Trans-Planckian Dark Energy?

TL;DR

This paper critically examines a trans-Planckian scalar field model for dark energy, demonstrating it faces fundamental issues like ill-defined vacuum states and excessive energy production, thus failing to explain dark energy.

Contribution

The paper provides a detailed critique showing the model's inability to produce reliable predictions and its problematic energy implications, challenging previous proposals linking trans-Planckian physics to dark energy.

Findings

No well-defined vacuum state in the considered wave-number region.

Model predicts excessive energy density incompatible with observations.

Fine-tuning of initial data is required but insufficient to resolve issues.

Abstract

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.