The Expanding Universe (Part 2)
Posted by Brian Ventrudo
Vesto Slipher startled astronomers in the early 20th century when he discovered the astonishing speed at which “spiral nebulae” moved through space. While his measurements suggested these nebulae lay outside our own galaxy, the true nature and distance to these objects remained unknown.
But in 1919, a young American just back from the First World War found himself assigned to the world’s largest telescope, the 100-inch reflector on Mount Wilson in California. With patience, dogged effort, and a keen and open mind, the young man made a discovery which expanded the size of the cosmos a hundred-fold, and lent observational evidence to the expansion of the fabric of space itself.
The young man’s name was Edwin Hubble.
Edwin Hubble was a fascinating, larger-than-life character during a time of breathtaking scientific advancement, and he was a study in ambition and contradiction …
He was a keen math and science student with an excellent mind, but also a track star in high school and university who was movie-star handsome (“too handsome for his profession”, according to a Hollywood script writer who befriended him in his prime).
He was a middle-class mid-westerner who won a Rhodes scholarship to Oxford and transformed himself into an affected faux-English dandy, took on a British accent, smoked a pipe, and sometimes wore knickers and a beret while observing at the big telescopes in California.
And he was a tireless self-promoter and master networker, who married into wealth and befriended Hollywood celebrities, yet who preferred to work by himself.
Hubble was also a dutiful son, who despite his intense interest in astronomy since boyhood, surrendered to his father’s request to study law, first at the University of Chicago and later at Oxford, though he managed to take a few math and science courses. After his father died in 1913, Edwin returned to the Midwest from Oxford, but found himself without direction or motivation to practice law. So he taught high school for a year before he resolved to start over, at the age of 25, to become a professional astronomer.
With the help of a former professor in Chicago, Hubble entered graduate school at Yerkes Observatory and secured his doctorate in 1917. His skill and reputation landed him a prestigious position at Mount Wilson Observatory near Pasadena. After an unexpected detour to serve in World War I (for which Hubble volunteered), Hubble showed up for work at Mount Wilson in 1919, just as the giant 100-inch telescope came into service.
With this big telescope, the ambitious Hubble tackled a big problem: the nature of the spiral nebulae. He measured their distribution in space. He took stock of their various shapes, developing a galaxy classification system that still bears his name. And he searched the nebulae for novae – stars that suddenly brighten – to estimate their distances.
In the fall of 1924, on a photographic plate of the Andromeda Nebula, Hubble discovered something even more remarkable: Cepheid variable stars. As was discovered by Henrietta Leavitt at Harvard, Cepheids varied in brightness with a period directly proportional to their intrinsic brightness. So once Hubble carefully measured the period of Andromeda’s Cepheids, and measured their apparent brightness, he could easily calculate their true brightness and the distance to the nebula. An image of the Cepheid variable on Hubble’s photographic plate is visible at the top of the page.
Hubble’s measurements yielded an astonishing result: the Andromeda Nebula was one million light years away, some ten times the size of our own Milky Way. (With better measurements and understanding, astronomers today estimate Andromeda lies at a distance of 2.3 million light years). In a single stroke, Hubble proved Andromeda and other spiral nebulae are massive “island universes” of stars in their own right, far outside yet quite similar to our own Milky Way. The discovery of these new universes, which astronomers came to call “galaxies”, made Hubble world-famous, and he gave lectures to packed halls of interested laypersons about his discoveries.
But Hubble and others noticed another surprise: a connection between the distance to a galaxy and its speed away from the solar system. Hubble and his skilled assistant Milton Humason, a former mule driver and janitor at Mount Wilson, began a painstaking study to measure the distance to many of the galaxies for which Vesto Slipher measured speeds. Using Slipher’s data, and data collected by Humason over hundreds of nights of meticulous and exhausting observation at Mt. Wilson, they found a direct relationship between a galaxy’s distance and its speed away from our solar system. Here’s an image of their initial results, published in 1929:
This completely unexpected relationship means the further a galaxy, the faster it recedes. The relationship is written as:
where V is velocity (in km/s), D is distance in parsecs, and Ho is a number now known as the “Hubble Constant”. This simple equation is known as “Hubble’s Law”. The Hubble constant is a critical number in the study of the universe. Its current most accurately measured value is 73.8 (km/s)/megaparsec, which means the expected speed of a galaxy at a distance of one megaparsec (about 3.26 million light years) is 73.8 km/s.
This deceptively simple relationship, gleaned from the hard work and skill of Hubble and Humason, completely baffled astronomers. Hubble himself, always the patient observationalist, was careful not to tarnish his reputation with reckless speculation about the meaning of his discovery. But there was an explanation… and it came from the imagination and mathematical skill of none other than Albert Einstein, with the help of an obscure Belgian priest and mathematician who had the insight and courage to see what Einstein refused to believe…
Until next time…