I have a quick question about the Special Relativity. Non-relativistically, we expect no doppler shift in the wavelength of the emitted light if the source is moving at right angles to the line of sight to the observer. However there is a transverse doppler shift even in this case, caused by time dilation of the source. Now, correct me if I'm wrong, but the mechanism for the cosmological redshift is not the same as the above relativistic shift. I'm confused because I often find the two associated together, like in this answer of anna v. On the opposite edge, I would reply that relativistic doppler effect is a more simple subject of a PHY206 (special relativity) while cosmological redshifts are discussed in PHY314 (relativity and cosmology) and PHY306 (introduction to cosmology). I think that the analogy between the Hubble expansion and a simple recession would be justified only if the scale factor would be increasing linearly: is it correct?