Trans-Planckian problem of inflationary cosmology - art. no. 123501

In most current models of inflation based on a weakly self-coupled scalar m atter field minimally coupled to gravity, the period of inflation lasts so long that, at the beginning of the inflationary period, the physical wavele ngths of comoving scales which correspond to the present large-scale struct ure of the Universe were smaller than the Planck length. Thus, the usual co mputations of the spectrum of fluctuations in these models involve extrapol ating low-energy physics (both in the matter and gravitational sector) into regions where this physics is not applicable. In this article we study the dependence of the usual predictions of inflation for the spectrum of cosmo logical fluctuations on the hidden assumptions about super-Planck scale phy sics. We introduce a class of modified dispersion relations to mimic possib le effects of super-Planck scale physics, and find that, given an initial s tate determined by minimizing the energy density, for dispersions relations introduced by Unruh the spectrum is unchanged, whereas for a class of disp ersion relations similar to those used by Corley and Jacobson (which involv e a more radical departure from the usual linear relation) important deviat ions from the usual predictions of inflation can be obtained. Some implicat ions of this result for the unification of fundamental physics and early Un iverse cosmology are discussed.