Photons have two aspects. On the one hand, they travel at the speed of light which is the same in all references. Photons also express wavelength and frequency, which are dependent on inertial reference. A moving source can impact wavelength/frequency, since the inertial reference changes, but it does not impact the speed of light. These are two different legs of the photon.

One leg of the photon is always in the ground state of the speed of light, with this ground state the same for all inertial references. The other leg is in inertial reference and will therefore change with reference. If we were to travel at the speed of light, we would not be able to see the all the variety of wavelength, since space-time will be contracted to a point-instant. Wavelength and frequency differentiation is an artifact of inertial reference and space-time.

Because of the two legs, a potential is set between the ground state at C, which does not see any variety in terms of wavelength and frequency; unbiased reference for all, and the wavelength and frequency, determined by space-time and inertial reference. A tug of war between the two; inertial and the C-ground state, results in the wave nature of photons. If we alter inertial reference, the parameters of the tug of war change, thereby altering the wavelength and frequency seen in inertial reference. The ground state never changes.

The red shift of the universe reflects that the C ground state wins all the tug of wars over time. The energy value of photons lose potential to the ground state and red shift.

As an analogy say we have a series of different weights hanging from a series of standard springs. Gravity defines the ground state common for all the springs; how they will hang at steady state. If we start at the top of all the springs and let them all fall, they will each vibrate; wavelength and frequency, based on the potential between the common ground state and the weight/tension in each spring.

Say we had one static spring/weight and we conserve the energy from one vibrating spring and transfer this energy to the static spring. The frequency will change base on its weight. Weight itself is dependent on the ground state of gravity.