity, and thus, the arrow of time
shall be explored in the context of thermodynamic irreversibility and quantum
physics. In Newtonian physics, time flows at a constant rate, the same for all
observers; however, it necessarily flows at different rates for different
observers in special and general relativity. Special relativity provides
important quantitative elucidations of the fundamental processes related to
time dilation effects, and general relativity provides a deep analysis to
effects of time flow, such as in the presence of gravitational fields. Through
the special theory of relativity, time became intimately related with space,
giving rise to the notion of spacetime, in which both parameters cannot be
considered as separate entities. As time is incorporated into the proper
structure of the fabric of spacetime, it is interesting to note that general
relativity is contaminated with non-trivial geometries that generate closed
timelike curves, and thus apparently violates causality. The notion of
causality is fundamental in the construction of physical theories; therefore
time travel and its associated paradoxes have to be treated with great caution.
These issues are briefly analyzed in this review paper.Comment: 14 pages, 11 figures. Book chapter to appear in 'Psychology of Time',
Elsevier Publishers, editor Simon Grondin. V2: comments, one figure and a
reference added; typos correcte
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