Enhancing inflationary tensor modes through spectator fields

We show that a large contribution to tensor modes during inflation can be generated by a spectator scalar field with a speed of sound lower than unity.

Figure 1

The normalized tensor-mode power spectrum as a function of ($-k\tau$). Either $k$ or $\tau$ can be kept fixed; both choices will generate the behavior above.

Figure 2

The normalized tensor-mode power spectrum as a function of $c_s$ on super-Hubble scales (red triangles) with the function with scaling $c_s^{-18/5}$ superimposed.