Can a spectator scalar field enhance inflationary tensor mode?
  We consider the possibility of enhancing the inflationary tensor mode by introducing a spectator scalar field with a small sound speed which induces gravitational waves as a second order effect. We analytically obtain the power spectra of gravitational waves and curvature perturbation induced by the spectator scalar field. We found that the small sound speed amplifies the curvature perturbation much more than the tensor mode and the current observational constraint forces the induced gravitational waves to be negligible compared with those from the vacuum fluctuation during inflation. 
Tomohiro Fujita, Jun'ichi Yokoyama and Shuichiro Yokoyama
astro-ph.CO gr-qc hep-ph hep-th