Examining the Viability of Phantom Dark Energy
  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.
