1929: Edwin Hubble Discovers the Universe is Expanding

Edwin Hubble observing on the 100-inch telescope. Image courtesy of the Observatories of the Carnegie Institution for Science.
Edwin Hubble observing on the 100-inch telescope. Image courtesy of the Observatories of the Carnegie Institution for Science.

When Edwin Hubble was hired to work at Mount Wilson Observatory in 1919 (part of the Observatories of the Carnegie Institution of Washington) as a junior astronomer, the most pressing question of the day concerned the nature of the cloudy patches called nebulae.  Most of Hubble’s colleagues at Mount Wilson thought they were all in the Milky Way, but he was not so sure.  He succeeded in answering this question by taking the best possible photos of these objects, providing convincing evidence that at least some of them were well beyond the Milky Way.  By discovering other galaxies Hubble expanded the known universe 100-fold.  But he didn’t stop there.  By measuring the distances and motions of the galaxies he surprised everyone, including Einstein, by discovering that the universe is expanding.

Hubble was lucky enough to arrive at Mt. Wilson soon after the 100-inch reflecting telescope was completed.  A careful and hard-working observer, Hubble took many photographs of the same set of spiral nebulae (now called galaxies).  Multiple images were needed in order to identify changes over time.  He observed several novas, or instances in which a dim star became much brighter as it attracted material from a nearby companion star.  Then, on October 4, 1923, while comparing a photograph that he had just taken of the Andromeda galaxy with photos taken on previous nights, Hubble identified a Cepheid variable star—the one kind of star that could provide a means of determining the distance to the galaxy.  Over the next several months Hubble determined that the star varied in brightness with a period of 31.45 days, which meant it was 7,000 times brighter than the Sun. Comparing its apparent brightness with its actual brightness Hubble determined that it was 900,000 light years away.
Since Harlow Shapley had previously measured the distance across the Milky Way to be about 100,000 light years, the new findings clearly indicated that the Andromeda galaxy was far beyond the Milky Way.  Later investigators found that there were two types of Cepheid variable stars, and that Hubble was comparing the bright kind of Cepheid in Andromeda with a dimmer kind of Cepheid in our own galaxy, which meant that Andromeda was actually twice as far away—approximately 2 million light years.  In subsequent decades, distances were measured to many other galaxies. Today, galaxies that are billions of light years distant have been observed.
But Hubble’s work was not done.  He was aware that a decade earlier astronomer Vesto Slipher had measured the Doppler shift of several galaxies, finding a few that were approaching our Milky Way and several that were moving away at very high speeds. Working with his assistant, Milton Humason, Hubble carefully measured the distance and Doppler shift of as many galaxies as possible. In 1929 Hubble published a paper that would lead to the realization that the universe was expanding.
As astronomers’ abilities to measure the distance and Doppler shift of galaxies improved over the years, Hubble’s and Humason’s discovery of the expanding universe was confirmed.  Further, it soon became obvious that by measuring the rate of expansion it was possible to determine the age of the universe by calculating when all the galaxies were in one place.  This was truly an amazing discovery—that human beings could not only grasp our place within a huge pinwheel of billions of stars, itself one of many such galaxies; but that we could also determine when the universe began.  Although Edwin Hubble is credited with these findings he acknowledged that the discoveries were not his alone.  They depended on the work many astronomers who had come before, building the knowledge and tools that were needed to reach the summit, and look out across the vast universe and innumerable galaxies to the very beginning of time and space.