Decomposable $(4,7)$ solutions in eleven-dimensional supergravity

Consider an oriented four-dimensional Lorentzian manifold $(\widetilde{M}^{3, 1}, \widetilde{g})$andanorientedseven-dimensionalRiemannianmanifold$(M^{7}, g)$. 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 ${\tilde{M}}^{3,1}$ 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 $\varphi ={\star }_7{F}^4$. Moreover, the supergravity equation reduces to the condition that $({\tilde{M}}^{3,1},\tilde{g})$ 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 $\varphi$. Whenever this 3-form is generic, the Maxwell equation induces a weak ${\mathrm{G}}_2$-structure on $M^7$ and then we obtain decomposable supergravity backgrounds given by the product of a weak ${\mathrm{G}}_2$-manifold $(M^7,\varphi ,g)$ with a Lorentzian Einstein manifold $({\tilde{M}}^{3,1},\tilde{g})$. We classify homogeneous 7-manifolds $M^7=G\mathrm{/}H$ of a compact Lie group $G$ and indicate the cosets which admit an invariant or non-invariant ${\mathrm{G}}_2$-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.