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."
Related Web Sites
Illuminating Black Holes
In addition to developing theories about the physics of black holes, galaxies, and other space objects, Professor Quataert also serves as the director of UC Berkeley's Theoretical Astrophysics Center. Image credit: courtesy Eliot Quataert
In a universe filled with improbable objects, supermassive black holes are among of the oddest of the odd. They lurk at the centers of galaxies like giant spiders, gobbling up much of the matter and energy within their gravitational reach. Such an appetite leaves black holes at the centers of galaxies quite corpulent, with most topping out at a few million to a few billion times the mass of our Sun. At the same time, black holes remain surprisingly compact; a fraction of a teaspoon could weigh billions of tons. This great density arms black holes with the most powerful gravity in the universe.
Yet these galactic hubs are tricky to study. Since supermassive black holes swallow rather than emit any light of their own, scientists must instead observe the objects that surround them and the radiation produced by gas in their vicinity.
As a theoretical physicist and professor of astronomy at UC Berkeley, Eliot Quataert uses these observations to deduce how black holes and other astronomical objects work. "One of the really attractive things about astrophysics research is you can take relatively basic concepts of physics and apply them to understand the vast array of things that we observe in the universe." Though Quataert studies a wide variety of astrophysical problems, one of his major areas of expertise is how supermassive black holes grow and affect the development of their galaxies.
According to Quataert, black holes have effects on their galaxies all out of proportion to their size and mass. "It's as if a speck of dust had a huge effect on everything in a room," Quataert says.
"A Ring of Fire": Gas falling into a black hole, based on numerical simulations. Most of the light is produced just before the gas crosses the event horizon, giving rise to a ringlike structure. Over the next decade, astronomers should be able to observe this event directly, testing this and other theoretical predictions of black hole behavior. Image credit: Josh Goldston-Peek and Eliot Quataert
This far-reaching influence stems from the halo of matter and radiation streaming outward from each black hole. "Together with neutron stars, black holes emit the brightest electromagnetic radiation in the universe," Quataert says. Yet the only part of this process scientists can observe is the amount and electromagnetic spectrum of these flares.
From these rudimentary clues, Quataert and other astronomers have sketched out a compelling picture of black hole operations. The black hole's powerful gravity pulls any gases drifting nearby into a spinning pancake known as an accretion disk with the black hole at its center. With every revolution, these gases moves closer to the black hole itself. Just before crossing the black hole's event horizon—the point of no return—the gases emits a tremendous burst of radiation in the most efficient energy-producing reaction known to mankind, Quataert says. "If you had a choice for energy on Earth and could choose between building a little black hole and throwing some gas into it and performing nuclear fusion, the black hole would be a much better energy source."
Quataert is now deciphering why black holes belch out the amount and frequency of electromagnetic radiation they do. The magnetic field around each black hole likely plays a starring role. "Energy from the gas gets stored in the magnetic field," he says, slowing the rotation of any orbiting gas. Eventually, the gas loses so much momentum that the gravity of the black hole can capture the gas particles and send them spiraling toward its gaping maw.
At the same time, Quataert has found the magnetic field returns some of the energy stolen from the gas as heat and turbulence. This accelerates the gas until the matter closest to the black hole is whirling around at nearly the speed of light. The gas in the disk grows hotter and hotter until it eclipses the temperature of the sun. Then, just before it crosses the event horizon, it turns into the blaze of radiation we see from Earth. "Understanding in detail the chain of energy—how it goes from inflowing material into magnetic fields, back into the particles via turbulence, and then into radiation—needs to be understood to interpret the radiation we see," Quataert says.
An X-ray image of two giant black holes at the center of a nearby galaxy. X-rays are produced when hot gases fall into a black hole. Supermassive black holes at the centers of galaxies produce so much radiation that they dramatically influence how their host galaxies form. For scale, this image is roughly 30 thousand light-years across&mdsah;nearly a billion times larger than the gas disk illustrated in the "Ring of Fire" above. Image Credit: NASA/CXC/MPE/S.Komossa et al.
Recent observations indicate that black holes in the center of galaxies weigh almost exactly a thousand times less than the mass of the stars in their galaxy. Quataert has developed a theory explaining this strange relationship. "A black hole at the center of a galaxy eventually gets so big that the amount of radiation and light outflowing from it stops more gas from falling in, and the black hole's not going to be able to grow." Quataert says.
This phenomenon, in turn, affects the size of the host galaxy itself. "If the black hole is pushing a lot of its galaxy's gas outward into the universe, there's less matter available to turn into stars," Quataert says. "The galaxy will end up being less massive than it otherwise would have been." The resulting equilibrium defines the limits of some of the largest structures in the universe.
"If we understand the gas around black holes, and its behavior and dynamics, we'll be ultimately able to say something quantitative about Einstein's predictions of what gravity looks like around a black hole," Quataert says—and help illuminate one of the darkest mysteries in the cosmos.
