Hyperboloidal approach for linear and non-linear wave equations in FLRW spacetimes

In this numerical work, we deal with two distinct problems concerning the propagation of waves in cosmological backgrounds. In both cases, we employ a spacetime foliation given in terms of compactified hyperboloidal slices. These slices intersect future null infinity, so our method is well-suited to study the long-time behaviour of waves. Moreover, our construction is adapted to the presence of the time--dependent scale factor that describes the underlying spacetime expansion. First, we investigate decay rates for solutions to the linear wave equation in a large class of expanding FLRW spacetimes, whose non--compact spatial sections have either zero or negative curvature. By means of a hyperboloidal foliation, we provide new numerical evidence for the sharpness of decay--in--time estimates for linear waves propagating in such spacetimes. Then, in the spatially-flat case, we present numerical results in support of small data global existence of solutions to semi-linear wave equations in FLRW spacetimes having a decelerated expansion, provided that a generalized null condition holds. In absence of this null condition and in the specific case of $\square_g \phi = (\partial_t \phi)^2$ (Fritz John's choice), the results we obtain suggest that, when the spacetime expansion is sufficiently slow, solutions diverge in finite time for every choice of initial data.