Abstract:
 Modifications of gravity at large scales are motivated by the
cosmological constant problem. One particular class of modifications is
massive gravity, where the graviton is assumed to have a mass of the
order of the present Hubble expansion rate, so to influence the
cosmological expansion only now. A consistent graviton mass was first
given by Fierz and Pauli in the linearized approximation around
Minkowsky background. I will present a simple covariant construction
which reduces to the Fierz-Pauli theory in the weak field limit. In the
second part of the talk I will discuss the phenomenology of a different
model recently introduced by Arkani-Hamed et al. (ghost condensation),
where Lorentz symmetry is spontaneously broken by the gradient of a
scalar field. In particular, I will show how the motion of a source
drastically affects the phenomenological signatures of this model.