Unexorcized ghost in DGP brane world
Keisuke Izumi, Kazuya Koyama, Takahiro Tanaka
TL;DR
The paper investigates ghost-free prospects in the DGP brane world by adding a second brane and by stabilizing brane separation with a bulk scalar. It shows a robust correlation between spin-2 and spin-0 ghosts: removing a spin-2 ghost via a two-brane setup transfers the ghost to a brane-bending scalar, and stabilization with a bulk scalar does not prevent the ghost from reappearing due to critical-mass degeneracy. The analysis derives the perturbation spectrum, boundary conditions, and the four-dimensional effective action, revealing that no ghost-free configuration exists within these linear two-brane constructions. The work highlights a fundamental obstacle posed by the Higuchi-like critical mass and brane-bending dynamics, underscoring the need for alternative mechanisms beyond simple two-brane extensions.
Abstract
The braneworld model of Dvali-Gabadadze-Porrati realizes the self-accelerating universe. However, it is known that this cosmological solution contains a spin-2 ghost. We study the possibility of avoiding the appearance of the ghost by slightly modifying the model, introducing the second brane. First we consider a simple model without stabilization of the separation of the brane. By changing the separation between the branes, we find we can erase the spin-2 ghost. However, this can be done only at the expense of the appearance of a spin-0 ghost instead. We discuss why these two different types of ghosts are correlated. Then, we examine a model with stabilization of the brane separation. Even in this case, we find that the correlation between spin-0 and spin-2 ghosts remains. As a result we find we cannot avoid the appearance of ghost by two-branes model.