One of the most important figures in modern cosmology is the American astronomer Edwin Hubble. He discovered a universe of galaxies outside the Milky Way and showed that the entire universe is expanding.
In the early 20th century, most astronomers believed that the universe did not extend beyond our Milky Way galaxy. Then in the early 1920s, Hubble pointed the largest telescope in the world (on Mt. Wilson in California) at the mysterious Andromeda nebula. He recognized individual stars within the “nebula” and concluded that it was not a cloud of interstellar gas within our Milky Way, but an entire galaxy very far away. Suddenly, the universe was far larger than most astronomers had imagined.
Hubble continued to find galaxies at greater and greater distances He developed the system for classifying galaxies by their structure, which is still used today.
Hubble also discovered that the distant galaxies are moving away from each other and the greater the distance between two galaxies, the faster they are moving away. This simple proportionality between the distance and the speed of galaxies moving away from each other, set forth in 1929, is now known as Hubble’s law. It shows that the universe is expanding.