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Riemannian Geometry: The moduli space of Ricci-flat manifolds May 10, 2016 (11:00 AM PDT - 12:00 PM PDT)
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Location: MSRI: Simons Auditorium
Speaker(s) Bernd Ammann (Universit├Ąt Regensburg)
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(Joint work with Klaus Kroencke, Hartmut Weiss and Frederik Witt.) We study the set of all Ricci-flat Riemannian metrics on a given compact manifold $M$. We say that a Ricci-flat metric on $M$ is structured if its pullback to the universal cover admits a parallel spinor. 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. 


The set of unstructured Ricci-flat metrics is poorly understood. Nobody knows whether unstructured compact Ricci-flat Riemannian manifolds exist, and if they exist, there is no reason to expect that the set of such metrics on a fixed compact manifold should have the structure of a smooth manifold. 


On the other hand, the set of structured Ricci-flat metrics on compact manifolds is now well-understood. 


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. 


Our work builds on previous work by J. Nordstr\"om, Goto, Tian \& Todorov, Joyce, McKenzie Wang and many others. The important step is to pass from irreducible to reducible holonomy groups.

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