4,335 research outputs found
Flat wormholes from straight cosmic strings
Special multi-cosmic string metrics are analytically extended to describe
configurations of Wheeler-Misner wormholes and ordinary cosmic strings. I
investigate in detail the case of flat, asymptotically Minkowskian,
Wheeler-Misner wormhole spacetimes generated by two cosmic strings, each with
tension .Comment: 5 pages, latex, no figure
Classification of BPS instantons in N=4 D=4 supergravity
This talk is based on the recent work in collaboration with M.
Azreg-A\"{\i}nou and G. Cl\'ement devoted to extremal instantons in the
one-vector truncation of the Euclidean theory. Extremal
solutions satisfying the no-force condition can be associated with null
geodesic curves in the homogeneous target space of the three-dimensional sigma
model arising in toroidal reduction of the four-dimensional theory. Here we
(preliminarily) discuss the case of two vector fields sufficient to find all
relevant metrics in the full theory. Classification of
instanton solutions is given along the following lines. The first is their
possible asymptotic structure: asymptotically locally flat (ALF),
asymptotically locally Euclidean (ALE) and ALF or ALE with the dilaton growing
at infinity. The second is the algebraic characterization of matrix generators
according to their rank and the nature of the charge vectors in an associated
Lorentzian space. Finally, solutions are distinguished by the number of
independent harmonic functions with unequal charges (up to four).Comment: Submitted to Proceedings of "Quantum Theory and Symmetries" (QTS-7),
Prague, August 7-13, 201
Wormhole cosmic strings
We construct regular multi-wormhole solutions to a gravitating model
in three space-time dimensions, and extend these solutions to cylindrical
traversable wormholes in four and five dimensions. We then discuss the
possibility of identifying wormhole mouths in pairs to give rise to Wheeler
wormholes. Such an identification is consistent with the original field
equations only in the absence of the -model source, but with possible
naked cosmic string sources. The resulting Wheeler wormhole space-times are
flat outside the sources and may be asymptotically Minkowskian.Comment: 17 pages, LaTeX, 4 figures (hard copy available on request
Topologically massive gravito-electrodynamics: exact solutions
We construct two classes of exact solutions to the field equations of
topologically massive electrodynamics coupled to topologically massive gravity
in 2 + 1 dimensions. The self-dual stationary solutions of the first class are
horizonless, asymptotic to the extreme BTZ black-hole metric, and regular for a
suitable parameter domain. The diagonal solutions of the second class, which
exist if the two Chern-Simons coupling constants exactly balance, include
anisotropic cosmologies and static solutions with a pointlike horizon.Comment: 15 pages, LaTeX, no figure
A simple spectral condition implying separability for states of bipartite quantum systems
For two qubits and for general bipartite quantum systems, we give a simple
spectral condition in terms of the ordered eigenvalues of the density matrix
which guarantees that the corresponding state is separable.Comment: 5 pages Revised 31 May 200
Rheology of a sonofluidized granular packing
We report experimental measurements on the rheology of a dry granular
material under a weak level of vibration generated by sound injection. First,
we measure the drag force exerted on a wire moving in the bulk. We show that
when the driving vibration energy is increased, the effective rheology changes
drastically: going from a non-linear dynamical friction behavior - weakly
increasing with the velocity- up to a linear force-velocity regime. We present
a simple heuristic model to account for the vanishing of the stress dynamical
threshold at a finite vibration intensity and the onset of a linear
force-velocity behavior. Second, we measure the drag force on spherical
intruders when the dragging velocity, the vibration energy, and the diameters
are varied. We evidence a so-called ''geometrical hardening'' effect for
smaller size intruders and a logarithmic hardening effect for the velocity
dependence. We show that this last effect is only weakly dependent on the
vibration intensity.Comment: Accepted to be published in EPJE. v3: Includes changes suggested by
referee
Black hole mass and angular momentum in 2+1 gravity
We propose a new definition for the mass and angular momentum of neutral or
electrically charged black holes in 2+1 gravity with two Killing vectors. These
finite conserved quantities, associated with the SL(2,R) invariance of the
reduced mechanical system, are shown to be identical to the quasilocal
conserved quantities for an improved gravitational action corresponding to
mixed boundary conditions. They obey a general Smarr-like formula and, in all
cases investigated, are consistent with the first law of black hole
thermodynamics. Our framework is applied to the computation of the mass and
angular momentum of black hole solutions to several field-theoretical models.Comment: 23 pages, 3 references added, to be published in Physical Review
Gravitating Chern-Simons vortices
The construction of self-dual vortex solutions to the Chern-Simons-Higgs
model (with a suitable eighth-order potential) coupled to Einstein gravity in
(2 + 1) dimensions is reconsidered. We show that the self-duality condition may
be derived from the sole assumption . Next, we derive a family of
exact, doubly self-dual vortex solutions, which interpolate between the
symmetrical and asymmetrical vacua. The corresponding spacetimes have two
regions at spatial infinity. The eighth-order Higgs potential is positive
definite, and closed timelike curves are absent, if the gravitational constant
is chosen to be negative.Comment: 11 pages, LaTe
Multi-Black-Holes in Three Dimensions
We construct time-dependent multi-centre solutions to three-dimensional
general relativity with zero or negative cosmological constant. These solutions
correspond to dynamical systems of freely falling black holes and conical
singularities, with a multiply connected spacetime topology. Stationary
multi-black-hole solutions are possible only in the extreme black hole case.Comment: 8 pages, \LaTex, 4 figures (available on request), GCR 94/02/0
Hidden symmetry of the three-dimensional Einstein-Maxwell equations
It is shown how to generate three-dimensional Einstein-Maxwell fields from
known ones in the presence of a hypersurface-orthogonal non-null Killing vector
field. The continuous symmetry group is isomorphic to the Heisenberg group
including the Harrison-type transformation. The symmetry of the
Einstein-Maxwell-dilaton system is also studied and it is shown that there is
the transformation between the Maxwell and the dilaton fields.
This transformation is identified with the Geroch
transformation of the four-dimensional vacuum Einstein equation in terms of the
Ka{\l}uza-Klein mechanism.Comment: 5 page
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