Alexandra Terrana (York University, Canada)
Massive bigravity, a theoretically consistent modification of general relativity with an additional dynamical rank two tensor, successfully describes the observed accelerated expansion of the Universe without a cosmological constant. Previous analyses of perturbations around a cosmological background have revealed power law instabilities in both the scalar and tensor sectors, leading to tremendous growth in the amplitude of perturbations. The amount of growth is strongly dependent on various parameters of the theory and the initial conditions, motivating an analysis of the initial conditions, evolution, and cosmological observables to determine the viability of these theories. Here we focus on the tensor sector. We compute observables, namely the tensor contribution to the temperature anisotropies in the Cosmic Microwave Background and the present-day stochastic gravitational wave background. Finding that our results depend heavily on the initial conditions, we analyze the primordial tensor perturbations generated in an inflationary cosmology. Despite the extreme growth of perturbations, we find that inflation generically yields initial conditions that, when evolved, give rise to a stochastic background observationally indistinguishable from standard General Relativity.