  It has recently been proposed by Mersini et al. 01, Bastero-Gil and Mersini
02 that the dark energy could be attributed to the cosmological properties of a
scalar field with a non-standard dispersion relation that decreases
exponentially at wave-numbers larger than Planck scale (k_phys > M_Planck). In
this scenario, the energy density stored in the modes of trans-Planckian
wave-numbers but sub-Hubble frequencies produced by amplification of the vacuum
quantum fluctuations would account naturally for the dark energy. The present
article examines this model in detail and shows step by step that it does not
work. In particular, we show that this model cannot make definite predictions
since there is no well-defined vacuum state in the region of wave-numbers
considered, hence the initial data cannot be specified unambiguously. We also
show that for most choices of initial data this scenario implies the production
of a large amount of energy density (of order M_Planck^4) for modes with
momenta of order M_Planck, far in excess of the background energy density. We
evaluate the amount of fine-tuning in the initial data necessary to avoid this
back-reaction problem and find it is of order H/M_Planck. We also argue that
the equation of state of the trans-Planckian modes is not vacuum-like.
Therefore this model does not provide a suitable explanation for the dark
energy.
