The New Ekpyrotic Ghost

TL;DR

The paper analyzes the new ekpyrotic scenario involving ghost condensate and finds that higher derivative terms introduce a ghost field causing vacuum instability, challenging the model's viability.

Contribution

It demonstrates that the higher derivative term in the model leads to a ghost field and vacuum instability, questioning the scenario's consistency.

Findings

Higher derivative terms affect cosmological dynamics.

Existence of a ghost field with negative energy.

Vacuum instability challenges the model's validity.

Abstract

The new ekpyrotic scenario attempts to solve the singularity problem by involving violation of the null energy condition in a model which combines the ekpyrotic/cyclic scenario with the ghost condensate theory and the curvaton mechanism of production of adiabatic perturbations of metric. The Lagrangian of this theory, as well as of the ghost condensate model, contains a term with higher derivatives, which was added to the theory to stabilize its vacuum state. We found that this term may affect the dynamics of the cosmological evolution. Moreover, after a proper quantization, this term results in the existence of a new ghost field with negative energy, which leads to a catastrophic vacuum instability. We explain why one cannot treat this dangerous term as a correction valid only at small energies and momenta below some UV cut-off, and demonstrate the problems arising when one attempts to construct a UV completion of this theory.