Horizon-flow off-track for inflation

Inflation can be parametrized by means of truncated flow equations. In this "horizon-flow" setup, generic results have been obtained, such as typical values for r/(1-nS). They are sometimes referred to as intrinsic features of inflation itself. In this paper we first show that the phenomenological class of inflationary potentials sampled by horizon flow is directly responsible for such predictions. They are therefore anything but generic. Furthermore, the horizon-flow setup is shown to rely on trajectories in phase space that differ from the slow roll. For a given potential, we demonstrate that this renders horizon flow blind to entire relevant inflationary regimes, for which the horizon-flow trajectory is shown to be unstable. This makes horizon flow a biased parametrization of inflation.