

6.6 Tensor perturbations
The covariant description of tensor modes on the brane is via the
shear, which satisfies the wave equation [219]
Unlike the density and vector perturbations, there is no closed
system on the brane for large scales. The KK anisotropic stress
is an unavoidable source for tensor modes on the
brane. Thus it is necessary to use the 5D metric-based formalism.
This is the subject of the next Section 7.

