  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.
