Hubble's Guide to the Expanding Universe



by cbrownlee{at}nas.edu
PNAS Staff writer


Color-composite image of spiral galaxy NGC 1512, created from images taken by Hubble Space Telescope cameras. Space Telescope Science Institute.
Think you've established your place in the universe? Think again. Your position right now isn't the same as it was just a second ago. As you read this article, our galaxy, the Milky Way, is hurtling through the universe at a speed of several hundred kilometers a second. At the same time, our neighboring galaxies, scattered throughout space, are speeding farther away from us and each other. In essence, our universe is expanding.

For such a fundamental assumption of modern astronomy, the realization that the universe is growing is relatively new. Only 74 years ago, scientists were aware of little motion outside the comparatively static interior of the Milky Way. The revelations of a single researcher, Edwin Hubble, rushed expansion straight to the forefront, opening up the entire branch of astronomy known as cosmology. His famous paper, published in PNAS in 1929 (1), proved with observational evidence that galaxies are moving away from us with a speed proportional to their distance. Hubble's revolutionary vision of a swelling cosmos gave astronomers new insight into the birth of our universe, as well as a glimpse of its future evolution.
Measuring Success in Light Years


Edwin Hubble, January 1940. © Copyright California Institute of Technology.
Hubble's early meanderings almost detoured him completely from a career in astronomy and gave no indication of his ultimate role as the "father of cosmology." After receiving an undergraduate degree in mathematics and astronomy from the University of Chicago, Hubble made a promise to his dying father to study law. His intelligence and athleticism helped him land a prestigious Rhodes scholarship to Oxford University, where he ran track, played baseball and basketball, and boxed while earning his law degree. As a new attorney, Hubble joined the Kentucky bar association upon his return to the United States, but switched tracks again to teach high school Spanish.

However, Hubble's heart was with the stars. After one year as a high school teacher, he signed on as a graduate student at Yerkes Observatory at the University of Chicago and earned his PhD in astronomy. Although a short term as an enlisted soldier in World War I almost sidetracked him once again, Hubble was invited to join the staff of Mt. Wilson Observatory in 1919.

The 100-inch telescope at Mt. Wilson, high above Pasadena, CA, was the most powerful in the world at the time. Hubble first set his sights on using the telescope to elucidate the nature of galactic nebulae--fuzzy, glowing patches then thought to be clouds of burning gas. After resolving individual bright spots in several of the nebulae, Hubble decided that the nebulae were remote clusters of stars. However, the exact distance between these clusters and the earth was a mystery.

Galaxy NGC 3310, as seen from the Hubble Space Telescope. Space Telescope Science Institute.
According to Martha Haynes, an Academy member and Cornell University astronomer, many contemporary scientists believed that the Milky Way was the entire universe. Were these nebulae at the edges of the Milky Way, or were they possibly "island universes" far away from our own? Hubble would later derive the answer from computing the distance to these star clusters and comparing it with the known edge of the Milky Way--not a minor task when distances can extend to millions of light years.

"It's not a trivial job in astronomy when you can't go out and walk across the field and measure how far it is, or send someone out to make a roadmap," said Haynes.

However, in the early 1900s, astronomer Henrietta Leavitt perfected a technique to gauge the distance to a type of star known as a Cepheid variable (2). These stars vary in brightness over a period related to their distance. With the powerful Mt. Wilson telescope, Hubble was able to spot Cepheid variable stars in the nebulae and calculate their approximate distance. The measurements far exceeded any distances for stars in the Milky Way, leading Hubble to conclude that the nebulae were spiral galaxies much like our own. He published this research in 1925 in The Astrophysical Journal (3).
A Universe Like Raisin Cake


Concurrent with Hubble's research, other scientists were studying Doppler shifts in the light coming from galaxies. Like the change in pitch from a passing car radio, light waves also change in frequency with velocity. The effect is invisible to the naked eye, but it is detectable with measurements taken from photographic plates. Hubble's contemporary researchers had noticed that many of the galaxies were redder than they should be--a possible indicator that the galaxies were moving away.

