That’s true, as with this cosmology light evidently isn’t affected by the increase in rest mass, as photons are massless. If this cosmology affected relativisti

That’s true, as with this cosmology light evidently isn’t affected by the increase in rest mass, as photons are massless. If this cosmology affected relativistic mass, then light would also be affected and would not be seemingly redshifted by the mass gain mechanism. So, yeah, we should be able to measure a gradual emission spectrum shift here on Earth with current methods - we can measure energies of photons to very high precision, and you’d likely only need a few years of data to prove or disprove the hypothetical cosmology.