727 research outputs found
A general class of braneworld wormholes
The brane cosmology scenario is based on the idea that our Universe is a
3-brane embedded in a five-dimensional bulk. In this work, a general class of
braneworld wormholes is explored with , where is the four
dimensional Ricci scalar, and specific solutions are further analyzed. A
fundamental ingredient of traversable wormholes is the violation of the null
energy condition (NEC). However, it is the effective total stress energy tensor
that violates the latter, and in this work, the stress energy tensor confined
on the brane, threading the wormhole, is imposed to satisfy the NEC. It is also
shown that in addition to the local high-energy bulk effects, nonlocal
corrections from the Weyl curvature in the bulk may induce a NEC violating
signature on the brane. Thus, braneworld gravity seems to provide a natural
scenario for the existence of traversable wormholes.Comment: 6 pages, Revtex4. V2: comments and references added, to appear in
Phys. Rev.
Evolving wormhole geometries within nonlinear electrodynamics
In this work, we explore the possibility of evolving (2+1) and
(3+1)-dimensional wormhole spacetimes, conformally related to the respective
static geometries, within the context of nonlinear electrodynamics. For the
(3+1)-dimensional spacetime, it is found that the Einstein field equation
imposes a contracting wormhole solution and the obedience of the weak energy
condition. Nevertheless, in the presence of an electric field, the latter
presents a singularity at the throat, however, for a pure magnetic field the
solution is regular. For the (2+1)-dimensional case, it is also found that the
physical fields are singular at the throat. Thus, taking into account the
principle of finiteness, which states that a satisfactory theory should avoid
physical quantities becoming infinite, one may rule out evolving
(3+1)-dimensional wormhole solutions, in the presence of an electric field, and
the (2+1)-dimensional case coupled to nonlinear electrodynamics.Comment: 17 pages, 1 figure; to appear in Classical and Quantum Gravity. V2:
minor corrections, including a referenc
Exact solutions of f(R) gravity coupled to nonlinear electrodynamics
In this work, exact solutions of static and spherically symmetric space-times
are analyzed in f(R) modified theories of gravity coupled to nonlinear
electrodynamics. Firstly, we restrict the metric fields to one degree of
freedom, considering the specific case of g_tt\g_rr = -1. Using the dual P
formalism of nonlinear electrodynamics an exact general solution is deduced in
terms of the structural function H_P. In particular, specific exact solutions
to the gravitational field equations are found, confirming previous results and
new pure electric field solutions are found. Secondly, motivated by the
existence of regular electric fields at the center, and allowing for the case
of g_tt\g_rr \= -1, new specific solutions are found. Finally, we outline
alternative approaches by considering the specific case of constant curvature,
followed by the analysis of a specific form for f(R).Comment: 14 pages, 6 figures, revtex4; minor changes & details added,
conclusions remain; accepted for publication in Phys. Rev.
Generalized dark gravity
The late-time cosmic acceleration may be due to infra-red modifications of
General Relativity. In particular, we consider a maximal extension of the
Hilbert-Einstein action and analyze several interesting features of the theory.
Generally, the motion is non-geodesic and takes place in the presence of an
extra force, which is orthogonal to the four-velocity. These models could lead
to some major differences, as compared to the predictions of General Relativity
or other modified theories of gravity, in several problems of current interest,
such as cosmology, gravitational collapse or the generation of gravitational
waves. The study of these phenomena may also provide some specific signatures
and effects, which could distinguish and discriminate between the various
gravitational models.Comment: 8 pages. Received an Honorable Mention in the Gravity Research
Foundation Essay Contest 201
Extra force in modified theories of gravity
The equation of motion for test particles in modified theories of
gravity is derived. By considering an explicit coupling between an arbitrary
function of the scalar curvature, , and the Lagrangian density of matter, it
is shown that an extra force arises. This extra force is orthogonal to the
four-velocity and the corresponding acceleration law is obtained in the weak
field limit. Connections with MOND and with the Pioneer anomaly are further
discussed.Comment: Revtex4 file, 5 pages. Version to appear in Physical Review
Comparação da tromboelastografia com a classificação de Child-Pugh em pacientes cirróticos
Linearized stability analysis of gravastars in noncommutative geometry
In this work, we find exact gravastar solutions in the context of
noncommutative geometry, and explore their physical properties and
characteristics. The energy density of these geometries is a smeared and
particle-like gravitational source, where the mass is diffused throughout a
region of linear dimension due to the intrinsic uncertainty
encoded in the coordinate commutator. These solutions are then matched to an
exterior Schwarzschild spacetime. We further explore the dynamical stability of
the transition layer of these gravastars, for the specific case of
, where M is the black hole mass, to linearized
spherically symmetric radial perturbations about static equilibrium solutions.
It is found that large stability regions exist and, in particular, located
sufficiently close to where the event horizon is expected to form.Comment: 6 pages, 3 figure
Restrictive strategy of intraoperative fluid maintenance during optimization of oxygen delivery decreases major complications after high-risk surgery
Traversable wormholes coupled to nonlinear electrodynamics
In this work we explore the possible existence of static, spherically
symmetric and stationary, axisymmetric traversable wormholes coupled to
nonlinear electrodynamics. Considering static and spherically symmetric (2+1)
and (3+1)-dimensional wormhole spacetimes, we verify the presence of an event
horizon and the non-violation of the null energy condition at the throat. For
the former spacetime, the principle of finiteness is imposed, in order to
obtain regular physical fields at the throat. Next, we analyze the
(2+1)-dimensional stationary and axisymmetric wormhole, and also verify the
presence of an event horizon, rendering the geometry non-traversable.
Relatively to the (3+1)-dimensional stationary and axisymmetric wormhole
geometry, we find that the field equations impose specific conditions that are
incompatible with the properties of wormholes. Thus, we prove the non-existence
of the general class of traversable wormhole solutions, outlined above, within
the context of nonlinear electrodynamics.Comment: 9 pages, Revtex4. V2: major change in title; considerable additions
in the Introduction and in the rotating solution, no physics changes;
correction of a reference, one reference added; now 10 pages. This version to
appear in Classical and Quantum Gravit
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