On the rigidity of the complex Grassmannians

We study the integrability to second order of the infinitesimal Einstein deformations of the symmetric metric $g$ on the complex Grassmannian of $k$-planes inside ${\mathbb{C}}^n$. By showing the nonvanishing of Koiso’s obstruction polynomial, we characterize the infinitesimal deformations that are integrable to second order as an explicit variety inside $\mathfrak{s}\mathfrak{u}(n)$. In particular we show that $g$ is isolated in the moduli space of Einstein metrics if $n$ is odd.