New Sources of Gravitational Waves during Inflation
  We point out that detectable inflationary tensor modes can be generated by particle or string sources produced during inflation, consistently with the requirements for inflation and constraints from scalar fluctuations. We show via examples that this effect can dominate over the contribution from quantum fluctuations of the metric, occurring even when the inflationary potential energy is too low to produce a comparable signal. Thus a detection of tensor modes from inflation does not automatically constitute a determination of the inflationary Hubble scale. 
Leonardo Senatore, Eva Silverstein, and Matias Zaldarriaga
hep-th astro-ph.CO gr-qc