Phantom Dirac-Born-Infeld Dark Energy

TL;DR

This paper develops a stable scalar field model for Dark Energy with an equation of state less than -1, addressing vacuum instability issues and analyzing its cosmological attractor behavior.

Contribution

It introduces a Hamiltonian-bounded scalar field model for phantom Dark Energy with w < -1, overcoming previous instability problems.

Findings

The model has a cosmological attractor solution.

Early-time boundary conditions involve a frozen scalar field.

Instabilities arise in highly boosted frames, related to NEC violation.

Abstract

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.