arXiv2018
Consider an oriented four-dimensional Lorentzian manifold $(\widetilde{M}^{3, 1}, \widetilde{g})$ and an oriented seven-dimensional Riemannian manifold $(M^{7}, g)$. We describe a class of decomposable eleven-dimensional supergravity backgrounds on the product manifold $({\mathcal{M}}^{10, 1}=\widetilde{M}^{3,1} \times M^7, g_{\mathcal{M}}=\widetilde{g}+g)$, endowed with a flux form given in terms of the volume form on $\widetilde{M}^{3, 1}$ and a closed $4$-form $F^{4}$ on $M^{7}$. We show that the Maxwell equation for such a flux form can be read in terms of the co-closed 3-form $ϕ=\star_{7}F^{4}$. Moreover, the supergravity equation reduces to the condition that $(\widetilde{M}^{3,1},\widetilde{g})$ is an Einstein manifold with negative Einstein constant and $(M^7, g, F)$ is a Riemannian manifold which satisfies the Einstein equation with a stress-energy tensor associated to the 3-form $ϕ$. Whenever this 3-form is generic, the Maxwell equation induces a weak ${\rm G}_2$-structure on $M^{7}$ and then we obtain decomposable supergravity backgrounds given by the product of a weak ${\rm G}_2$-manifold $(M^7, ϕ, g)$ with a Lorentzian Einstein manifold $(\widetilde{M}^{3,1},\widetilde{g})$. We classify homogeneous 7-manifolds $M^{7}=G/H$ of a compact Lie group $G$ and indicate the cosets which admit an invariant or non-invariant ${\rm G}_2$-structure, or even no ${\rm G}_2$-structure. Then we construct examples of compact homogeneous Riemannian 7-manifolds endowed with non-generic invariant 3-forms which satisfy the Maxwell equation, but the construction of decomposable homogeneous supergravity backgrounds of this type remains an open problem.