| dc.description |
The model of Dvali, Gabadadze, and Porrati (DGP) gives a simple geometrical setup in which gravity becomes 5-dimensional at distances larger than a length scale λ<sub>DGP</sub>. We show that this theory has strong interactions at a length scale λ<sub>3</sub> ∼ (λ<sub>DGP</sub><sup>2</sup>/M<sub>P</sub>)<sup>1/3</sup>. If λ<sub>DGP</sub> is of order the Hubble length, then the theory loses predictivity at distances shorter than λ<sub>3</sub> ∼ 1000 km. The strong interaction can be viewed as arising from a longitudinal `eaten Goldstone' mode that gets a small kinetic term only from mixing with transverse graviton polarizations, analogous to the case of massive gravity. We also present a negative-energy classical solution, which can be avoided by cutting off the theory at the same scale scale λ<sub>3</sub>. Finally, we examine the dynamics of the longitudinal Goldstone mode when the background geometry is curved. |
