Effect of torsion in Dirac equation for Coulomb potential in Robertson-Walker space-time

The Dirac equation with Coulomb-like potential and self-interaction term, that originates from torsion, is studied in the Robertson-Walker space-time. It is shown that the angular dependence of the equation can be separated also in presence of nonlinear terms. Under reasonable physical assumptions, the time dependence is also separated. An extended perturbative calculation can then be applied qualitatively. The conclusion is that the perturbation of the energy levels of the system, as consequence of the self-interacting term, is not relevant on physical grounds.