Additionally, > light never changes is wrong. A photon has a momentum, which can increase or decrease as it travels through free space. Using general relati

Additionally, > light never changes is wrong. A photon has a momentum, which can increase or decrease as it travels through free space. Using general relativity, it's straightforward to define an affine time on a null geodesic (even if one cannot define a proper time on one), and a momentum that is a function of the affine time at each point. The Lyman-alpha forest is a straightforward "laboratory" for this approach, which can calculate the Lyα absorption lines seen when a bright distant quasar or luminous Lyα emitter has atomic hydrogen clouds between the distant source and us (the absorption lines indicate photons with a very narrow range of momenta participated in hydrogen atoms' electrons transitioning from ground to the first excited state, with the later relaxation photons radiating in random directions). The pattern of such dark spectral lines tells us how redshifted the background light and earlier absorption lines are at each gas cloud along the way. https://www.astro.ucla.edu/~wright/Lyman-alpha-forest.html https://www.astro.ucla.edu/~wright/Lyman-alpha-forest.html https://en.wikipedia.org/wiki/Lyman-alpha_emitter https://en.wikipedia.org/wiki/Lyman-alpha_emitter And over much-shorter-than-cosmological distances, https://en.wikipedia.org/wiki/Pound%E2%80%93Rebka_experiment https://en.wikipedia.org/wiki/Pound%E2%80%93Rebka_experiment