A short review of “DGP Specteroscopy”

In this paper we provide a short review of the main results developed in hep-th/0604086. We focus on linearised vacuum perturbations about the selfaccelerating branch of solutions in the DGP model. These are shown to contain a ghost in the spectrum for any value of the brane tension. We also comment on hep-th/0607099, where some counter arguments have been presented. Recent observations of high redshift supernovae suggest that dark energy accounts for roughly 70% of the energy content of our universe [1]. This dark energy is consistent with a small positive cosmological constant, Λ ∼ 10−12 (eV), exerting negative pressure on the universe, causing its expansion to accelerate. If we wish to resort to effective field theory methods to explain the origin of the cosmological constant, we typically run into an horrendous fine tuning problem. For a field theory cut off at the Planck scale, mpl, the natural value of the cosmological constant would be of the order mpl, which is 10 120 orders of magnitude larger than the observed value. This problem has inspired a search for alternative explanations of the current cosmic acceleration. One possibility is that it is due to new gravitational physics kicking in at the current Hubble scale, H ∼ 10−34eV (see, for example [2]). In this paper we will focus on the DGP model [3], which has arguably received more attention than any other model in which gravity is modified on ultra large scales. The model consists of a Z2 symmetric 3-brane embedded in 5D Minkowski space, described by the following action