Edwin Hubble at the 48" Schmidt Telescope, Palomar Observatory, 1949. © Copyright California Institute of Technology.
By measuring the extent of these Doppler shifts with his assistant, Martin Humason, Hubble came to the startling conclusion that the galaxies weren't shifting around in a random way. Instead, most were moving away from the Milky Way at the speed of hundreds of kilometers a second. Although he had a relatively small amount of data, Hubble was able to model the data with a simple calculation in his 1929 PNAS paper. Combined with Hubble's earlier measurements of galaxy distance, the conclusion he drew became what is now known as Hubble's Law: the farther away a galaxy is from the Milky Way, the faster it's racing off into the cosmos.

The usual example for Hubble's law is a raisin cake, says Neta Bahcall, an Academy member and astronomer at Princeton University. "Put a raisin cake in the oven, and it's very small. Then you let it go, and the distance between the raisins is like the distance between the galaxies - it gets larger and larger with time," she said. The inescapable deduction is that the universe, like the cake, is also growing larger and larger, expanding indefinitely with time.
An Uncertain Future


Since Hubble's 1929 PNAS paper, the science of cosmology has expanded almost as much as the universe. Today, thousands of scientists all over the world study the tenets of cosmology, including the origin, structure, and relationships of objects in the universe. At least 31 universities in the United States offer doctoral programs in the field, and several journals, including The Astrophysical Journal, The Astronomical Journal, and PNAS, regularly publish new cosmological research.

Currently, researchers who have continued Hubble's study of the expanding universe have come to their own surprising conclusion--that the rate of expansion seems to be accelerating. Results from two independent teams of scientists, published in 1998 (4, 5), show that galaxies are moving apart more quickly with time. Hubble originally estimated the rate of expansion--found by dividing velocity by distance--to be about 500 kilometers/second/megaparsec (a megaparsec is 3.26 million light years). However, this number, known as Hubble's Constant, has since been radically revised because initial assumptions about stars yielded underestimated distances. The most recent estimate of the expansion rate, computed with data from the Wilkinson Microwave Anisotropy Probe, is about 71 kilometers/second/megaparsec, and accelerating.

"Very nearby, Hubble's Law is very good. But we've found that if you look over big enough spans of distance, the distance and velocity are not exactly proportional to each other," said Robert Kirshner, an Academy member and astronomer at Harvard University.

Researchers aren't sure why the expansion is accelerating, but it may have to do with a substance known as dark energy. Completely undetectable by scientists and currently only a theory, dark energy may be driving the acceleration and defying gravity's pull. Kirshner says that the amount of invisible dark energy in the universe may far exceed other visible material, such as stars and planets.

"The universe may be like Los Angeles," Kirshner jokes. "It's one-third substance and two-thirds energy."

A false-color image of the Hubble Space Telescope's "Deep Field." Red regions correspond to galaxies and stars. Space Telescope Science Institute.
However, scientists studying this phenomenon are equipped with better tools than in Hubble's day. Mt. Wilson Observatory is rarely used now due to light pollution, but several 200-inch telescopes have since been built, and a pair of 400-inch telescopes currently operate at Keck Observatory in Mauna Kea, Hawaii. Additionally, a satellite launched in 1990 carries the Hubble Space Telescope, capable of viewing galaxies more than 10 billion light years away. Each of these telescopes is equipped with digital light detectors far more sensitive than Hubble's photographic plates.

Currently, researchers are using another powerful telescope to conduct the Sloan Digital Sky Survey, the largest study of galactic velocities to date. The results of this research may provide some much-needed insight into the future of our universe, continuing Hubble's goal of understanding our cosmos. However, new findings are likely to spur an additional round of questions, similar to the uproar that followed publication of Hubble's original 1929 research.

"We used to be able to explain what the universe was doing to our students and now we can't anymore," said Haynes.


References


1. Hubble, E. (1929) Proc. Natl. Acad. Sci. USA15, 168-173.|Article|

2. Leavitt, H. (1912) Harv. Coll. Obs. Circ. 173.

3. Hubble, E. (1925) Astrophys. J.62, 409.

4. Schmidt, B. P., Suntzeff, N. B., Phillips, M. M., Schommer, R. A., Clocchiatti, A.
Kirshner, R. P., Garnavich, P., Challis, P., Leibundgut, B., Spyromilio, J., et al. (1998) Astrophys. J.507, 46-63. |Article|

5. Perlmutter, S., Aldering, G., Della Valle, M., Deustua, S., Ellis, R. S., Fabbro, S., Fruchter, A., Goldhaber, G., Goobar, A., Groom, D. E. et al. (1998) Nature (London)391, 51-54. |Article|
 
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