Les observations des dernières décennies ont fait émerger un modèle standard en cosmologie, appelé modèle de concordance. Le paradigme qui s'est imposé grâce aux observations aux échelles cosmologiques est celui d'un univers contenant deux phases d'expansion accélérée. La première permet d'expliquer l'origine des galaxies et amas de galaxies que l'on observe de nos jours, tandis que la deuxième rend compte de l'expansion accélérée actuelle, confirmée par de nombreuses observations. Le mécanisme physique à l'origine de cette expansion accélérée reste une des plus grandes questions ouvertes en physique. Néanmoins, des observations provenant du rayonnement fossile cosmologique (Cosmic microwave background - CMB), des oscillations acoustiques baryoniques (Baryon Acoustic Oscillations -BAO), des supernovae du type Ia (SNeIa) et de nombreuses autres observations, semblent en accord avec l'introduction d'une constante cosmologique Lambda dans les équations du champ de la gravitation (les équations d'Einstein). Par ailleurs, une composante de matière ``sombre'' froide (non-relativiste) -appelée Cold Dark Matter ou CDM- est un ingrédient nécessaire pour expliquer la croissance et la formation des grandes structures dans l'Univers. Ce modèle repose aussi sur des hypothèses concernant les fluctuations cosmologiques primordiales (supposées d'origine inflationnaire). En fait, le modèle LCDM de base est entièrement caractérisé par 6 paramètres libres.

Euclidean D-branes wrapped on non-trivial cycles of a Calabi-Yau threefold are known to affect the metric on the hypermultiplet moduli space determining the effective action of type II strings on such manifolds. After reviewing the existing results on the instanton corrected metric following from a combination of constraints imposed by supersymmetry and dualities, I'll show how the D-instanton effects can be computed by a direct worldsheet approach. It required solving several conceptual and technical problems and can now be extended to compactifications with lower supersymmetry. The talk is based on joint work with A. Sen and B. Stefanski.

I'll explain several lessons suggested for the issues of emergence of spacetime and background independence in quantum gravity by models of non-critical and topological strings.

Euclidean D-branes wrapped on non-trivial cycles of a Calabi-Yau threefold are known to affect the metric on the hypermultiplet moduli space determining the effective action of type II strings on such manifolds. After reviewing the existing results on the instanton corrected metric following from a combination of constraints imposed by supersymmetry and dualities, I'll show how the D-instanton effects can be computed by a direct worldsheet approach. It required solving several conceptual and technical problems and can now be extended to compactifications with lower supersymmetry.The talk is based on joint work with A. Sen and B. Stefanski.

Type IIA string theory compactified on a Calabi-Yau threefold has a hypermultiplet moduli space whose metric is known to receive non-perturbative corrections from Euclidean D2-branes wrapped on 3-cycles. These corrections have been computed earlier by making use of mirror symmetry, S-duality and twistorial description of quaternionic geometries. In this paper we compute the leading corrections in each homology class using a direct world-sheet approach without relying on any duality symmetry or supersymmetry. Our results are in perfect agreement with the earlier predictions.

We exhibit a set of six explicit higher nonlocal integral invariants of free-surface deep-water gravity waves conservedin lowest nontrivial orders of perturbation in the amplitude of the surface displacement.