The Huygens principle and cosmological gravitational waves in the Regge-Wheeler gauge

We study the propagation of axial gravitational waves in Friedmann 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.