Trans-Planckian Modes, Back-Reaction, and the Hawking Process
  Hawking's prediction of black-hole evaporation depends on the application of known physics to fantastically high energies -- well beyond the Planck scale. Here, I show that before these extreme regimes are reached, another physical effect will intervene: the quantum backreaction on the collapsing matter and its effect on the geometry through which the quantum fields propagate. These effects are estimated by a simple thought experiment. When this is done, it appears that there are no matrix elements allowing the emission of Hawking quanta: black holes do not radiate. 
Adam D. Helfer