Symmetries and assisted inflation in the Dvali-Gabadadze-Porrati braneworld model

We present here the general transformation that leaves unchanged the form of the field equations for perfect fluid cosmologies in the Dvali-Gabadadze-Porrati (DGP) brane-world model. Specifically, a prescription for relating exact solutions with different equations of state is provided, and the symmetries found can be used as algorithms for generating new cosmological models from previously known ones. We also present, implicitly, the first known exact DGP perfect fluid spacetime. A particular case of the general transformation is used to illustrate the crucial role played both by the number of scalar fields and the extra-dimensional effects in the occurrence of inflation. In particular, we see that assisted inflation does not proceed at all times for one of the two possible ways in which the brane can be embedded into the bulk