5,734 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
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
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
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
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
Variants of Plane Diameter Completion
The {\sc Plane Diameter Completion} problem asks, given a plane graph and
a positive integer , if it is a spanning subgraph of a plane graph that
has diameter at most . We examine two variants of this problem where the
input comes with another parameter . In the first variant, called BPDC,
upper bounds the total number of edges to be added and in the second, called
BFPDC, upper bounds the number of additional edges per face. We prove that
both problems are {\sf NP}-complete, the first even for 3-connected graphs of
face-degree at most 4 and the second even when on 3-connected graphs of
face-degree at most 5. In this paper we give parameterized algorithms for both
problems that run in steps.Comment: Accepted in IPEC 201
Bifurcation-based parameter tuning in a model of the GnRH pulse and surge generator
We investigate a model of the GnRH pulse and surge generator, with the
definite aim of constraining the model GnRH output with respect to a
physiologically relevant list of specifications. The alternating pulse and
surge pattern of secretion results from the interaction between a GnRH
secreting system and a regulating system exhibiting fast-slow dynamics. The
mechanisms underlying the behavior of the model are reminded from the study of
the Boundary-Layer System according to the "dissection method" principle. Using
singular perturbation theory, we describe the sequence of bifurcations
undergone by the regulating (FitzHugh-Nagumo) system, encompassing the rarely
investigated case of homoclinic connexion. Basing on pure dynamical
considerations, we restrict the space of parameter search for the regulating
system and describe a foliation of this restricted space, whose leaves define
constant duration ratios between the surge and the pulsatility phase in the
whole system. We propose an algorithm to fix the parameter values to also meet
the other prescribed ratios dealing with amplitude and frequency features of
the secretion signal. We finally apply these results to illustrate the dynamics
of GnRH secretion in the ovine species and the rhesus monkey
Ring Wormholes in D-Dimensional Einstein and Dilaton Gravity
On the basis of exact solutions to the Einstein-Abelian gauge-dilaton
equations in -dimensional gravity, the properties of static axial
configurations are discussed. Solutions free of curvature singularities are
selected; they can be attributed to traversible wormholes with cosmic
string-like singularities at their necks. In the presence of an electromagnetic
field some of these wormholes are globally regular, the string-like singularity
being replaced by a set of twofold branching points. Consequences of wormhole
regularity and symmetry conditions are discussed. In particular, it is shown
that (i) regular, symmetric wormholes have necessarily positive masses as
viewed from both asymptotics and (ii) their characteristic length scale in the
big charge limit () is of the order of the ``classical radius"
.Comment: Latex file, 15 page
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