Volume 4, Issue 26 March 2007
The Seeds of Structure in the Universe
For his work discovering the origins of structure in the universe, George Smoot was awarded the 2006 Nobel Prize in physics. Image credit: Nobel Foundation
To the uninitiated, the night sky is a chaotic place. The stars and planets are sprinkled randomly across the inky backdrop of space, sugar tossed upon a velvet drape. Astronomers; however, see a completely different view. Their universe is a well-ordered place, finely structured on every scale. Scores of moons and planets orbit suns. Squadrons of star systems circle galaxies. Galaxies themselves clump into galactic clusters that are scattered evenly through the universe as far as we can see.
But how did all of this structure come to be? Current theories suggest that the Big Bang filled the nascent universe with an intense and perfectly uniform radiation. Known as the cosmic microwave background, or CMB, ghostly remnants of this outpouring of energy still permeate the universe. When first detected in 1964, it appeared to be an unvarying across the sky at 2.7 degrees Kelvin.
Scientists realized that, under closer scrutiny, the CMB might yield minute temperature variations. Over the next 13.7 billion years, with the help of gravity, these infinitesimal energy fluctuations would have grown into today's planets, stars, and galaxies.
But the Earth is a warm and thermally noisy place—too noisy to study the tiny temperature differences predicted in the CMB. So in 1974, NASA took the study of the CMB to a new level by commissioning the Cosmic Background Explorer (COBE) satellite. The mission carried two experiments probing the CMB. One, led by John Mather of NASA's Goddard Space Center, aimed to measure the intensity of the wavelengths in the CMB radiation spectrum. The other, led by George Smoot, was designed to locate actual temperature fluctuations in the CMB.
An image of the cosmic microwave background obtained by COBE. The colors represent temperature differences of several hundred-thousandths of a degree. These temperature fluctuations correspond to minute density differences that gravity amplified into galaxies and other large-scale structures in space. Image credit: George Smoot
"The technical challenges were extremely high," says Smoot, now a Professor of Physics at UC Berkeley. "We were looking for a signal that varied by just a hundred thousandth of a degree." The satellite would use exquisitely sensitive thermal detectors to compare the relative temperature of every corner of space.
Due to delays caused by the tragic explosion of the space shuttle Challenger, COBE didn't launch until 1989. But Smoot's experimental results, published in 1992, were well worth the wait: The data they collected was nothing less than a map of the first seeds of structure in the universe.
"We found that the CMB contained information about the epoch at which the universe became fertile and began developing all of the structure it contains today," Smoot says. The minute thermal variations they found correspond to density differences that grew into the galaxies, galaxy clusters, and other large-scale structures we see in space today. Meanwhile, the spectrum detected in Mather's experiment indicated that virtually all of the Universe's radiant energy was released within a year of the Big Bang. For their groundbreaking work on COBE, Smoot and Mather shared the 2006 Nobel Prize in Physics.
"COBE was very important as a pathfinder," Smoot says. "We set the standard for this field to become serious and quantitative." Since then, scientists have been busy mapping the CMB with ever finer precision. Smoot is now involved in developing the third-generation mapping mission, known as the Planck Surveyor; it's scheduled to launch in 2008.
Now Smoot and other astronomers plan to use the data gathered by COBE and its progeny to test the limits of our understanding of the universe.
"We're getting to the point now where we can do calculations that match the data we've collected so far, which have described the universe down the few percent level. But within the next few years, we should get to within the one percent level. When you get there, you have a qualitative change in what you can do," Smoot says.
The cosmic microwave background radiation is a remnant of the Big Bang. It is essentially a baby picture of the early universe when it was approximately 400,000 years old. Over the next 13.7 billion years, its mosaic of temperatures evolved into the pattern of voids, galaxies, and galaxy clusters we see today. Image credit: NASA WMAP
Smoot compares it to buying trousers from a department store. "The waists come in one-inch increments. But if I could buy them in increments of one percent, 1/3 of an inch—that's finely tailored. You have to be careful what you eat so that your pants will still fit. What that means is, we're going to move from just trying to determine the general parameters of the universe, to probing whether the physics we're using fits what we know about the universe as a whole."
A number of factors make astronomers question whether they are using the right physics to understand the universe. Equations describing the behavior of the universe require four pieces of new science in order to match observations. "Dark matter" accounts for why galaxies have stronger gravity than predicted by their observed mass, "dark energy" accelerates the expansion of the universe, another factor explains the preponderance of matter over antimatter, and lastly, an epoch of exponential expansion called "inflation" is considered to have occurred at the very start of the universe. But no one has ever detected direct evidence of these mysterious components. Other open questions include whether the Big Bang really occurred, the existence of extra dimensions, and whether other relics of the Big Bang remain to be discovered.
The Center for Cosmological Physics, with Smoot at its helm, focuses on answering these questions. "We have at Berkeley a unique set of people who are studying the microscopic and macroscopic theory, making observations, and doing high-level computing. I'm trying to put those people together."
"It's really an exciting time in cosmology because you can talk about these things and not have people laugh and say, 'that sounds like science fiction,'" Smoot says. "With the level of detail in our observations, and the fact that we can do tremendous calculations on our computers, we're now in a position to probe for some of the answers."