| Abstract: The so-called trans-Planckian problem is the statement that in present-day cosmological observables there could be signatures of physics beyond the Planck scale, because of the exponential "stretching" of modes that took place during inflation. A popular counterargument is that, as long the Hubble rate during inflation is sub-Planckian, the expansion of the universe is adiabatic as far as trans-Planckian modes are concerned, and thus can only have exponentially small effects on them. We give quantitative support to this view by performing explicit computations in a new set of coordinates, which highlights that, for local physics, the expansion of the universe can be dealt with in perturbation theory. |