Hubble's Law and the expanding universe.

In one of the most famous classic papers in the annals of science, Edwin Hubble’s 1929 PNAS article on the observed relation between distance and recession velocity of galaxies—the Hubble Law—unveiled the expanding universe and forever changed our understanding of the cosmos. It inaugurated the field of observational cosmology that has uncovered an amazingly vast universe that has been expanding and evolving for 14 billion years and contains dark matter, dark energy, and billions of galaxies. It is difficult to imagine that only 90 years ago, we did not know about the existence of most of the universe around us. From today’s perspective, the reality of a very large, old, expanding universe, filled with billions of galaxies that are receding from each other as the cosmic space expands from an initial “Big Bang” billions of years ago seems so obvious that we expect it must have been known for centuries. Not so. It was Edwin Hubble’s seminal 1929 PNAS paper, “A relation between distance and radial velocity among extra-galactic nebulae” (1), that led to a turning point in our understanding of the universe. In his short paper, Hubble presented the observational evidence for one of science’s greatest discoveries—the expanding universe. Hubble showed that galaxies are receding away from us with a velocity that is proportional to their distance from us: more distant galaxies recede faster than nearby galaxies. Hubble’s classic graph of the observed velocity vs. distance for nearby galaxies is presented in Fig. 1; this graph has become a scientific landmark that is regularly reproduced in astronomy textbooks. The graph reveals a linear relation between galaxy velocity (v) and its distance (d)