8,930 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
Stable Gravastars of Anisotropic Dark Energy
Dynamical models of prototype gravastars made of phantom energy are
constructed, in which an infinitely thin spherical shell of a perfect fluid
with the equation of state divides the whole spacetime
into two regions, the internal region filled with a dark energy (or phantom)
fluid, and the external Schwarzschild region. It is found that in some cases
the models represent the "bounded excursion" stable gravastars, where the thin
shell is oscillating between two finite radii, while in other cases they
collapse until the formation of black holes or normal stars. In the phase
space, the region for the "bounded excursion" gravastars is very small in
comparison to that of black holes, but not empty, as found in our previous
papers. Therefore, although the existence of gravastars can not be completely
excluded from current analysis, the opposite is not possible either, that is,
even if gravastars exist, they do not exclude the existence of black holes.Comment: 35 pages, 43 figures, added some clarifying texts and corrected some
typos, accepted for publication in JCA
Good Regulatory Lags for Price Cap and Rolling Cap contracts
Price caps are a popular form of monopoly price regulation. One of its disadvantages is the perverse incentives that regulated firms might have to scamp on cost reducing effort during the last years before a price review. In order to avoid this problem a ĂąâŹĆrolling capù⏠contract was introduced in the United Kingdom that overcomes this last problem. In spite of their popularity, there is scant research on the optimal regulatory lag (number of years between price reviews) of a price cap or rolling cap contract. In practice, around the world most price cap or rolling cap contracts have a lag of 4 to 5 years, but this is not based on any optimality consideration. As is well known, the regulatory lag determines the power of an incentive contract and thus the incentives to undertake cost reducing effort. Schmalensee (1989) studied the optimal power of regulatory contracts in a static model with uncertainty and asymmetric information. She finds that medium powered contracts are generally superior to the polar cases of high or low powered contracts. In this paper, we extend Schmalensee (1989) model used to study the optimal power of regulatory contracts to a dynamic framework. We use numerical simulation to study the optimal regulatory lag for different combinations of demand and cost parameters under a particular linear quadratic structure. We find that in general a 2 year lag is optimal under both a price cap and rolling cap contracts and that a benevolent regulator prefers the rolling cap over the price cap contract in almost all the casesPrice Cap, Rolling Cap, Regulatory Lag, Dynamic Programming
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.
Security policy refinement using data integration: a position paper.
In spite of the wide adoption of policy-based approaches for security management, and many existing treatments of policy verification and analysis, relatively little attention has been paid to policy refinement: the problem of deriving lower-level, runnable policies from higher-level policies, policy goals, and specifications. In this paper we present our initial ideas on this task, using and adapting concepts from data integration. We take a view of policies as governing the performance of an action on a target by a subject, possibly with certain conditions. Transformation rules are applied to these components of a policy in a structured way, in order to translate the policy into more refined terms; the transformation rules we use are similar to those of global-as-view database schema mappings, or to extensions thereof. We illustrate our ideas with an example. Copyright 2009 ACM
Errors on the inverse problem solution for a noisy spherical gravitational wave antenna
A single spherical antenna is capable of measuring the direction and
polarization of a gravitational wave. It is possible to solve the inverse
problem using only linear algebra even in the presence of noise. The simplicity
of this solution enables one to explore the error on the solution using
standard techniques. In this paper we derive the error on the direction and
polarization measurements of a gravitational wave. We show that the solid angle
error and the uncertainty on the wave amplitude are direction independent. We
also discuss the possibility of determining the polarization amplitudes with
isotropic sensitivity for any given gravitational wave source.Comment: 13 pages, 4 figures, LaTeX2e, IOP style, submitted to CQ
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