Abstract
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 λDGP. We show that this theory has strong interactions at a length scale λ3 ∼ (λDGP2/MP)1/3. If λDGP is of order the Hubble length, then the theory loses predictivity at distances shorter than λ3 ∼ 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 λ3. Finally, we examine the dynamics of the longitudinal Goldstone mode when the background geometry is curved.
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