  We compute the universal generic corrections to the inflationary power
spectrum due to unknown high-energy physics. We arrive at this result via a
careful integrating out of massive fields in the "in-in" formalism yielding a
consistent and predictive low-energy effective description in time-dependent
backgrounds. We find that the power spectrum is universally modified at order
H/M, where H is the scale of inflation. This is qualitatively different from
the universal corrections in time-independent backgrounds, and it suggests that
such effects may be present in upcoming cosmological observations.
