  With an energy scale that can be as high as $10^{14}\,{\rm GeV}$, inflation
may provide a unique probe of high-energy physics. Both scalar and tensor
fluctuations generated during this early accelerated expansion contain crucial
information about the particle content of the primordial universe. The advent
of ground- and space-based interferometers enables us to probe primordial
physics at length-scales much smaller than those corresponding to current CMB
constraints. One key prediction of single-field slow-roll inflation is a
red-tilted gravitational wave spectrum, currently inaccessible at
interferometer scales. Interferometers probe directly inflationary physics that
deviates from the minimal scenario and in particular additional particle
content with sizeable couplings to the inflaton field. We adopt here an
effective description for such fields and focus on the case of extra spin-2
fields. We find that a time-dependent sound speed for the helicity-2 modes can
generate primordial gravitational waves with a blue-tilted spectrum,
potentially detectable at interferometer scales.
