Trans-Planckian particle creation in cosmology and ultra-high energy cosmic rays

We consider observational constraints on creation of particles induced by hypothetical trans-Planckian effects during the current stage of the Universe expansion. We show that compatibility with the diffuse $\gamma$-ray background measured by the EGRET experiment strongly restricts this creation. In particular, it rules out the possibility to detect signatures of such short distance effects in anisotropies of the cosmic microwave background radiation. On the other hand, a possibility that some part of ultra-high energy cosmic rays originates from new trans-Planckian physics remains open.