Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: 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.

4
The categories of this paper are: cond-mat
33
The categories of this paper are: plasm-ph
35
The categories of this paper are: mtrl-th
17
The categories of this paper are: q-fin
2
The categories of this paper are: math
18
The categories of this paper are: econ
16
The categories of this paper are: stat
5
The categories of this paper are: gr-qc
22
The categories of this paper are: chao-dyn
21
The categories of this paper are: adap-org
10
The categories of this paper are: q-bio
3
The categories of this paper are: physics
20
The categories of this paper are: acc-phys
23
The categories of this paper are: solv-int
30
The categories of this paper are: q-alg
27
The categories of this paper are: comp-gas