  We investigate the presence of instabilities in the Cascading DGP model. We
start by discussing the problem of the cosmological late time acceleration, and
we introduce the modified gravity approach. We then focus on brane induced
gravity models and in particular on the Cascading DGP model. We consider
configurations of the latter model where the source term is given simply by
vacuum energy (pure tension), and we study perturbations at first order around
these configurations. We perform a four-dimensional scalar-vector-tensor
decomposition of the perturbations, and show that, regarding the scalar sector,
the dynamics in a suitable limit can be described by a master equation. This
master equation contains an energy scale (critical tension) which is related in
a non-trivial way to the parameters of the model. We give a geometrical
interpretation of why this scale emerges, and explain its relevance for the
presence of ghost instabilities in the theory. We comment on the difference
between our result and the one present in the literature, and stress its
importance regarding the phenomenological viability of the model. We finally
provide a numerical check which confirms the validity of our analysis.
