We confirm the validity of our analysis by performing numerically the integration in a particular case where the solution inside the thick cod-2 brane is known exactly. We suggest that the difference lies in the procedure used to evaluate the pillbox integration across the codimension-2 brane.
GRAVITY CIRCUIT GHOST TREE FREE
Differently from the results in the literature, our analysis implies that, choosing the background tension suitably, we can construct ghost-free models for any value of the free parameters of the theory. We however find a value of the critical tension which is different from the one already found in the literature.
We give a geometrical interpretation of the existence of the critical tension and of the reason why the relevant field is a ghost or not depending on the background tension. We show that, more » in a suitable 4D limit, the trace part of the 4D metric perturbations obeys a decoupled equation which suggests that it is a ghost for background tensions smaller than a critical tension, while it is a healthy field otherwise. We adopt a 4D scalar-vector-tensor decomposition, and focus on the scalar sector of perturbations.
We consider the solutions corresponding to pure tension sources on the 4D brane, and study perturbations at first order around these background solutions. To deal with the ambiguity in the thin limit of branes of codimension higher than one, we consider a specific regularization of the internal structure of the branes where the 5D brane can be considered thin with respect to the 4D one. We study the behaviour of weak gravitational fields in the 6D Cascading DGP model using a bulk-based approach.
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