431 research outputs found
Asymptotically (anti)-de Sitter solutions in Gauss-Bonnet gravity without a cosmological constant
In this paper we show that one can have asymptotically de Sitter (dS),
anti-de Sitter (AdS) and flat solutions in Gauss-Bonnet gravity without any
need to a cosmological constant term in field equations. First, we introduce
static solutions whose 3-surfaces at fixed and have constant positive
(), negative (), or zero () curvature. We show that for
, one can have asymptotically dS, AdS and flat spacetimes, while for
the case of , one has only asymptotically AdS solutions. Some of these
solutions present naked singularities, while some others are black hole or
topological black hole solutions. We also find that the geometrical mass of
these 5-dimensional spacetimes is , which is different from
the geometrical mass, , of the solutions of Einstein gravity. This feature
occurs only for the 5-dimensional solutions, and is not repeated for the
solutions of Gauss-Bonnet gravity in higher dimensions. We also add angular
momentum to the static solutions with , and introduce the asymptotically
AdS charged rotating solutions of Gauss-Bonnet gravity. Finally, we introduce a
class of solutions which yields an asymptotically AdS spacetime with a
longitudinal magnetic field which presents a naked singularity, and generalize
it to the case of magnetic rotating solutions with two rotation parameters.Comment: 13 pages, no figur
Governance in Service Delivery in the Middle East and North Africa. World Development Report Background Paper
This paper examines the clientelistic equilibrium that remains prevalent in much of the Middle East and North Africa (MENA) region during the post-independence period, undermining service delivery and creating inequality in access. Political institutions and social practices that shape incentives for policymakers, service providers, and citizens create what can be called a potentially tenuous, “clientelistic equilibrium.” Service delivery is influenced by political institutions that allow for the capture of public jobs and service networks, and by social institutions that call upon individuals to respond more readily to members of their social networks than to others. The result is poor quality service delivery (e.g., absenteeism, insufficient effort), difficulties in access (e.g., need for bribes, connections), and inequalities in the provision of services
Planet Formation in the Outer Solar System
This paper reviews coagulation models for planet formation in the Kuiper
Belt, emphasizing links to recent observations of our and other solar systems.
At heliocentric distances of 35-50 AU, single annulus and multiannulus
planetesimal accretion calculations produce several 1000 km or larger planets
and many 50-500 km objects on timescales of 10-30 Myr in a Minimum Mass Solar
Nebula. Planets form more rapidly in more massive nebulae. All models yield two
power law cumulative size distributions, N_C propto r^{-q} with q = 3.0-3.5 for
radii larger than 10 km and N_C propto r^{-2.5} for radii less than 1 km. These
size distributions are consistent with observations of Kuiper Belt objects
acquired during the past decade. Once large objects form at 35-50 AU,
gravitational stirring leads to a collisional cascade where 0.1-10 km objects
are ground to dust. The collisional cascade removes 80% to 90% of the initial
mass in the nebula in roughly 1 Gyr. This dust production rate is comparable to
rates inferred for alpha Lyr, beta Pic, and other extrasolar debris disk
systems.Comment: invited review for PASP, March 2002. 33 pages of text and 12 figure
On the linear independence of spikes and sines
The purpose of this work is to survey what is known about the linear
independence of spikes and sines. The paper provides new results for the case
where the locations of the spikes and the frequencies of the sines are chosen
at random. This problem is equivalent to studying the spectral norm of a random
submatrix drawn from the discrete Fourier transform matrix. The proof involves
depends on an extrapolation argument of Bourgain and Tzafriri.Comment: 16 pages, 4 figures. Revision with new proof of major theorem
Time Scales for transitions between free energy minima of a hard sphere system
Time scales associated with activated transitions between glassy metastable
states of a free energy functional appropriate for a dense hard sphere system
are calculated by using a new Monte Carlo method for the local density
variables. We calculate the time the system,initially placed in a shallow
glassy minimum of the free energy, spends in the neighborhood of this minimum
before making a transition to the basin of attarction of another free energy
minimum. This time scale is found to increase with the average density. We find
a crossover density near which this time scale increases very sharply and
becomes longer than the longest times accessible in our simulation. This scale
shows no evidence of dependence on sample size.Comment: 25 pages, Revtex, 6 postscript figures. Will appear in Phys Rev E,
March 1996 or s
Experiments on local search for bi-objective unconstrained binary quadratic programming
International audienceThis article reports an experimental analysis on stochastic local search for approximating the Pareto set of bi-objective unconstrained binary quadratic programming problems. First, we investigate two scalarizing strategies that iteratively identify a high-quality solution for a sequence of sub-problems. Each sub-problem is based on a static or adaptive definition of weighted-sum aggregation coefficients, and is addressed by means of a state-of-the-art single-objective tabu search procedure. Next, we design a Pareto local search that iteratively improves a set of solutions based on a neighborhood structure and on the Pareto dominance relation. At last, we hybridize both classes of algorithms by combining a scalarizing and a Pareto local search in a sequential way. A comprehensive experimental analysis reveals the high performance of the proposed approaches, which substantially improve upon previous best-known solutions. Moreover, the obtained results show the superiority of the hybrid algorithm over non-hybrid ones in terms of solution quality, while requiring a competitive computational cost. In addition, a number of structural properties of the problem instances allow us to explain the main difficulties that the different classes of local search algorithms have to face
Perturbing gauge/gravity duals by a Romans mass
We show how to produce algorithmically gravity solutions in massive IIA (as
infinitesimal first order perturbations in the Romans mass parameter) dual to
assigned conformal field theories. We illustrate the procedure on a family of
Chern--Simons--matter conformal field theories that we recently obtained from
the N=6 theory by waiving the condition that the levels sum up to zero.Comment: 30 page
Two-time scales, two-temperature scenario for nonlinear rheology
We investigate a general scenario for ``glassy'' or ``jammed'' systems driven
by an external, non-conservative force, analogous to a shear force in a fluid.
