Redshifts and the Hubble Law

In 1924, Edwin Hubble demonstrated that the small, hazy patches of light we see in the sky on a dark night—the galaxies—are really enormous islands of billions of stars, like our own Milky Way galaxy seen at a great distance. Study with large telescopes revealed that the fainter and smaller a galaxy appeared, the higher, in general, was its redshift. Redshift describes the fact that the characteristic lines in its spectrum due to hydrogen, calcium, and other elements appear at longer (redder) wavelengths than in a terrestrial laboratory. This effect was most simply attributed to a recession velocity of the emitting source—like the falling pitch of a receding train whistle. It was therefore concluded that the fainter and smaller the galaxy, the more distant it was, and the faster it was flying away from us. This is the velocity interpretation of the redshift-apparent brightness relation, the standard interpretation of the so-called Hubble law.

About this time, Einstein was writing equations that attempted to describe the behavior of the entire universe, the totality of what exists. His equations pointed to its probable instability. Gravitation was either strong enough to be in the process of contracting the universe or too weak to prevent its expansion. In view of the extant conclusions about galaxy recession velocities, it was natural to interpret them as due to expansion of the universe.