We should all know that photons only exist in quantum mechanics. In fact, the idea that energy comes bundled in discrete units is what actually caused QM in the first place. For some reason, however, I've spent my entire life under the assumption that cosmologists make true statements about the nature of light. But then I realized that GR consists of field equations that are only meant to model gravity fields. Other than gravity, these equations cannot be used to describe the nature of any other phenomena. Of course, it is possible for people with advanced degrees in cosmology to speak in rather informal ways about the nature of EM propagation within inter-galactic space. But this is a long way from the assertion that they are speaking in ways that are universally accepted by the scientific mainstream. Really, the only "mainstream" when it comes to the nature of EM propagation is the branch of QM known as quantum electrodynamics. Cosmologists are stuck in the 19th century if they think that Hubble's law can possibly be explained by a Doppler effect. In QM, a photon is simply an invariant unit of energy. There is absolutely no spacetime description involved in this concept, so relativistic arguments (either SR or GR) that depend on reference frames simply do not apply. In order to establish a mechanism for a Doppler effect on EM propagation, we have to eliminate the photon concept entirely, and go back to a classical, wave-based theory. But in this case, we revert back to the blackbody problem circa 1890's, and all of the issues of "ultraviolet catastrophe" that surrounded it. It is for all of these reasons that Einstein spent his later decades trying to come up with a unified field theory. If he had succeeded, we would have been given field equations that could be solved in order to yield cosmological solutions that include the EM field. But he didn't succeed, so we are left with immaterial metric systems that are supposed to expand over time. Cosmologists use flawed logic in the arguments concerning the reason for redshift. They say it cannot possibly be any quantum mechanical cause, and so it must be a relativistic one. But when it comes to offering any reasoning as to how a fundamentally quantum mechanical entity such as a photon can possibly suffer relativistic effects, they simply point to the empirical phenomenon of redshift itself. In other words: The phenomenon of redshift is caused by relativistic effects, and these effects are proven by the phenomenon of redshift itself. This is a vicious circle.