How Are Galaxies Moving?
• Facts: In the 1920’s Edwin Hubble found that, except for some nearby ones, the light from all galaxies is redshifted, and the redshift z (or velocity V) is proportional to the distance d, or v = H0 d, out to hundreds of millions of ly! The exceptions include M31 [and M86, moving toward us at 1 million mph!]. • The pattern: Distances to galaxies are increasing because the universe is expanding or “stretching”, at an average rate H0 of about 71 km/s for every Mpc of distance (as of 2006); So, for the farthest galaxies, redshifts indicate “motions” that are much faster than the speed of light! • What is going on? The expansion is like that of an infinitely large raising cake, with galaxies being the raisins; There is no “center”, distances between all distant pairs of galaxies are increasing in the same way [if we accept the special theory of relativity, we cannot interpret the redshifts to mean that galaxies are actually moving away while space itself is unchanged]; Galaxies themselves
