I have seen over and over a description of how the Copernican Principle led people to believe Hubble's redshifts were cosmological (due to metric expansion) rather than typical Doppler shifts. I'm trying to understanding the logic of this, but it appears like just a revision of history. From what I can tell, the Copernican Principle (by itself) gives us no reason to suspect Hubble's red shifts are due to cosmological effects rather than plain, vanilla velocity-induced Doppler Shifts. Hubble's observations showed all galaxies are red-shifted (receding from Earth) and the magnitudes of these redshifts (the speed) was proportional to their distance away from the Earth. The argument I've seen (implicitly) goes something like this. 1. Either the redshift seen by Hubble was i)due to the Earth honestly being at the center of the universe and everything was receding from it due to ordinary velocity, in which case the redshift is from the Doppler Effect. OR ii) The redshift is not from the Doppler effect but is due to metric expansion of the universe itself (the stretching of space...in which case the balloon analogy is used to explain how this is different from mundane velocity that would show up as the Doppler effect) 2. The Copernican Principle rules out option "i)" above because we are not at the center of the universe. 3. Hence, "ii)" is correct and the red shift is due to metric expansion. QED But this just looks like shoddy logic. From what I can tell, there is absolutely nothing in option "i" that requires Earth to be at the center of the universe. Indeed, simple vector algebra should show that if everything is doppler-shifted away from any point in space in a way consistent with Hubble's law, then the same applies to all points in the space without resorting to metric expansion. If Alex if flying away from me at 45 miles per hour, then (in Alex's rest frame) I am flying away from Alex at the same speed. If Bob is halfway between me and Alex, and I see Bob running away from me toward Alex, but only at half the speed, then Alex (in his rest frame) see's Bob running away from him toward me, but only at half the speed. This reminds me of one way of seeing part of the Coriolis effect. We normally think that the North Pole is "special' in that someone sitting on a frictionless dais would see the Earth rotate underneath him. But it turns out that the same is true for ANY point on the Earth (other than the Equator). If you took a frictionless seat mounted on a pole anywhere at all and sat down, you would see the earth rotate under you (less rotational speed the closer you are to the equator). Having a net rotation under your seat does not mean you are at the north pole, and seeing everything moving away from you (even if it is just due to standard, everyday velocity-induced redshift) does not mean you are the center of the universe....in fact, it means that you see the same thing that everyone else does. Nothing here is meant to suggest that Hubble's redshifts are not cosmological. Rather, I am replying to the common argument mentioned above that presumes to show that the Copernican Principle dictates which explanation is used. Since neither Doppler nor Cosmological red shifts would require the earth to be near the center of the universe, the Copernican principle has no relevance whatsoever. Of course, if I'm wrong about the above, I'd be interested in seeing what I am missing...but at the very least it looks like people who make this argument (which I see everywhere) are trying to sweep things under the rug, hoping their audience will just blindly accept their argument without thinking it through.