### 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.