Failures of homogeneous and isotropic cosmologies in Extended Quasi-Dilaton Massive Gravity

Understand

We analyze the Extended Quasi-Dilaton Massive Gravity model around a Friedmann-Lema\^itre-Robertson-Walker cosmological background.

• We present a careful stability analysis of asymptotic fixed points.
• We find that the traditional fixed point cannot be approached dynamically, except from a perfectly fine-tuned initial condition involving both the quasi-dilaton and the Hubble parameter.
• A less-well examined fixed-point solution, where the time derivative of the 0-th St\"uckelberg field vanishes $\dot\phi^0=0$, encounters no such difficulty, and the fixed point is an attractor in some finite region of initial conditions.