On the Null Energy Condition and Cosmology

TL;DR

This paper examines models violating the null energy condition in cosmology, analyzing their theoretical consistency, stability issues, and potential for UV completion, with implications for singularity resolution and dark energy.

Contribution

It evaluates two recent NEC-violating models, highlighting their stability challenges and exploring their theoretical foundations and implications for cosmological theories.

Findings

Ghost condensate model has unbounded energy and ghost fields.

Stringy nonlocal model admits UV completion from superstring theory.

Both models face stability and energy spectrum issues.

Abstract

Field theories which violate the null energy condition (NEC) are of interest for the solution of the cosmological singularity problem and for models of cosmological dark energy with the equation of state parameter $w<-1$. We discuss the consistency of two recently proposed models that violate the NEC. The ghost condensate model requires higher-order derivative terms in the action. It leads to a heavy ghost field and unbounded energy. We estimate the rates of particles decay and discuss possible mass limitations to protect stability of matter in the ghost condensate model. The nonlocal stringy model that arises from a cubic string field theory and exhibits a phantom behavior also leads to unbounded energy. In this case the spectrum of energy is continuous and there are no particle like excitations. This model admits a natural UV completion since it comes from superstring theory.