Enhancing Inflationary Tensor Modes through Spectator Fields

In the inflationary scenario, a spectator scalar field is a light field that plays no role whatsoever in the inflationary dynamics, neither at the background level (it does not give a significant contribution to the energy density during or after inflation), nor at the level of generating the final curvature perturbations (which we assume to be predominantly sourced by the inflaton field). We show that a large contribution to tensor modes during inflation can be generated by a spectator scalar field with speed of sound lower than unity.