Null Energy Condition and Dark Energy Models

TL;DR

This paper investigates how the Null Energy Condition constrains dark energy models, showing that while eternal phantom dark energy is excluded, transient phantom phases are compatible with NEC in Quintom models.

Contribution

It demonstrates that NEC restricts dark energy to not be phantom forever, and explores how Quintom models can satisfy NEC during transient phantom phases.

Findings

NEC excludes eternal phantom dark energy.

Transient phantom phases can satisfy NEC in Quintom models.

Different Quintom models demonstrate compatibility with NEC during short phantom periods.

Abstract

Null Energy Condition (NEC) requires the equation of state (EoS) of the universe $w_u$ satisfy $w_u\geq-1$, which implies, for instance in a universe with matter and dark energy dominating $w_u=w_m\Omega_m+w_{de}\Omega_{de}=w_{de}\Omega_{de}\geq-1$. In this paper we study constraints on the dark energy models from the requirement of the NEC. We will show that with $\Omega_{de}\sim0.7$, $w_{de}<-1$ at present epoch is possible. However, NEC excludes the possibility of $w_{de}<-1$ forever as happened in the Phantom model, but if $w_{de}<-1$ stays for a short period of time as predicted in the Quintom theory NEC can be satisfied. We take three examples of Quintom models of dark energy, namely the phenomenological EoS, the two-scalar-field model and the single scalar model with a modified Dirac-Born-Infeld (DBI) lagrangian to show how this happens.