"”But when you look at the aggregate effects of what happens to many, many galaxies, you find something else: your overall view of the Universe gets distorted. Wherever you have a large amount of mass in one location in space — a large-scale overdensity like a galaxy cluster, cosmic filament, or something even larger — it’s going to impart large peculiar velocities, including along the line-of-sight, to every mass that’s bound to it. With many observable objects all at the same distance, but with widely-varying redshifts, mapping out objects becomes challenging."
You've heard of Hubble's law: the law that the Universe is expanding, and that the speed at which we'd infer a distant galaxy to be receding from us is proportional to its distance away from us. That "speeding away" makes the light appear redshifted, and that's what we observe.
So, measure the redshift, learn the distance, and you can map out the whole Universe, right? Not quite; here's why that isn't the way to do it!”
