2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/2258There are several works searching for a clue of trans-Planckian physics on the primordial density perturbation spectrum. Here we would like to point out an important aspect which has been overlooked so far. When we consider a model in which the primordial density perturbation spectrum is modified due to trans-Planckian physics, the energy density of fluctuations of the inflaton field necessarily becomes significantly large, and hence its back reaction to the cosmic expansion rate cannot be neglected.3 pages, one reference is addedAstrophysicstext