Critical metrics for the Dirac-Yamabe eigenvalue ratio and Einstein-Dirac equations

Abstract

Let $(M^n,g)$ be a closed spin manifold of dimension $n\geq 3$ with positive Yamabe constant. We prove that critical metrics for the mixed eigenvalue ratio $\mathcal H(g)=\frac{λ_1(D_g)^2}{λ_1(L_g)},$ where $D_g$ is the Dirac operator and $L_g$ is the conformal Laplacian are characterized by a harmonic section whose energy-momentum tensor satisfies an Einstein--Dirac equation. We discuss consequences of this result for Hijazi's inequality, and we construct an example to prove that $\mathcal H$ is unbounded above within the conformal class of the round sphere.