Jeans' Ghost

TL;DR

The paper demonstrates that a massless scalar field coupled to gravity can exhibit a low-energy tachyonic ghost, which manifests as a Jeans-like instability in large-wavelength modes, without causing quantum vacuum instability.

Contribution

It reveals that low-energy tachyonic ghosts can be reinterpreted as fluid instabilities, challenging the notion that all ghosts lead to catastrophic quantum instabilities.

Findings

Identifies conditions for a scalar field to become a tachyonic ghost.

Shows the ghost can be recast as a fluid with Jeans-like instability.

Demonstrates low-energy ghosts do not necessarily cause vacuum instability.

Abstract

We show that a massless canonical scalar field minimally coupled to general relativity can become a tachyonic ghost at low energies around a background in which the scalar's gradient is spacelike. By performing a canonical transformation we demonstrate that this low energy ghost can be recast, at the level of the action, in a form of a fluid that undergoes a Jeans-like instability affecting only modes with large wavelength. This illustrates that low-energy tachyonic ghosts do not lead to a catastrophic quantum vacuum instability, unlike the usual high-energy ghost degrees of freedom.