Notes on flat pseudo-Riemannian manifolds

Abstract

In these notes we survey basic concepts of affine geometry and their interaction with Riemannian geometry. We give a characterization of affine manifolds which has as counterpart those pseudo-Riemannian manifolds whose Levi-Civita connection is at. We show that no connected semisimple Lie group admits a left invariant flat affine connection. We characterize at pseudo-Riemannian Lie groups. For a at left-invariant pseudo-metric on a Lie group, we show the equivalence between the completeness of the Levi-Civita connection and unimodularity of the group.
We emphasize the case of at left invariant hyperbolic metrics on the cotangent bundle of a simply connected at affine Lie group. We also discuss Lie groups with bi-invariant pseudo-metrics and the construction of orthogonal Lie algebras.