Can the cosmological redshift be interpreted as atomic frequencies increasing by the scale factor as the Universe expands? This explanation seems closer to the truth than the popular idea that a photon's wavelength somehow expands while it travels to us from a distant galaxy. Metric expansion only occurs with proper distances (between events at the same cosmological time). I think people take the standard derivation of the cosmological redshift to imply that the photon's oscillation period increases with the scale factor. But this again is equivalent to assuming its wavelength increases.