UV physics and the speed of sound during inflation
  We consider inflation as an effective field theory and study the effects of
the addition to the Lagrangian of irrelevant operators with higher powers of
first derivatives on its dynamics and observables. We find that significant
deviations from the two-derivative dynamics are possible within the regime of
validity of the effective field theory. Focusing on monomial potentials we show
that the main effect of the terms under consideration is to reduce the speed of
sound thereby reducing the tensor fraction, while having little impact on the
scalar tilt. Crucially, these effects can arise even when the UV cut-off is
well above the inflationary Hubble parameter.
