Cohomologically calibrated affine connections and the Einstein condition on $S^2\times T^2$

Abstract

This paper applies the recently developed framework of cohomologically calibrated affine connections to the fundamental problem of constructing non-Riemannian Einstein manifolds. In this framework, the torsion of a connection is intrinsically related to the global topology of the manifold, represented by the de Rham cohomology class specified by a set of real parameters. We focus on the product manifold $S^2\times T^2$, whose third cohomology group is $H^3(S^2\times T^2;\mathbb{R})\cong {\mathbb{R}}^2$. We analyze how the geometry is modeled by the choice of the torsion tensor $T$ within the family ${\mathcal{T}}_{\omega }$, defined by the property that each member of this family must have an associated 3-form $T^{♭}$ such that it represents the nontrivial cohomology class via Hodge decomposition. Our analysis reveals a dependence on this choice. First, we show that using a torsion tensor, which produces a strictly positive biorthogonal curvature, leads to a non-diagonal Ricci tensor, creating a structural obstacle to any Einstein solution. Conversely, we then show that using the torsion tensor associated with the purelly harmonic 3-form allow us the construction of an explicit non-Riemannian Einstein solution. Our work thus demonstrates that the cohomologically calibrated affine connections allow a family of feasible connections rich enough to allow for several geometries intrinsically justified by the differential topology of the manifold.

16 pages