  We show that a general but shear-free perturbation of homogeneous and
isotropic universes are necessarily silent, without any gravitational waves. We
prove this in two steps. First we establish that a shear free perturbation of
these universes are acceleration-free and the fluid flow geodesics of the
background universe maps onto themselves in the perturbed universe. This effect
then decouples the evolution equations of the electric and magnetic parts of
the Weyl tensor in the perturbed spacetimes and the magnetic part no longer
contains any tensor modes. Although the electric part, that drives the tidal
forces, do have tensor modes sourced by the anisotropic stress, these modes
have homogeneous oscillations at every point on a time slice without any wave
propagation. We also show the presence of vorticity vector waves that are
sourced by the curl of heat flux. This analysis shows the critical role of the
shear tensor in generating cosmological gravitational waves in an expanding
universe.
