A non Ricci-flat Einstein pseudo-Riemannian metric on a 7-dimensional nilmanifold

Abstract

We answer in the affirmative the question posed by Conti and Rossi on the existence of nilpotent Lie algebras of dimension 7 with an Einstein pseudo-metric of nonzero scalar curvature. Indeed, we construct a left-invariant pseudo-Riemannian metric $g$ of signature $(3,4)$ on a nilpotent Lie group of dimension 7, such that $g$ is Einstein and not Ricci-flat. We show that the pseudo-metric $g$ cannot be induced by any left-invariant closed $G_2^{\ast }$-structure on the Lie group. Moreover, some results on closed and harmonic $G_2^{\ast }$-structures on an arbitrary 7-manifold $M$ are given. In particular, we prove that the underlying pseudo-Riemannian metric of a closed and harmonic $G_2^{\ast }$-structure on $M$ is not necessarily Einstein, but if it is Einstein then it is Ricci-flat.

20 pages