In this scenario, the drive results in the suppression of the usual aging
process, and the correlation and response functions become time translation
invariant. The relaxation time and the response functions are then dependent on
the intensity of the drive and on temperature. We investigate this dependence
within the framework of a dynamical closure approximation that becomes exact
for disordered, fully-connected models. The relaxation time is shown to be a
decreasing function of the drive (``shear thinning'' effect). The correlation
functions below the glass transition temperature (Tc) display a two time scales
relaxation pattern, similar to that observed at equilibrium slightly above Tc.
We also study the violation of the fluctuation dissipation relationship in the
driven system. This violation is very reminiscent of the one that takes place
in a system aging below Tc at zero drive. It involves in particular the
appearance of a two-temperatures regime, in the sense of an effective
fluctuation dissipation temperature. Although our results are in principle
limited to the closure relations that hold for mean-field models, we argue that
a number of the salient features are not inherent to the approximation scheme,
and may be tested in experiments and simulations.Comment: Version accepted for publication - Physical Review
St\"uckelino Dark Matter in Anomalous U(1)' Models
We study a possible dark matter candidate in the framework of a minimal
anomalous extension of the MSSM. It turns out that in a suitable
decoupling limit the St\"uckelino, the fermionic degree of freedom of the
St\"uckelberg multiplet, is the lightest supersymmetric particle (LSP). We
compute the relic density of this particle including coannihilations with the
next to lightest supersymmetric particle (NLSP) and with the next to next to
lightest supersymmetric particle (NNLSP) which are assumed almost degenerate in
mass. This assumption is needed in order to satisfy the stringent limits that
the Wilkinson Microwave Anisotropy Probe (WMAP) puts on the relic density. We
find that the WMAP constraints can be satisifed by different NLSP and NNLSP
configurations as a function of the mass gap with the LSP. These results hold
in the parameter space region where the model remains perturbative.Comment: 21 pages, 6 figures, improved section 5, revised version published on
EPJ
Invariant Differential Operators and Characters of the AdS_4 Algebra
The aim of this paper is to apply systematically to AdS_4 some modern tools
in the representation theory of Lie algebras which are easily generalised to
the supersymmetric and quantum group settings and necessary for applications to
string theory and integrable models. Here we introduce the necessary
representations of the AdS_4 algebra and group. We give explicitly all singular
(null) vectors of the reducible AdS_4 Verma modules. These are used to obtain
the AdS_4 invariant differential operators. Using this we display a new
structure - a diagram involving four partially equivalent reducible
representations one of which contains all finite-dimensional irreps of the
AdS_4 algebra. We study in more detail the cases involving UIRs, in particular,
the Di and the Rac singletons, and the massless UIRs. In the massless case we
discover the structure of sets of 2s_0-1 conserved currents for each spin s_0
UIR, s_0=1,3/2,... All massless cases are contained in a one-parameter
subfamily of the quartet diagrams mentioned above, the parameter being the spin
s_0. Further we give the classification of the so(5,C) irreps presented in a
diagramatic way which makes easy the derivation of all character formulae. The
paper concludes with a speculation on the possible applications of the
character formulae to integrable models.Comment: 30 pages, 4 figures, TEX-harvmac with input files: amssym.def,
amssym.tex, epsf.tex; version 2 1 reference added; v3: minor corrections;
v.4: minor corrections, v.5: minor corrections to conform with version in J.
Phys. A: Math. Gen; v.6.: small correction and addition in subsections 4.1 &
4.
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