Phantom Dirac-Born-Infeld Dark Energy
  Motivated by the apparent discrepancy between Cosmic Microwave Background
measurements of the Hubble constant and measurements from Type-Ia supernovae,
we construct a model for Dark Energy with equation of state $w = p / \rho <
-1$, violating the Null Energy Condition. Naive canonical models of so-called
"Phantom" Dark Energy require a negative scalar kinetic term, resulting in a
Hamiltonian unbounded from below and associated vacuum instability. We
construct a scalar field model for Dark Energy with $w < -1$, which nonetheless
has a Hamiltonian bounded from below in the comoving reference frame, {\it
i.e.} in the rest frame of the fluid. We demonstrate that the solution is a
cosmological attractor, and find that early-time cosmological boundary
conditions consist of a "frozen" scalar field, which relaxes to the attractor
solution once the Dark Energy component dominates the cosmological energy
density. We consider the model in an arbitrary choice of gauge, and find that,
unlike the case of comoving gauge, the fluid Hamiltonian is in fact unbounded
from below in the reference frame of a highly boosted observer, corresponding
to a nonlinear gradient instability. We discuss this in the context of general
NEC-violating perfect fluids, for which this instability is a general property.
