Dynamics of sub-horizon tensor modes in bimetric gravity

In this talk I will examine the propagation of gravitational waves in bimetric gravity, a theory formulated in terms of two metrics with, in principle, different Planck masses. In particular, I will consider sub-horizon tensor modes on a homogeneous and isotropic cosmological background with non-zero spatial curvature. These waves can be described by a system of two coupled oscillators with time-dependent frequencies and interaction terms. By introducing action and angle variables, the dynamical equations are rewritten as a system of four first-order differential equations. Making use of two dimensionless quantities of the system (the ratio between the Planck masses and the dimensionless mode number), which provide two natural scales, we apply specific approximate analytical methods, suited to each scale, to study the behavior of the solutions in different regimes.