Modified Gravity: living without Birkhoff I. DGP

We consider the consequences of the absence of Birkhoff's theorem in theories of modified gravity. As an example, we calculate the gravitational force on a test particle due to a spherical mass shell in the Dvali-Gabadaze-Porrati model (DGP). We show that unlike in General Relativity, the force depends on the mass distribution. In particular, the gravitational force within a spherical mass shell depends on the geometric structure of the bulk, and is likely non-zero.