  Inflation typically predicts a quasi scale-invariant spectrum of
gravitational waves. In models of slow-roll inflation, the amplitude of such a
background is too small to allow direct detection without a dedicated
space-based experiment such as the proposed BBO or DECIGO. In this paper we
note that particle production during inflation can generate a feature in the
spectrum of primordial gravitational waves. We discuss the possibility that
such a feature might be detected by ground-based laser interferometers such as
Advanced LIGO and Advanced Virgo, which will become operational in the next few
years. We also discuss the prospects of detection by a space interferometer
like LISA. We first study gravitational waves induced by nonperturbative,
explosive particle production during inflation: while explosive production of
scalar quanta does not generate a significant bump in the primordial tensor
spectrum, production of vectors can. We also show that chiral gravitational
waves produced by electromagnetic fields amplified by an axion-like inflaton
could be detectable by Advanced LIGO.
