Hubble Confirms the Expanding Universe

Date 1929

Edwin Powell Hubble established that distant galaxies are moving away from the Milky Way galaxy at speeds that are determined by their distance from the Milky Way.

Locale Mount Wilson, California

Key Figures

• Edwin Powell Hubble (1889-1953), American astronomer
• Vesto Melvin Slipher (1875-1969), American astronomer
• Henrietta Swan Leavitt (1868-1921), American astronomer
• Georges Lemaître (1894-1966), Belgian cosmologist
• Walter Baade (1893-1960), German American astronomer

Summary of Event

In 1929, Edwin Powell Hubble announced that the greater the distance to a given galaxy, the faster it is traveling away from the Milky Way galaxy. This discovery was of major importance because it implied that the universe was expanding; the discovery, in turn, supported a theory proposed by Georges Lemaître in 1927 that would be developed into the “big bang” theory of the creation of the universe by the American physicistGeorge Gamow in 1948.

Hubble made his discovery by studying photographs of stellar spectra that Vesto Melvin Slipher had taken as a way of measuring the distances to those stars. The initial key in measuring the distances to the galaxies was the work of Henrietta Swan Leavitt. In 1911 and 1912, Leavitt analyzed Cepheid variables, which are stars that change their brightness according to a predictable cycle. Leavitt arranged the stars in order according to the periods (durations) of their cycles. She noticed that arranging them by period placed them in order of actual, or absolute, brightness. She developed what became known as the “period-luminosity scale,” by means of which, once the period of a Cepheid was measured, the star’s actual brightness could be determined and compared to its apparent brightness, which in turn would reveal its distance.

The “redshift” measurements that Slipher had begun in 1913 suggested that the farther away a galaxy was, the faster it was receding. (The phenomenon is called “redshift” because, as the galaxy moves away, the light it emits has longer wavelengths; that is, its light moves, or shifts, toward the red end of the spectrum of visible light.) Slipher had no reliable way of measuring distances, however, and thus no means of proving the relationship. It was left to Hubble to put together the redshift results with the measurements of distance, leading to what is now called “Hubble’s law.”

Hubble began work in 1919 with the 60-inch (152-centimeter) telescope on Mount Wilson, near Pasadena, California, when he returned from service in World War I; he then moved to the 100-inch (254-centimeter) Hooker telescope at the same location. He studied objects within the Milky Way, such as novas (exploding stars), stars associated with gaseous nebulas, and variable stars. By 1922, he had published a paper noting the differences between the gaseous nebulae and those that were suspected of being more remote.

By 1928, using Leavitt’s period-luminosity scale, Hubble estimated that the Andromeda nebula was more than 900,000 light-years away (a light-year is the distance that light, moving in a vacuum, travels in one year—at the rate of 299,000 kilometers, or 185,790 miles, per second). This figure was far higher than both the Dutch astronomer Jacobus Cornelius Kapteyn’s 50,000 light-year diameter for the Milky Way and the American astronomer Harlow Shapley’s estimate of 200,000 light-years. Hubble later adjusted his estimate to 750,000 light-years. It is now known that Hubble’s estimates were too small, because, in 1952, Walter Baade was able to demonstrate that there are two types of Cepheid variables with different absolute brightnesses. As a consequence, modern estimates of the distance to the Andromeda nebula are more than 2 million light-years. Hubble had, however, established that Andromeda is outside the Milky Way.

Later, using the Hooker telescope, Hubble was able to resolve some of the fringes of nebulae into stars. By 1929, he had measured twenty-three galaxies out to a distance of about 20 million light-years. By 1931, Hubble and Milton L. Humason had measured some forty new galactic velocities out to a distance of 100 million light-years. Their major contribution to reliable measurement had to do with the fact that twenty-six of these velocities occurred within eight clusters. Because stars in the same cluster are assumed to be moving at the same speed, measuring redshifts for different stars within the same cluster is a good way to check the accuracy of their results.

