Backreaction in trans-Planckian cosmology: Renormalization, trace anomaly, and self-consistent solutions

We analyze the semiclassical Einstein equations for quantum scalar fields satisfying modified dispersion relations. We first discuss in detail the renormalization procedure based on adiabatic subtraction and dimensional regularization. We show that, contrary to what is expected from power counting arguments, in 3+1 dimensions the subtraction involves up to the fourth adiabatic order even for dispersion relations containing higher powers of the momentum. Then we analyze the dependence of the trace of the renormalized energy-momentum tensor with the scale of new physics, and we recover the usual trace anomaly in the appropriate limit. We also find self-consistent de Sitter solutions for dispersion relations that contain up to the fourth power of the momentum. Using this particular example, we also discuss the possibility that the modified dispersion relation can be mimicked at lower energies by an effective initial state in a theory with the usual dispersion relation.