Examining the Viability of Phantom Dark Energy

TL;DR

This paper investigates whether phantom dark energy necessarily implies negative kinetic energy, considering observational constraints and perturbations, and finds that negative kinetic energy is not categorically unavoidable for phantom or quintessence models.

Contribution

It challenges the standard assumption that phantom dark energy must have negative kinetic energy by exploring more accurate models with perturbations and dynamic equations of state.

Findings

Phantom dark energy does not necessarily have negative kinetic energy at all scales and times.

Quintessence dark energy does not necessarily have positive kinetic energy at all scales and times.

Perturbative and dynamic models can alter the kinetic energy interpretation of dark energy.

Abstract

In the standard cosmological framework of the 0th-order FLRW metric and the use of perfect fluids in the stress-energy tensor, dark energy with an equation-of-state parameter $w < -1$ (known as phantom dark energy) implies negative kinetic energy and vacuum instability when modeled as a scalar field. However, the accepted values for present-day $w$ from Planck and WMAP9 include a significant range of values less than $-1$. We find that it is not as obvious as one might think that phantom dark energy has negative kinetic energy categorically. Analogously, we find that field models of quintessence dark energy ($w_\phi>-1$) do not necessarily have positive kinetic energy categorically. Staying within the confines of observational constraints and general relativity, for which there is good experimental validation, we consider a few reasonable departures from the standard 0th-order framework in an attempt to see if negative kinetic energy can be avoided in these settings despite an apparent $w<-1$. We consider a more accurate description of the universe through the perturbing of the isotropic and homogeneous FLRW metric and the components of the stress-energy tensor, and we consider dynamic $w$ and primordial isocurvature and adiabatic perturbations. We find that phantom dark energy does not necessarily have negative kinetic energy for all relevant length scales at all times, and we also find that, by the same token, quintessence dark energy does not necessarily have positive kinetic energy for all relevant length scales at all times.