What can we learn by probing Trans-Planckian physics

In this talk we address the issue of how the observables in our present Universe are affected by processes that may have occured at superplanckian energies (referred to as the {\it transplanckian regime}). For example, the origin of the cosmological perturbation spectrum. We model the transplanckian regime by introducing a 1-parameter family of smooth non-linear dispersion relations which modify the frequencies at very short distances. For this family of dispersions, we present the exact solutions and show that the CMBR spectrum is that of a (nearly) black body, and that the adiabatic vacuum is the only choice for the initial conditions. A particular feature of the family of dispersion functions chosen is the production of ultralow frequencies at very high momenta $k$ (for $k>M_P$). Modes with ultralow frequencies equal or less than the current Hubble rate are still frozen today. Therefore, their energy today provides a strong candidate for the dark energy of the Universe.