Here’s something I never really understood before. Edwin Hubble is most famous for the idea that galaxies have a redshift proportional to their distance, which gives rise to the idea of the expanding universe since the Big Bang. But the redshift that we measure for galaxies is not a Doppler shift, and you can’t really say that distant galaxies are moving away from us at a certain velocity. Instead, space itself is expanding and that stretches out light waves en route. This cosmological redshift effect looks just like a Doppler shift, so it’s easy to confuse the two, but they’re two unrelated causes that have the same effect.
h/t Sean Carroll for his blog that clarifies this topic in the context of “superluminal expansion” — http://www.preposterousuniverse.com/blog/2015/10/13/the-universe-never-expands-faster-than-the-speed-of-light