Quantum-like interpretation of Dirac wave equation in Robertson-Walker geometry

The Dirac wave equation is separated in the Robertson-Walker metric. The resulting radial equation is interpreted as a one dimensional quantum-like equation that is explicitly solved. There results that the energy spectrum, that is determined in the flat, open and closed universe, is independent of the mass of the particle. Moreover it is the same of the massless neutrino case previously studied. In the closed metric case the discrete positive spectrum is asymptotically determined. The separation of the energy levels is however very far from being experimentally tested.