Hubble found the speed of recession to be directly proportional to distance by a factor that came to be called the “Hubble constant,” which he estimated to be 170 kilometers (roughly 106 miles) per second for each million light-years of distance. The modern value is 15 kilometers (9.32 miles) per second per million light-years. His original value for the constant indicated an age for the universe of only 2 billion years, much less than the 3 or 4 billion years that geologists had derived for the age of the earth. His figure created an anomaly that persisted until Walter Baade’s discovery of two stellar populations, which reduced the size of the constant and increased the estimated age of the universe. (Today, through measurements taken by the Wilkinson Microwave Anisotropy Probe the age of the universe has been measured at 13.7 billion years.)

Significance

The establishment of the expansion of the universe is one of the most significant achievements of twentieth century astronomy. Establishing the scale of distance was a key to understanding the nature of the universe, which has led to Hubble’s being considered the founder of extragalactic astronomy.

Hubble was not the first to presume that there were objects of interest beyond the Milky Way galaxy. Many astronomers had suspected that Sir William Herschel was correct in his opinion that “nebulae,” those faint patches of light scattered throughout space, were “island universes” of stars, located outside the bounds of the Milky Way. In the mid-1920’s, Lemaître theorized that the universe originated from an original superdense “cosmic egg” that had expanded into the present universe; two decades later, his idea would expand into the big bang theory.

Bibliography

Adams, Walter S. “Obituary: Dr. Edwin P. Hubble.” Observatory 74 (February, 1954): 32-35. A short biographical sketch in which the main emphasis is the distance-redshift relationship. Reliable and interesting account of Hubble’s life (although the chronology surrounding his doctoral degree and service in World War I is somewhat confusing).

Clark, David H., and Matthew D. H. Clark. Measuring the Cosmos: How Scientists Discovered the Dimensions of the Universe. New Brunswick, N.J.: Rutgers University Press, 2004. Relates the stories of the scientists who have contributed to current knowledge about the size, mass, and age of the universe. Chapters 4 and 5 include discussion of the work of Hubble, Slipher, and Leavitt. Features glossary, bibliography, and index.

Gribbin, John. In Search of the Big Bang: The Life and Death of the Universe. Rev. ed. New York: Penguin Books, 1998. Accurate and comprehensive statement of the currently accepted cosmology presents an excellent summary of Hubble’s contribution. One of the most readable books on the subject for a general audience. Includes illustrations, bibliography, and index.

Hetherington, N. “Edwin Hubble: Legal Eagle.” Nature 39 (January 16, 1986): 189-190. A fascinating short analysis of the influence of Hubble’s legal training on the manner in which he presented his astronomical evidence. Also considers the way in which he conducted his conflicts with other scientists. Presents a perspective different from that found in most other sources.

Hubble, Edwin. “The Exploration of Space.” In Theories of the Universe, edited by Milton Karl Munitz. Glencoe, Ill.: Free Press, 1957. Short discussion excerpted from Hubble’s 1936 book The Realm of the Nebulae and from an article that appeared in Proceedings of the American Philosophy Society. An attractive summary and retrospect of his work that pays attention to the contributions of other astronomers working on related topics.

‗‗‗‗‗‗‗. The Realm of the Nebulae. 1936. Reprint. Mineola, N.Y.: Dover, 1991. Presents a comprehensive summary of Hubble’s work concerning the distant galaxies. Chapters 4 and 5 contain a statement of his discovery of Hubble’s law and the procedures used for determining Hubble’s constant.

Mayall, N. U. “Edwin Powell Hubble.” Biographical Memoirs of the National Academy of Sciences 41 (1970): 175-214. An extensive laudatory biography that describes each of Hubble’s major accomplishments. Contains some interesting anecdotal material not found in other sources. An abbreviated version of this article appeared in Sky and Telescope in January, 1954.

Struve, Otto, and Velta Zebergs. Astronomy of the Twentieth Century. New York: Macmillan, 1962. Offers an excellent nontechnical summary of Hubble’s work.