2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/177223The gravitational field produced by a domain wall acts as a medium with spacetime-dependent permittivity ε. Therefore, the fine structure constant α= e^2/4 πεwill be a time-dependent function at fixed position. The most stringent constraint on the time-variation of αcomes from the natural reactor Oklo and gives |\dotα/α| < few 10^{-17} yr^{-1}. This limit constrains the tension of a cosmic domain wall to be less than σ\lesssim 10^{-2} MeV^3, and then represents the most severe limit on the energy density of a cosmic wall stretching our Universe.New version, accepted for publication in MPLAAstrophysicsGeneral Relativity and Quantum CosmologyHigh Energy Physics - PhenomenologyHigh Energy Physics - Theorytext