Minimalism as a good resolution for 2021 : introducing the Minimal Theory of Bigravity

Abstract: In 2011, F. Hassan and R. Rosen achieved the construction of the first ghost-free theory of two interacting spin-2 fields. But despite its great elegance and interesting phenomenological implications, this theory of "bigravity" suffers from a gradient-type instability. Moreover, it postulates the existence of vectorial and scalar gravitational modes, the latter being severely constrained by observations of binary pulsars. This stimulated us to construct a "minimal" theory of bigravity, ie. a theory of two interacting spin-2 fields that propagate only four tensorial degrees of freedom. This talk will first introduce the motivations that led to this "minimal" theory of bigravity, and review its construction by a Hamiltonian procedure. I will then present its cosmological phenomenology and show that it provides a stable non-linear completion of the cosmology of the Hassan-Rosen bigravity. I will end by discussing interesting phenomenological features and possible ways to test the theory.