  The tensor to scalar ratio is affected by the evolution of the large-scale
gauge fields potentially amplified during an inflationary stage of expansion.
After deriving the exact evolution equations for the scalar and tensor modes of
the geometry in the presence of dynamical gauge fields, it is shown that the
tensor to scalar ratio is bounded from below by the dominance of the adiabatic
contribution and it cannot be smaller than one thousands whenever the
magnetogenesis is driven by a single inflaton field.
