Einstein metrics on homogeneous spaces $H\times H\mathrm{/}\mathrm{\Delta }K$

Given any compact homogeneous space $H\mathrm{/}K$ with $H$ simple, we consider the new space $M=H\times H\mathrm{/}\mathrm{\Delta }K$, where $\mathrm{\Delta }K$ denotes diagonal embedding, and study the existence, classification and stability of $H\times H$-invariant Einstein metrics on $M$, as a first step into the largely unexplored case of homogeneous spaces of compact non-simple Lie groups. We find unstable Einstein metrics on $M$ for most spaces $H\mathrm{/}K$ such that their standard metric is Einstein (e.g., isotropy irreducible) and the Killing form of $\mathfrak{k}$ is a multiple of the Killing form of $\mathfrak{h}$ (e.g., $K$ simple), a class which contains $17$ families and $50$ individual examples. A complete classification is obtained in the case when $H\mathrm{/}K$ is an irreducible symmetric space with $K$ simple. We also study the behavior of the scalar curvature function on the space of all normal metrics on $M=H\times H\mathrm{/}\mathrm{\Delta }K$ (none of which is Einstein), obtaining that the standard metric is a global minimum.