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...."
http://cosmology.carnegiescience.edu/timeline/1929
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