A general theory of linear cosmological perturbations: stability conditions, the quasistatic limit and dynamics

Understand

We analyse cosmological perturbations around a homogeneous and isotropic background for scalar-tensor, vector-tensor and bimetric theories of gravity.

• Building on previous results, we propose a unified view of the effective parameters of all these theories.
• Based on this structure, we explore the viable space of parameters for each family of models by imposing the absence of ghosts and gradient instabilities.
• We then focus on the quasistatic regime and confirm that all these theories can be approximated by the phenomenological two-parameter model described by an effective Newton's constant and the gravitational slip.