Discovering The Expanding Universe. Hubble and Humason Observed Redshifts of Distant Galaxies. Edwin Hubble along with his assistant Milton Humason made the observations of distant galaxy redshifts that showed the expansion of the universe.
Edwin Hubble was the key person who discovered the expansion of the universe. Born in Missouri in 1889, Hubble moved to Chicago as a young child. As a student Hubble showed more promise in athletics than academics. In high school he set a state record in the high jump. He played basketball in college and was also an accomplished amateur boxer.
After studying mathematics and astronomy as an undergraduate, Hubble traveled to Oxford on a Rhodes scholarship to study law. Apparently his interest in astronomy was greater. He returned to Chicago and earned a doctorate in astronomy in 1917.
Before starting his work at Mt. Wilson Observatory, Hubble served in World War I and became a Major in the US Army. He also interrupted his work to serve in World War II. Hubble died of a stroke in 1953.
Hubble collaborated with Milton Humason. Humason had only an 8th grade formal education, but was both curious and intelligent. He started working as a mule driver up the narrow winding road to the summit of Mt. Wilson. He soon became a janitor and started asking the astronomers lots of questions. Humason learned enough to become an observer and eventually Hubble’s assistant.
Using 100″ Hooker telescope on Mt. Wilson, the largest telescope in the world at the time, Hubble added to Slipher’s spectra of galaxies. More importantly Hubble figured out what the redshifts meant. The universe is expanding. Hubble devoted most of his career to studying external galaxies. Among other things, he measured their distances and studied their spectra.
Hubble and Humason noticed something curious. Not only are virtually all galaxies moving away from us, as Slipher had earlier noticed, but the more distant galaxies are moving away faster.
They graphed these quantities: the recessional speed versus the distance. The graph, which we call the Hubble Plot, shows a straight line relationship. A galaxy twice as far away as another will be moving away from us twice as fast. Three times as far away, three times as fast, and so on.
What does Hubble’s plot tell us about nature? Imagine making some raisin bread. Mix the dough. Put in lots of yeast, and completely mix the raisins. Then let it rise. If you had dirty hands, there will be lots of germs on the raisins.
Suppose there is one particularly intelligent microbe that studies the other raisins in the dough, by measuring how far away they are. If the dough rises for an hour and doubles in size, What does the microbe see?
A raisin that was one centimeter away at the beginning of the hour is two centimeters away at the end. It moved away at a rate of one centimeter per hour. A raisin that was originally two centimeters away is four centimeters away at the end. It moved two centimeters per hour. Similarly a raisin originally three centimeters away moves away at a rate of three centimeters per hour, and so on.
A Hubble plot for raisins would look the same as a Hubble plot for galaxies. It would simply be on a much smaller scale.
Hubble grasped this meaning of his plot. His graph told him that we live in an expanding universe. By 1929 Hubble had shown that the universe is expanding.
There is a very common misconception about this expansion. Many people envision this expansion as analogous to an explosion. In an explosion matter flies out to fill in space that is already there. This analogy is misleading. The raisin bread is the better analogy. The yeast filled dough is analogous to the space in the universe. The space in the universe, like the dough, is expanding causing the galaxies, or raisins, to move farther apart. The galaxies, or raisins, are not rushing out to fill in space, or dough, that is already there.