# Agenda de l’IDP

## Séminaire de Géométrie

Parallel spinors on Riemannian and Lorentzian manifolds
Bernd Ammann (Université de Regensbrug)
vendredi 22 mars 2019 14:00 -  Tours -  1180 (Bât E2)

Résumé :

The talk describes results in joint articles with Klaus Kröncke, Olaf Müller, Hartmut Weiss, and Frederik Witt.

We say that a Riemannian metric on $$M$$ is structured if its pullback to the universal cover admits a parallel spinor. All such metrics are Ricci-flat. The holonomy of these metrics is special as these manifolds carry some additional structure, e.g. a Calabi-Yau structure or a  $$G_2$$-structure. All known compact Ricci-flat manifolds are structured.

The set of structured Ricci-flat metrics on compact manifolds is now well-understood, and we will explain this in the first part of the talk.

The set of structured Ricci-flat metrics is an open and closed subset in the space of all Ricci-flat metrics. The holonomy group is constant along connected components. The dimension of the space of parallel spinors as well. The structured Ricci-flat metrics form a smooth Banach submanifold in the space of all metrics. Furthermore the associated premoduli space is a finite-dimensional smooth manifold, and the parallel spinors form a natural bundle with metric and connection over this premoduli space.

Lorentzian manifolds with a parallel spinor are not necessarily Ricci-flat, however the rank of the Ricci tensor is at most $$1$$, the image of the Ricci-endomorphism is lightlike. Helga Baum, Thomas Leistner and Andree Lischewski showed the well-posedness for an associated  Cauchy problem. Here well-posedness means that a (local) solutions exist if and only if the initial conditions satisfy some constraint equations.

We are now able to prove a conjecture by Leistner and Lischewski which states that solutions of the constraint equations on an $$n$$-dimensional Cauchy hypersurface can be obtained from curves in the moduli space of structured Ricci-flat metrics on an $$(n-1)$$-dimensional closed manifold.

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