The frequency of a photon changes due to relative velocity of the source and observer (red shift). This means that the energy of the photon has changed at the observer.
This theory says that if a photon loses energy after travelling great distances then either Plancks constant has to change or the frequency has to change.
Or it could be expansion of the universe and expansion of the fabric of space that causes all observed red shift. It is difficult to set up an experiment.
“This theory ...”
Repeat: Not a theory. No mention of a mechanism. Purely a mathematical exercise.
“Or it could be expansion of the universe and expansion of the fabric of space that causes all observed red shift. It is difficult to set up an experiment.”
At least three types of redshift occur in the universe — from the universe’s expansion, from the movement of galaxies relative to each other and from “gravitational redshift,” which happens when light is shifted due to the massive amount of matter inside of a galaxy.
This latter redshift is the subtlest of the three,”
https://www.space.com/25732-redshift-blueshift.html
“Or it could be expansion of the universe and expansion of the fabric of space that causes all observed red shift. It is difficult to set up an experiment.”
Experiment? It is quantified in our everyday life. GPS, speed detectors, Lidar, etc.