title: Massive gravity: nonlinear instability of a homogeneous and isotropic universe.
abstract: We argue that all homogeneous and isotropic solutions in nonlinear massive gravity are unstable. For this purpose, we study the propagating modes on a Bianchi type-I manifold. We analyze their kinetic terms and dispersion relations as the background manifold approaches the homogeneous and isotropic limit. We show that in this limit, at least one ghost always exists and that its frequency tends to vanish for large scales, meaning that it cannot be integrated out from the low energy effective theory. This ghost mode is interpreted as a leading nonlinear perturbation around a homogeneous and isotropic background.
authors.affiliation: ThEP's CRL, NEP, The Institute for Fundamental Study, Naresuan University, Phitsanulok 65000, Thailand.
authors.forename: Antonio
authors.initials: A
authors.lastname: De Felice
authors.forename: A Emir
authors.initials: AE
authors.lastname: Gümrükçüoğlu
authors.forename: Shinji
authors.initials: S
authors.lastname: Mukohyama