2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/195787Conditions for the existence and stability of de Sitter space in modified gravity are derived by considering inhomogeneous perturbations in a gauge-invariant formalism. The stability condition coincides with the corresponding condition for stability with respect to homogeneous perturbations, while this is not the case in scalar-tensor gravity. The stability criterion is applied to various modified gravity models of the early and the present universe.22 pages, LaTeX, to appear in Phys. Rev. DGeneral Relativity and Quantum CosmologyAstrophysicsHigh Energy Physics - Theorytext