2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/188014New boundary conditions are imposed on the familiar cylindrical gravitational wave vacuum spacetimes. The new spacetime family represents cylindrical waves in a flat expanding (Kasner) universe. Space sections are flat and nonconical where the waves have not reached and wave amplitudes fall off more rapidly than they do in Einstein-Rosen solutions, permitting a more regular null inifinity.Minor corrections to references. A note added in proofGeneral Relativity and Quantum Cosmologytext