No Cosmological D=11 Supergravity

We show, in two complementary ways, that D=11 supergravity---in contrast to all its lower dimensional versions---forbids a cosmological extension. First, we linearize the putative model about an Anti de Sitter background and show that it cannot even support a "global" supersymmetry invariance; hence there is no Noether construction that can lead to a local supersymmetry. This is true with the usual 4-form field as well as for a "dual", 7-form, starting point. Second, a cohomology argument, starting from the original full nonlinear theory, establishes the absence of deformations involving spin 3/2 mass and cosmological terms. In both approaches, it is the form field that is responsible for the obstruction. ``Dualizing'' the cosmological constant to an 11-form field also fails

A Note on the D-2-Brane of the Massive Type IIA Theory and Gauged Sigma Models

Gauging the M-2-brane effective action with respect to an Abelian isometry in such a way that the invariance under gauge transformations of the 3-form potential is maintained (slightly modified) we obtain a fully covariant action with 11-dimensional target space that gives the massive D-2-brane effective action upon dimensional reduction in the direction of the gauged isometry.Comment: Latex file. A reference adde

Liouville Lost, Liouville Regained: Central Charge in a Dynamical Background

Several recent approaches to black hole entropy obtain the density of states from the central charge of a Liouville theory. If Liouville theory is coupled to a dynamical spacetime background, however, the classical central charge vanishes. I show that the central charge can be restored by introducing appropriate constraints, which may be interpreted as fall-off conditions at a boundary such as a black hole horizon.Comment: 8 pages, LaTeX, no figures; reference adde

Holonomies, anomalies and the Fefferman-Graham ambiguity in AdS3 gravity

Using the Chern-Simon formulation of (2+1) gravity, we derive, for the general asymptotic metrics given by the Fefferman-Graham-Lee theorems, the emergence of the Liouville mode associated to the boundary degrees of freedom of (2+1) dimensional anti de Sitter geometries. Holonomies are described through multi-valued gauge and Liouville fields and are found to algebraically couple the fields defined on the disconnected components of spatial infinity. In the case of flat boundary metrics, explicit expressions are obtained for the fields and holonomies. We also show the link between the variation under diffeomorphisms of the Einstein theory of gravitation and the Weyl anomaly of the conformal theory at infinity.Comment: LaTeX, 26 pages, 1 figure, modified version that will appear in Nucl. Phys.

On M-9-branes and their dimensional reductions

The M-9-brane Wess-Zumino action is constructed, and by using it, consistency of the relation of p-branes for $p \ge 8$, suggested on the basis of superalgebra, is discussed.Comment: 6 pages, Latex. Contribution to the proceedings of the D.V.Volkov's memorial conference ``Supersymmetry and Quantum Field Theory'',Kharkov,July 25-29,2000, to be published in Nuclear Physics B Conference Supplement

Quand le discours pédagogique entrave la construction des usages littéraciés du langage

Le texte des savoirs disciplinaires fondés dans l’écrit, construit et énoncé dans une logique de transmission des savoirs, s’affaiblit dans le quotidien des classes au profit d’un curriculum largement cognitif et langagier, compétences littéraciées que les élèves sont censés mettre en œuvre dès les classes pré-élémentaires. Le travail scolaire lui-même, les situations d’apprentissage reposent sur une raison graphique, sur des modes de penser, d’apprendre, de raisonner, de traiter les documents qui supposent dès les premiers moments de la scolarité la familiarité avec la spécificité de la littéracie étendue. Cependant, le discours pédagogique, les interactions maître-élèves, les situations de travail aident-ils les élèves à entrer dans cette familiarité ? L’école prend-elle en charge l’apprentissage de ce qu’elle suppose partagé et qui ne l’est pas ? Si tel n’est pas le cas, l’école accentue les inégalités sociales d’apprentissage. Après la présentation d’un cadre théorique et conceptuel issu des travaux de Bernstein, le propos analyse le discours pédagogique contemporain dans ses caractéristiques les plus récurrente

La Télé-démocratie

Les implications des nouvelles technologies de la communication dans le domaine politique sont souvent considérées sous l'angle de l'amélioration qu'elles peuvent apporter au fonctionnement de la démocratie. Ainsi, les discours prospectifs et/ou récapitulatifs s'intéressant à ce qu'on peut appeler la télé-démocratie insistent tantôt sur le développement de la communication verticale (référendum électronique), tantôt sur celui de la communication horizontale (convivialité). Cependant un troisième courant de réflexion met plutôt l'accent sur les conditions institutionnelles nécessaires à la délibération électronique. La confrontation des trois tendances amène à poser le problème des relations entre la citoyenneté et les systèmes de communication.The implications of the new comunication technologies in the political field are often viewed in the way of an improvement brought to the working of democracy. Thus the prospective and/or recapitulative dis course about what can be called « tele-democracy » insists either on the development of vertical communication (electronic consultation) or on the horizontal aspect (convivial relations). However a third direction of thoughts rather goes toward the kind of institutions necessary to electronic debates. The confrontation of those three trends leads to the problem of relations between citizenship and the systems of communication