Energy Loss by Gravitational Viscosity
  Due to Lorentz invariance of General Relativity gravitational interaction is limited to the speed of light. Thus for particles, moving within a matter field, retardation leads to loss of energy by emission of gravitational radiation. This 'gravitomagnetic' effect, applied to motion in homogeneous mass filled space, acts like a viscous force, slowing down every motion in the universe on the Hubble time scale. The energy loss rate exactly equals the red shift of photons in an expanding universe, thus showing the equivalence of wavelength stretching in the wave picture and energy loss in the photon picture. The loss mechanism is not restricted to an expanding universe, however, but would also be present in a static Einstein universe. 
Ernst Fischer