Stability of cosmological solutions in extended quasidilaton massive gravity

Understand

We consider the stability of self-accelerating solutions to extended quasidilaton massive gravity in the presence of matter.

• By making a second or extended fiducial metric dynamical in this model, matter can cause it to evolve from a Lorentzian to Euclidean signature, triggering a ghost instability.
• We study this possibility with scalar field matter as it can model a wide range of cosmological expansion histories.
• For the $\Lambda$CDM expansion history, stability considerations substantially limit the available parameter space while for a kinetic energy dominated expansion, no choice of quasidilaton parameters is stable.