Rigidity and stability of Einstein metrics for quadratic curvature functionals

We investigate rigidity and stability properties of critical points of quadratic curvature functionals on the space of Riemannian metrics. We show it is possible to “gauge” the Euler-Lagrange equations, in a self-adjoint fashion, to become elliptic. Fredholm theory may then be used to describe local properties of the moduli space of critical metrics. We show a number of compact examples are infinitesimally rigid, and consequently, are isolated critical points in the space of unit-volume Riemannian metrics. We then give examples of critical metrics which are strict local minimizers (up to diffeomorphism and scaling). A corollary is a local “reverse Bishop’s inequality” for such metrics. In particular, any metric $g$ in a $C^{2,\alpha }$-neighborhood of the round metric $(S^n,g_S)$ satisfying $Ric(g)\le Ric(g_S)$ has volume $Vol(g) \geq Vol(g_S)$,withequalityholdingifandonlyif$g$isisometricto$g_S$.