Am I understanding Cosmological Redshift rightly now? My first way of understanding was... The space expands between two galaxies A and B. light leaves A and travels to B. Notice that whilst the light is being emitted from A it will be red shifted (that is, the light is going in the opposite direction to which A is moving). So, that's one red shift. Now, when the light nears B the light will be red shifted again as it has to catch up with a receding B. That's another red shift. Notice it is the expansion of space which is making A and B receded from each other, but it is not space ''stretching'' the wavelength. It's the emitting and catching up parts that ''stretches'' the wavelength. My error here is interpreting the Doppler effect as a velocity of the galaxy itself in the same way as Hubble. ******* Now, after reading this Sean Carroll recommended pdf... https://arxiv.org/abs/astro-ph/0310808 Notice they say special relativistic velocity Doppler shift formula should not be used with anything to do with space expansion. The equation to use is a General Relativity equation... the pdf gives this equation.. Another help was the wiki redshift article and a couple of PMs to a member here, won't mention name incase I wrong in my understanding. So, if the pdf tells us we cannot use any SR velocity equations and only use GR, are we left with the following way of understanding cosmological redshift... Back to galaxies A and B. In the rest frame of galaxy A, there is just a light emitted into space at such and such a frequency, ie. no red shift in A's frame of rest. In B's frame of rest, a light is received at such and such a frequency, the observer at B then allows for the expansion of space by using the General Relativity equation. Can only use GR because of the metric expansion of space between A and B. Can't use SR because galaxies are not moving through space with a velocity. Is this right? The thing is, I have known for along time space expansion was responsible for recession of distant galaxies, but 'now' know how to interpret that expansion in the case of the local rest frames of A and B. Ps. galaxies read galaxy clusters.