Postulated the Expansion of the Universe Based on Redshift Observations.
Introduction
Edwin Hubble (circa 1889–1953) was an American astronomer. He played a crucial role in establishing the field of extragalactic astronomy and is generally regarded as one of the most important observational cosmologists of the 20th century.
Early Life and Education
In 1906, Hubble won a scholarship to the University of Chicago, where he served for a year as a student laboratory assistant for physicist Robert Millikan, a future Nobel Prize winner. Hubble graduated in 1910 and was selected as a Rhodes Scholar from Illinois. He spent three years at the University of Oxford and was awarded a B.A. in jurisprudence, a subject he had taken at his father’s insistence. After his father’s death in 1913, he was free to pursue a scientific career.
Upon his return to the United States later in 1913, Hubble taught high school in Indiana for a year. He then entered the University of Chicago and embarked on graduate studies in astronomy. Hubble conducted his observational research at the Yerkes Observatory in Williams Bay, Wisconsin, under the supervision of astronomer Edwin Frost. By this time, Yerkes was no longer at the forefront of astronomy, but Hubble had access to a powerful telescope, an innovative 24-inch (61-cm) reflector.
Hubble’s timing was fortunate, as he was completing his graduate studies just as George Ellery Hale, the director of the Mount Wilson Observatory in California, was recruiting new staff. The observatory housed the 100-inch (254-cm) Hooker telescope, then the world’s most powerful.
At Mount Wilson, Hubble initially studied reflection nebulae within the Milky Way. However, he soon returned to the problem of the spiral nebulae, objects he had investigated for his doctorate. The nature of these spirals was unclear: were they distant star systems (galaxies) comparable to the Milky Way, or were they clouds of gas or sparse star clusters within the Milky Way?
Hubble’s Law
In 1923, Hubble found Cepheid variable stars in the Andromeda Nebula, a well-known spiral. The fluctuations in the light of these stars enabled Hubble to determine the nebula’s distance using the relationship between the period of the Cepheid fluctuations and its luminosity. Although there was no clear consensus on the size of the Milky Way, Hubble’s distance estimate placed the Andromeda Nebula approximately 900,000 light-years away. If Hubble was correct, the Nebula clearly lay far beyond the borders of the Milky Way Galaxy. The Andromeda Nebula, therefore, had to be a galaxy, not a nebulous cloud or sparse star cluster within the Milky Way. Hubble’s findings in the Andromeda Nebula and in other relatively nearby spiral nebulae quickly convinced most astronomers that the universe contains a multitude of galaxies. (The current distance estimate of the Andromeda Galaxy is 2.48 million light-years.)
Hubble is known for showing that the recessional velocity of a galaxy increases with its distance from the Earth, implying the universe is expanding, a relationship known as “Hubble’s law.” Georges Lemaître had previously discovered this relation based on information from the American astronomer Vesto Slipher, a decade earlier. Slipher had provided the first evidence that the light from many of these nebulae was strongly red-shifted, indicative of high recession velocities.
Hubble interpreted the “redshift” seen in the light from distant stars as evidence that the Universe is expanding. He attributed redshift entirely to the Doppler effect from the movement of stars away from each other, without considering other possible explanations. He called this observation “Hubble’s Law.” His law implies that the universe is expanding, and this “expansion” drives our estimates of its size and age.
The reason for the redshift was unclear. Georges Lemaître found that Hubble’s observations supported the Friedmann model of an expanding universe based on Einstein’s equations for General Relativity, which is now known as the Big Bang theory. Hubble himself remained doubtful about Lemaître’s interpretation.
Although some of his more renowned colleagues were skeptical of Hubble’s idea of an expanding universe, Hubble published his findings on nebulae.
Today, the “apparent velocities” are understood as an increase in proper distance 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).
Contributions
Edwin Hubble also provided substantial evidence that many objects then classified as “nebulae” were actually galaxies beyond the Milky Way. American astronomer Vesto Slipher had provided the first evidence for this argument almost a decade before.
Using the Hooker Telescope at Mt. Wilson, Hubble identified Cepheid variables in several spiral nebulae, including the Andromeda Nebula and Triangulum. His observations, made in 1922–1923, proved these nebulae were much too distant to be part of the Milky Way and were, in fact, entire galaxies outside our own. Hubble’s findings fundamentally changed the scientific view of the universe. Supporters of the expanding universe theory state that the discovery of nebulae outside of our galaxy helped pave the way for future astronomers.
Hubble also devised the most commonly used system for classifying galaxies, grouping them according to their appearance in photographic images.
In 1929, Hubble examined the relation between distance and redshift of galaxies, combining his own measurements of galaxy distances based on Henrietta Swan Leavitt’s period-luminosity relationship for Cepheids with earlier data from fellow astronomer Vesto Slipher and Milton L. Humason’s measurements. He found a rough proportionality of these objects’ distances with their redshifts, now called Hubble’s law.
This meant that the greater the distance between any two galaxies, the greater their relative speed of separation. Interpreted this way, Hubble’s measurements of 46 galaxies led to a value for the Hubble Constant of 500 km/s/Mpc, significantly higher than the currently accepted value due to errors in their distance calibrations.
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 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 the 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 learned of Hubble’s redshifts, he immediately realized that the expansion predicted by General Relativity must be real, and later in life, he said that changing his equations was “his biggest blunder.”
Hubble’s name is most widely recognized for the Hubble Space Telescope, which was named in his honor.
Conjecture
Since Einstein’s gravity under his “Special theory of relativity” is “equivalent”, he needed a method of showing how the organization of the universe was accomplished. This led to his acceptance of the Big Bang as a possibility.
Legacy
At the time, the Nobel Prize in Physics did not recognize work done in astronomy. Hubble spent much of the latter part of his career attempting to have astronomy considered an area of physics, instead of being its own science.
Since then he received the following awards: Legion of Merit, Gold Medal of the Royal Astronomical Society, Franklin Medal, Bruce Medal. Now the Hubble telescope is named after him.