Can a brane fluctuate freely?
  No, it cannot in the following sense if a self-gravitating vacuum brane is concerned. Once we write down the full set of linear perturbation equations of the system containing a self-gravitating brane, we will see that such a brane does not have its own dynamical degrees of freedom independent of those of gravitational waves which propagate in the surrounding spacetime. This statement seems to contradict with our intuition that a brane fluctuates freely on a given background spacetime in the lowest order approximation. Based on this intuition, we usually think that the dynamics of a brane can be approximately described by the equations derived from the Nambu-Goto action. In this paper we fill the gap residing between these two descriptions, showing that the dynamics of a self-gravitating brane is in fact similar to that described by a non-gravitating brane on a fixed background spacetime when the weak backreaction condition we propose in this paper is satisfied. 
Akihiro Ishibashi and Takahiro Tanaka