More on ghosts in DGP model

TL;DR

The paper demonstrates that the self-accelerating cosmological solution in the DGP model contains a ghost in its perturbative spectrum. Through an explicit Hamiltonian analysis of the helicity-0 sector and a parallel Abelian-Stückelberg toy model, the authors show the instability arises from mixing between localized gravitational modes and brane bending modes, leading to an energy unbounded from below. They derive a four-dimensional effective action for the localized modes and reveal a non-diagonalizable, resonant structure at the self-accelerating limit Hr_c=1, which can be recast with Stückelberg fields. The result casts serious doubt on the viability of the self-accelerating branch for cosmological acceleration and motivates seeking ghost-free IR modifications of gravity.

Abstract

It is shown by an explicit calculation that the excitations about the self-accelerating cosmological solution of the Dvali--Gabadaze--Porrati model contain a ghost mode. This raises serious doubts about viability of this solution. Our analysis reveals the similarity between the quadratic theory for the perturbations around the self-accelerating Universe and an Abelian gauge model with two Stueckelberg fields.