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.
