Role of matter in extended quasidilaton massive gravity

Understand

The extended quasidilaton theory is one of the simplest Lorentz-invariant massive gravity theories which can accommodate a stable self-accelerating vacuum solution.

• In this paper we revisit this theory and study the effect of matter fields.
• For a matter sector that couples minimally to the physical metric, we find hints of a Jeans type instability in the IR.
• In the analogue k-essence field set-up, this instability manifests itself as an IR ghost for the scalar field perturbation, but this can be interpreted as a classical instability that becomes relevant below some momentum scale in terms of matter density perturbations.