2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/34543We consider an Einstein-Maxwell action modified by the addition of three terms coupling the electromagnetic strength to the curvature tensor. The corresponding generalized Maxwell equations imply a variation of the speed of light in a vacuum. We determine this variation in Friedmann-Robertson-Walker spacetimes. We show that light propagates at a speed greater than $c$ when a simple condition is satisfied.Invited lecture given at the Meeting on Electromagnetism organized by the Fondation Louis de Broglie in Peyresq, September 2-7, 2002; 13 pages. The new version gives the explicit definition of $\tilde{a}$ in Theorem 1General Relativity and Quantum Cosmologytext