Spinning charged BTZ black holes and self-dual particle-like solutions

We generate from the static charged BTZ black hole a family of spinning charged solutions to the Einstein-Maxwell equations in 2+1 dimensions. These solutions go over, in a suitable limit, to self-dual spinning charged solutions, which are horizonless and regular, with logarithmically divergent mass and spin. To cure this divergence, we add a topological Chern-Simons term to the gauge field action. The resulting self-dual solution is horizonless, regular, and asymptotic to the extreme BTZ black hole.Comment: 10 pages, LaTex, no figure

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 $-1/4G$.Comment: 5 pages, latex, no figure

Critical collapse in 2+1 dimensional AdS spacetime: quasi-CSS solutions and linear perturbations

We construct a one-parameter family of exact time-dependent solutions to 2+1 gravity with a negative cosmological constant and a massless minimally coupled scalar field as source. These solutions present a continuously self-similar (CSS) behaviour near the central singularity, as observed in critical collapse, and an asymptotically AdS behaviour at spatial infinity. We consider the linear perturbation analysis in this background, and discuss the crucial question of boundary conditions. These are tested in the special case where the scalar field decouples and the linear perturbations describe exactly the small-mass static BTZ black hole. In the case of genuine scalar perturbations, we find a growing mode with a behavior characteristic of supercritical collapse, the spacelike singularity and apparent horizon appearing simultaneously and evolving towards the AdS boundary. Our boundary conditions lead to the value of the critical exponent $\gamma = 0.4$.Comment: 33 pages, 6 figures. Nuclear Physics B (in press

Generalized Fleming-Viot processes with immigration via stochastic flows of partitions

The generalized Fleming-Viot processes were defined in 1999 by Donnelly and Kurtz using a particle model and by Bertoin and Le Gall in 2003 using stochastic flows of bridges. In both methods, the key argument used to characterize these processes is the duality between these processes and exchangeable coalescents. A larger class of coalescent processes, called distinguished coalescents, was set up recently to incorporate an immigration phenomenon in the underlying population. The purpose of this article is to define and characterize a class of probability-measure valued processes called the generalized Fleming-Viot processes with immigration. We consider some stochastic flows of partitions of Z_{+}, in the same spirit as Bertoin and Le Gall's flows, replacing roughly speaking, composition of bridges by coagulation of partitions. Identifying at any time a population with the integers $\mathbb{N}:=\{1,2,...\}$, the formalism of partitions is effective in the past as well as in the future especially when there are several simultaneous births. We show how a stochastic population may be directly embedded in the dual flow. An extra individual 0 will be viewed as an external generic immigrant ancestor, with a distinguished type, whose progeny represents the immigrants. The "modified" lookdown construction of Donnelly-Kurtz is recovered when no simultaneous multiple births nor immigration are taken into account. In the last part of the paper we give a sufficient criterion for the initial types extinction.Comment: typos and corrections in reference

An Enduring Philosophical Agenda. Worldview Construction as a Philosophical Method\ud

Is there such a thing as a philosophical method? It seems that there are as many philosophical methods as there are philosophies. A method is any procedure employed to achieve a certain aim. So, before proposing a method, we have to tackle the delicate question: “what is the aim of philosophy?”. At the origin of philosophy, there is a questioning about the world. The worldview approach developed by Leo Apostel elegantly explicit those fundamental questions. As we answer them, we come up with a worldview. Using this framework, this paper consider answering this enduring philosophical agenda as the primary aim of philosophy. We illustrate the approach by pointing out the limitations of both a strictly scientific worldview and a strictly religious worldview. We then argue that philosophical worldviews constitute a particular class of possible worldviews. With the help of three analogies, we give guidelines to construct such worldviews. The next step is to compare the relative strength of philosophical worldviews. Precise evaluation standards to compare and confront worldviews are proposed. Some problems for worldview diffusion are then expounded. We close with basic hypotheses to build a comprehensive philosophical worldview

An Enduring Philosophical Agenda. Worldview Construction as a Philosophical Method

Is there something like a philosophical method? It seems that there are as many methods as there are philosophies. A method is any procedure employed to attain a certain end. So, before going to a method, we have to ask: what is the aim of philosophy? At the origin of philosophy, there is a questioning about the world. Leo Apostel and Jan Van der Veken made more precise and explicit those fundamental questions (Apostel, Van der Veken 1991). The primarily aim of philosophy can be seen as answering this philosophical agenda; with the answers, one come up with a worldview. We'll argue that the philosophical worldviews constitute a particular class of the possible worldviews. With the help of three analogies, we'll give some guidelines to construct such worldviews. But, what are the best philosophical worldviews? We'll see how we can compare and confront them; and also some problems for their diffusion. The last section will propose some basic hypotheses to build such integrative worldviews

The Orlik-Solomon model for hypersurface arrangements

We develop a model for the cohomology of the complement of a hypersurface arrangement inside a smooth projective complex variety. This generalizes the case of normal crossing divisors, discovered by P. Deligne in the context of the mixed Hodge theory of smooth complex varieties. Our model is a global version of the Orlik-Solomon algebra, which computes the cohomology of the complement of a union of hyperplanes in an affine space. The main tool is the complex of logarithmic forms along a hypersurface arrangement, and its weight filtration. Connections with wonderful compactifications and the configuration spaces of points on curves are also studied.Comment: 23 pages; presentation simplified, results unchange