Great empirical discoveries in science have provided some of the most moving moments in human history. The discovery of the astronomical effect known as redshift, which implies an expanding universe, is one such moment. It reportedly brought tears to the eyes of Albert Einstein.
Einstein introduced time as the fourth dimension in his theory of special relativity (1905). A decade later, he extended this theory into the theory of general relativity which contains the idea that the four-dimensional space-time of special relativity has curvature, a non-Euclidean space in which space is curved and stretched by gravity despite it being the weakest of the four fundamental forces.
The decisive proof of an expanding universe that general relativity theory predicts did not come about until the early 1930s when observations from a new 100-inch telescope at Mount Wilson in California revealed the redshift in light from distant galaxies. The man at the center of discovery was American astronomer, Edwin Hubble (1889-1953) who played a crucial role in establishing the fields of extragalactic astronomy and observational cosmology.
Hubble used the Wilson telescope to study the pace at which stars are either moving away or toward us (the so-called redshift and blueshift effects respectively). He measured some of the most distant stars in the galaxy, some more than two million light-years away from Earth. What he found astonished him: almost every galaxy was redshifted, leading him to conclude that galaxies are moving away from each other, which implies that the universe is expanding, just as Einstein has predicted in general relativity.
When Einstein met with Hubble at the Mount Wilson Observatory in January and February 1931, he was visibly moved with Hubble’s discovery and reportedly said, with tears in his eyes that “It was the most beautiful and satisfying interpretation of astronomical science.” In light of the new evidence, Einstein published a paper two months later renouncing the concept of a cosmological constant (his idea) and said that he agreed with notion of an expanding universe that essentially counters the pull of gravity.