2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/36099We study in a Brill-Hartle type of approximation the back-reaction of a superposition of linear gravitational waves in an Einstein-de Sitter background up to the second order in the small wave amplitudes $h_{ik}$. The wave amplitudes are assumed to form a homogeneous and isotropic stochastic process. No restriction for the wavelengths is assumed. The effective stress-energy tensor $T^{e}_{μν}$ is calculated in terms of the correlation functions of the process. We discuss in particular a situation where $T^{e}_{μν}$ is the dominant excitation of the background metric. Apart from the Tolman radiation universe, a solution with the scale factor of the de Sitter universe exists with $p = -ρ$ as effective equation of state.3 pages, Latex (requires mprocl.sty). To appear in the Proceedings of the Eighth Marcel Grossmann meeting (Jerusalem, June 22-27, 1977)General Relativity and Quantum Cosmologytext