  We study the propagation of axial gravitational waves in Friedman universes.
The evolution equation is obtained in the Regge-Wheeler gauge. The
gravitational waves obey the Huygens principle in the radiation dominated era,
but in the matter dominated universe their propagation depends on their
wavelengths, with the scale fixed essentially by the Hubble radius. Short waves
practically satisfy the Huygens principle while long waves can backscatter off
the curvature of a spacetime.
