Gravitational birefringence of light in Robertson-Walker cosmologies

The spacetime evolution of massless spinning particles in a Robertson-Walker background is derived using the deterministic system of equations of motion due to Papapetrou, Souriau and Saturnini. A numerical integration of this system of differential equations in the case of the standard model of cosmology is performed. The deviation of the photon worldlines from the null geodesics is of the order of the wavelength. Perturbative solutions are also worked out in a more general case. An experimental measurement of this deviation would test the acceleration of our expanding universe.