Unexorcized ghost in DGP brane world

TL;DR

This paper investigates whether modifying the DGP brane world model with a second brane can eliminate ghost instabilities, finding that such ghosts persist due to inherent correlations, despite attempts to avoid them.

Contribution

It demonstrates that adding a second brane to the DGP model cannot remove ghost instabilities because of intrinsic correlations between spin-0 and spin-2 ghosts.

Findings

Changing brane separation can remove spin-2 ghosts

Removing spin-2 ghosts introduces spin-0 ghosts

Ghost correlation persists even with stabilized brane separation

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.