Speed of light is a constant in a given region of space
The basic underlaying mechanism is that the motion of the photon and the time measuring device are both produced by the expansion of the space. The relationship of the clock and speed of light is fixed at a basic level in a given region of space. Clocks may measure a different speed of light if located in a region of space which is different from where light is being observed. For example a clock on earth measuring speed of light in a region next to a distant neutron star where time is slower will measure a slower speed. While a clock located on the surface of neutron star will measure a faster speed of light in distant space.
Moving objects produce moving wave like curvature in space. Once within the vicinity of the moving object mass less particles like the photons are picked up and carried forward by the time differential which is present in these waves. Also the velocity of light is decreased by the slower time within the moving wave. Within the moving object like earth light is pulled along in the direction of motion and it also slows down to conform to the slower time around the earth.
Photon acceleration is a well known term in plasma physics. (PHOTON ACCELERATION) Frequency of light can be increased in accelerated plasma. This can also be interpreted as increase in the speed of light due to the drag effect of moving particles. This information can be used to construct a modern experiment which will prove this concept. For information on what effect will motion have on speed of light contact the author. (mkhan@timephysics.com)