3,491 research outputs found

### 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

### On thin-shell wormholes evolving in flat FRW spacetimes

We analize the stability of a class of thin-shell wormholes with spherical
symmetry evolving in flat FRW spacetimes. The wormholes considered here are
supported at the throat by a perfect fluid with equation of state
$\mathcal{P}=w\sigma$ and have a physical radius equal to $aR$, where $a$ is a
time-dependent function describing the dynamics of the throat and $R$ is the
background scale factor. The study of wormhole stability is done by means of
the stability analysis of dynamic systems.Comment: 8 pages; to appear in MPL

### Theoretical construction of stable traversable wormholes

It is shown in this paper that it is possible, at least in principle, to
construct a traversable wormhole that is stable to linearized radial
perturbations by specifying relatively simple conditions on the shape and
redshift functions.Comment: 5 pages; 1 figur

### An anti-Schwarzshild solution: wormholes and scalar-tensor solutions

We investigate a static solution with an hyperbolic nature, characterised by
a pseudo-spherical foliation of space. This space-time metric can be perceived
as an anti-Schwarzschild solution, and exhibits repulsive features. It belongs
to the class of static vacuum solutions termed "a degenerate static solution of
class A". In the present work we review its fundamental features, discuss the
existence of generalised wormholes, and derive its extension to scalar-tensor
gravity theories in general.Comment: 3 pages, contribution to the proceedings of the Spanish Relativity
Meeting-ERE200

### Phantom energy traversable wormholes

It has been suggested that a possible candidate for the present accelerated
expansion of the Universe is ''phantom energy''. The latter possesses an
equation of state of the form $\omega\equiv p/\rho<-1$, consequently violating
the null energy condition. As this is the fundamental ingredient to sustain
traversable wormholes, this cosmic fluid presents us with a natural scenario
for the existence of these exotic geometries. Due to the fact of the
accelerating Universe, macroscopic wormholes could naturally be grown from the
submicroscopic constructions that originally pervaded the quantum foam. One
could also imagine an advanced civilization mining the cosmic fluid for phantom
energy necessary to construct and sustain a traversable wormhole.
In this context, we investigate the physical properties and characteristics
of traversable wormholes constructed using the equation of state $p=\omega
\rho$, with $\omega<-1$. We analyze specific wormhole geometries, considering
asymptotically flat spacetimes and imposing an isotropic pressure. We also
construct a thin shell around the interior wormhole solution, by imposing the
phantom energy equation of state on the surface stresses. Using the ''volume
integral quantifier'' we verify that it is theoretically possible to construct
these geometries with vanishing amounts of averaged null energy condition
violating phantom energy. Specific wormhole dimensions and the traversal
velocity and time are also deduced from the traversability conditions for a
particular wormhole geometry. These phantom energy traversable wormholes have
far-reaching physical and cosmological implications. For instance, an advanced
civilization may use these geometries to induce closed timelike curves,
consequently violating causality.Comment: 9 pages, Revtex4. V2: Considerable comments and references added, no
physics changes, now 10 pages. Accepted for publication in Physical Review

### The Equation of State of a Low-Temperature Fermi Gas with Tunable Interactions

Interacting fermions are ubiquitous in nature and understanding their
thermodynamics is an important problem. We measure the equation of state of a
two-component ultracold Fermi gas for a wide range of interaction strengths at
low temperature. A detailed comparison with theories including Monte-Carlo
calculations and the Lee-Huang-Yang corrections for low-density bosonic and
fermionic superfluids is presented. The low-temperature phase diagram of the
spin imbalanced gas reveals Fermi liquid behavior of the partially polarized
normal phase for all but the weakest interactions. Our results provide a
benchmark for many-body theories and are relevant to other fermionic systems
such as the crust of neutron stars.Comment: 28 pages, 7 figure

### Radial stability analysis of the continuous pressure gravastar

Radial stability of the continuous pressure gravastar is studied using the
conventional Chandrasekhar method. The equation of state for the static
gravastar solutions is derived and Einstein equations for small perturbations
around the equilibrium are solved as an eigenvalue problem for radial
pulsations. Within the model there exist a set of parameters leading to a
stable fundamental mode, thus proving radial stability of the continuous
pressure gravastar. It is also shown that the central energy density possesses
an extremum in rho_c(R) curve which represents a splitting point between stable
and unstable gravastar configurations. As such the rho_c(R) curve for the
gravastar mimics the famous M(R) curve for a polytrope. Together with the
former axial stability calculations this work completes the stability problem
of the continuous pressure gravastar.Comment: 17 pages, 5 figures, References corrected, minor changes wrt v1,
matches published versio

### The mathematical theory of resonant transducers in a spherical gravity wave antenna

The rigoruos mathematical theory of the coupling and response of a spherical
gravitational wave detector endowed with a set of resonant transducers is
presented and developed. A perturbative series in ascending powers of the
square root of the ratio of the resonator to the sphere mass is seen to be the
key to the solution of the problem. General layouts of arbitrary numbers of
transducers can be assessed, and a specific proposal (PHC), alternative to the
highly symmetric TIGA of Merkowitz and Johnson, is described in detail.
Frequency spectra of the coupled system are seen to be theoretically recovered
in full agreement with experimental determinations.Comment: 31 pages, 7 figures, LaTeX2e, \usepackage{graphicx,deleq

### 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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