We investigate what can be said about the interaction of scalar fields with
Abelian gauge fields during a quasi-de Sitter phase of expansion and under the
assumption that the electric and the magnetic susceptibilities do not coincide.
The duality symmetry, transforming the magnetic susceptibility into the inverse
of the electric susceptibility, exchanges the magnetic and electric power
spectra. The mismatch between the two susceptibilities determines an effective
refractive index affecting the evolution of the canonical fields. The
constraints imposed by the duration of the inflationary phase and by the
magnetogenesis requirements pin down the rate of variation of the
susceptibilities that is consistent with the observations of the magnetic field
strength over astrophysical and cosmological scales but avoids back-reaction
problems. The parameter space of this magnetogenesis scenario is wider than in
the case when the susceptibilities are equal, as it happens when the inflaton
or some other spectator field is solely coupled to the standard gauge kinetic
term.Comment: 29 pages, 4 figure
