Figure 5: Evolution of the parameters for metric perturbations (scalar and tensorial parts), respectively, , , , , as defined in the text versus the scale factor, for . As found theoretically, the initial high velocity of the field drives both the speed of (scalar and tensor) metric perturbations to imaginary values, thus signaling a breakdown of hyperbolicity for metric perturbations (here it happens when the corresponding curves terminate, since the axis is in logarithmic scale).