Edwin Powell Hubble (November 20, 1889 – September 28, 1953) was an American astronomer who profoundly changed the understanding of the universe by confirming the existence of galaxies other than the Milky Way. He also considered the idea that the loss in frequency — the redshift — observed in the spectra of light from other galaxies increased in proportion to a particular galaxy's distance from Earth. This relationship became known as Hubble's law.

Hubble doubted the Doppler shift interpretation of the observed redshift that had been proposed earlier by Vesto Slipher, whose data he used, and that led to the theory of the metric expansion of space. He tended to believe the frequency of any beam of light could, by some so far unknown means, be diminished ever stronger, the longer the beam travels through space.

Hubble was born to an insurance executive, John Powell Hubble, and Virginia Lee James in Marshfield, Missouri, and moved to Wheaton, Illinois in 1900. In his younger days he was noted more for his athletic prowess than his intellectual abilities, although he did earn good grades in every subject except for spelling. He won seven first places and a third place in a single high school track and field meet in 1906. That year he also set the state high school record for the high jump in Illinois. Another of his personal interests was dry - fly fishing, and he practiced amateur boxing as well.

His studies at the University of Chicago were concentrated on mathematics, astronomy and philosophy, which led to a bachelor of science degree in 1910. Hubble also became a member of the Kappa Sigma Fraternity (and in 1948 was named the Kappa Sigma "Man of the Year"). He spent the three years at The Queen's College, Oxford, after earning his bachelors as one of the university's first Rhodes Scholars, initially studying jurisprudence (instead of science as a promise to his dying father) and later added literature and Spanish, and earning his master's degree. Some of his acquired British mannerisms and dress stayed with him all his life, occasionally irritating his American colleagues.

Hubble's father had in 1909 moved his family from Chicago to Shelbyville, Kentucky, so that the family could live in a small town, ultimately settling in nearby Louisville. His father died in the winter of 1913, while Edwin was still in England, and in the summer of 1913, he returned to care for his mother, two sisters, and younger brother, as did his brother William. The family moved once more to Everett Avenue, in Louisville's Highlands neighborhood, to accommodate Edwin and William.

Upon returning to the United States, Hubble taught Spanish, physics and mathematics at the New Albany High School in New Albany, Indiana. He also coached the boys' basketball team there. Hubble's early biographers uniformly noted that he had passed the Kentucky bar examination and briefly practiced law in Louisville, but he did neither. There is no evidence that Hubble ever handled a legal case. After a year of high school teaching, he returned to astronomy at the Yerkes Observatory of the University of Chicago, where he received his PhD in 1917. His dissertation was titled Photographic Investigations of Faint Nebulae.

Hubble then served in the United States Army in World War I, and he quickly advanced to the rank of major. In 1919, Hubble was offered a staff position in California by George Ellery Hale, the founder and director of the Carnegie Institution's Mount Wilson Observatory, near Pasadena, California, where he remained on the staff until his death. Hubble also served in the U.S. Army at the Aberdeen Proving Ground during World War II. For his work there he received the Legion of Merit award. Shortly before his death, Mount Palomar's giant 200 inch (5.1 m) reflector Hale Telescope was completed, and Hubble was the first astronomer to use it. Hubble continued his research at the Mount Wilson and Mount Palomar Observatories, where he remained active until his death.

Hubble experienced a heart attack on July 1949 while on vacation in Colorado. He was taken care of by Grace Hubble and continued on a modified diet and work schedule. He died of cerebral thrombosis (a spontaneous blood clot in his brain) on September 28, 1953, in San Marino, California. No funeral was held for him, and his wife Grace Hubble, did not reveal the disposition of his body.

Edwin Hubble's arrival at Mount Wilson, California, in 1919 coincided roughly with the completion of the 100 inch (2.5 m) Hooker Telescope, then the world's largest telescope. At that time, the prevailing view of the cosmos was that the universe consisted entirely of the Milky Way Galaxy. Using the Hooker Telescope at Mt. Wilson, Hubble identified Cepheid variables (a kind of star) in several spiral nebulae, including the Andromeda Nebula and Triangulum. His observations, made in 1922 – 1923, proved conclusively that these nebulae were much too distant to be part of the Milky Way and were, in fact, entire galaxies outside our own. This idea had been opposed by many in the astronomy establishment of the time, in particular by the Harvard University based Harlow Shapley. Despite the opposition, Hubble, then a thirty - five year old scientist, had his findings first published in The New York Times on November 23, 1924, and then more formally presented in the form of a paper at the January 1, 1925 meeting of the American Astronomical Society. Hubble's findings fundamentally changed the scientific view of the universe.

