When it comes to the distant galaxies in the Universe, one of the most profound discoveries in all of history is also one of the most puzzling: the fact that they’re almost all mutually receding from one another. It was only in 1923 that we firmly established that extragalactic objects — objects beyond our own Milky Way — even existed, with Hubble’s detailed measurements of Andromeda placing its distance far beyond the Milky Way itself. Just a few years later, from 1927-1929, enough evidence had accumulated for scientists to establish the redshift-distance relation, also known as Hubble’s Law: where a distant galaxy’s observed recession speed is proportional to its distance from us. If you think about that in detail, however, something puzzling emerges. If you look to great enough distances, that observed recession speed can get very fast indeed. Potentially, those speeds could approach, reach, or even exceed the speed of light! Is that a problem for physics?…