According to relativity, light bends in presence of gravity. it implies that light experiences gravitational force of attraction.
Also wavelength=h/mc
=> m=h/(c*wavelength)
SO retardation on light must be given by F/m => F*c*wavelength/h. But force of gravitation varies in space. It acts as accelerating force if light approaches a mass and retards it if it tends to get away. So, i think due to varying force of gravitation and it's nature of acc and retardation. It can't be exactly quoted if light is accelerating or retardingin universe.