Hubble also devised the most commonly used system for classifying galaxies, grouping them according to their appearance in photographic images. He arranged the different groups of galaxies in what became known as the Hubble sequence.

Combining his own measurements of galaxy distances based on Henrietta Swan Leavitt's period - luminosity relationship for Cepheids with Vesto Slipher and Milton L. Humason's measurements of the redshifts associated with the galaxies, he discovered a rough proportionality of the objects' distances with their redshifts. Though there was considerable scatter (now known to be due to peculiar velocities), he was able to plot a trend line from the 46 galaxies and obtained a value for the Hubble Constant of 500 km/s/Mpc, which is much higher than the currently accepted value due to errors in their distance calibrations. In 1929 Hubble formulated the empirical Redshift Distance Law of galaxies, nowadays termed simply Hubble's law, which, if the redshift is interpreted as a measure of recession speed, is consistent with the solutions of Einstein’s equations of general relativity for a homogeneous, isotropic expanding space. Although concepts underlying an expanding universe were well understood earlier, this statement by Hubble and Humason led to wider scale acceptance for this view. The law states that the greater the distance between any two galaxies, the greater their relative speed of separation. But two years before, in 1927, Georges Lemaître, a Belgian Catholic priest and physicist, published a paper in an obscure Belgian journal, Annales de la Societe Scientifique de Bruxelles. In that paper, he showed that the data collected by Hubble and two other astronomers up to that time was enough to derive a linear velocity - distance relation between the galaxies, and that this supported a model of an expanding universe based on Einstein’s equations for General Relativity.

This discovery was the first observational support for the Big Bang theory which had been proposed by Georges Lemaître in 1927. The observed velocities of distant galaxies, taken together with the cosmological principle appeared to show that the Universe was expanding in a manner consistent with the Friedmann - Lemaître model of general relativity. In 1931 Hubble wrote a letter to the Dutch cosmologist Willem de Sitter expressing his opinion on the theoretical interpretation of the redshift - distance relation:

[W]e use the term "apparent velocities" in order to emphasize the empirical feature of the correlation. The interpretation, we feel, should be left to you and the very few others who are competent to discuss the matter with authority.

Today, the "apparent velocities" in question are understood as an increase in proper distance that occurs due to the expansion of space. Light traveling through stretching space will experience a Hubble - type redshift, a mechanism different from the Doppler effect (although the two mechanisms become equivalent descriptions related by a coordinate transformation for nearby galaxies).

In the 1930s Hubble was involved in determining the distribution of galaxies and spatial curvature. These data seemed to indicate that the universe was flat and homogeneous, but there was a deviation from flatness at large redshifts. According to Allan Sandage,

Hubble believed that his count data gave a more reasonable result concerning spatial curvature if the redshift correction was made assuming no recession. To the very end of his writings he maintained this position, favoring (or at the very least keeping open) the model where no true expansion exists, and therefore that the redshift "represents a hitherto unrecognized principle of nature."

There were methodological problems with Hubble's survey technique that showed a deviation from flatness at large redshifts. In particular the technique did not account for changes in luminosity of galaxies due to galaxy evolution. Earlier, in 1917, Albert Einstein had found that his newly developed theory of general relativity indicated that the universe must be either expanding or contracting. Unable to believe what his own equations were telling him, Einstein introduced a cosmological constant (a "fudge factor") to the equations to avoid this "problem". When Einstein heard of Hubble's discovery, he said that changing his equations was "the biggest blunder of [his] life."

Hubble discovered the asteroid 1373 Cincinnati on August 30, 1935. He also wrote The Observational Approach to Cosmology and The Realm of the Nebulae approximately during this time.

Hubble spent much of the later part of his career attempting to have astronomy considered an area of physics, instead of being its own science. He did this largely so that astronomers — including himself — could be recognized by the Nobel Prize Committee for their valuable contributions to astrophysics. This campaign was unsuccessful in Hubble's lifetime, but shortly after his death the Nobel Prize Committee decided that astronomical work would be eligible for the physics prize. (The prize cannot be awarded posthumously.)

On March 6, 2008, the United States Postal Service released a 41 cent stamp honoring Hubble on a sheet titled "American Scientists". His citation reads:

Often called a "pioneer of the distant stars," astronomer Edwin Hubble (1889 – 1953) played a pivotal role in deciphering the vast and complex nature of the universe. His meticulous studies of spiral nebulae proved the existence of galaxies other than our own Milky Way. Had he not died suddenly in 1953, Hubble would have won that year's Nobel Prize in Physics.

The other scientists on the "American Scientists" sheet include Gerty Cori, biochemist; Linus Pauling, chemist, and John Bardeen, physicist.