Related Web Sites
When Galaxies Collide
Astrophysical theorist Chung-Pei Ma making observations at the Keck Telescopes on the Big Island of Hawaii. Ma is one of the deputy directors of UC Berkeley's new Center for Cosmological Physics. Image credit: Alison Coil
Crash testing might seem more suited to a Detroit carmaker than a UC Berkeley academic. However, that hasn't stopped astronomy professor Chung-Pei Ma, who stages some of the most spectacular crashes imaginable. But cars and crash-test dummies aren't her area of expertise. Instead, this astrophysical theorist arranges collisions between entire galaxies.
In a cosmos that consists largely of vast stretches of nothingness, galaxies possess the strongest gravity around. This force not only keeps the planets, stars, dust, and gas in each galaxy together as they sail through space, it plows galaxies into one another like jalopies in a demolition derby.
The resulting collisions are awe-inspiring. Gases get accelerated to supersonic speeds, triggering the formation of new stars. Stars are hurled about at ever increasing speeds, until some are slung straight into outer space. Supermassive black holes, normally ensconced at the center of their own galaxies, end up circling one another like wary panthers. Their hulking movements set off powerful ripples in the membrane of spacetime known as gravitational waves.
Though such titanic pyrotechnics are intrinsically appealing, Ma has more serious reasons for studying galactic collisions. Her findings are revealing how some of the largest structures in the universe, elliptical galaxies and galaxy clusters, form and continue to evolve as the universe ages.
Four of the five galaxies in this image are involved in a violent collision. Located in a galaxy cluster known as Stephan's Quintet, the crash has produced one of the largest shock waves ever seen (green arc). Made up of superheated atoms of green gas, the wave covers an area greater than our own Milky Way. This false-color image was compiled using data from NASA's Spitzer Space Telescope and a ground-based telescope in Spain. Image credit: NASA/JPL-Caltech/Max-Planck Institute/P. Appleton (SSC/Caltech)
Elliptical galaxies are the most massive galaxies in the universe. They are shaped more like an ovoid vitamin pill than the pinwheel of spiral galaxies like our Milky Way. Many of the biggest elliptical galaxies observed sit at the center of galaxy clusters, agglomerations of thousands of galaxies gradually drawn together by the force of gravity.
"We believe that these larger galaxies came about not just by pulling in matter locally and condensing them through gravity," Ma says. "The first galaxies probably came about that way, but later on, bigger ones arose from the mergers of smaller ones. So there were a few startups when the universe was young, but you form the biggest ones gradually out of many generations of galaxy cannibalism."
When two galaxies collide, what transpires is very different from, say, one billiard ball smacking into another. Instead of ricocheting away in opposite directions, galaxies are much more likely to meld together. After all, Ma points out, "Galaxies are mostly empty, so the stars and dark matter mostly just pass each other by. The chances of two stars hitting each other is tiny." In fact, only one percent of the masses of these galaxies consists of matter we can see, such as stars and gases. The rest consists of dark matter—material we can't see but astronomers have inferred from many observations must exist.
Actual galaxy mergers are hard to find and even harder to view. So Ma is doing the next best thing—simulating galaxy collisions using computer models. This way, she can specify the types of mergers she wants to analyze—head-ons versus glancing blows; galaxies of different masses and shapes; even the occasional threesome—and analyze their fates with mathematical precision.
In her simulations, Ma and her UC Berkeley collaborators Michael Boylan-Kolchin and Eliot Quataert use virtual particles to represent chunks of galactic mass, taking into account their density profiles, shapes, and initial orbits. The computer then calculates the mutual gravity between every pair of particles in a simulation, and tracks their resulting velocities and positions. It takes thousands of iterations of this process on hundreds of interconnected computers to portray a merger event from start to finish.
At smaller scales, such as those between individual galaxies, gravity dominates the interactions between objects. But in the universe as a whole, other forces play pivotal roles in molding the present shape of the cosmos.
Ma is now trying to understand how these forces—in particular dark energy, believed to be responsible for accelerating the expansion of the universe—can affect individual galaxy mergers. For this, she uses a second type of model, known as a cosmological simulation, which recapitulates the large-scale evolution of the universe. "The cosmological simulation will tell us at the end of the day how many smaller young galaxies were gobbled up to form a galaxy of its present size and mass," Ma says.
Because tracking the many billions of particles needed to represent the entire universe taxes even the fastest supercomputers, cosmological models can only provide coarse-grained data about individual galaxies. To work around this problem, Ma and her collaborators are designing a scheme to combine large cosmological simulations with results from their more detailed individual galaxy collision simulations. The results should provide more realistic, higher-resolution depictions of what galaxy collisions really look like.
Learning how such large-scale structures have affected the overall formation of the universe, Ma says, is what keeps her arranging still more galactic smashups. "Galaxy mergers are exciting to me because they are the basic processes responsible for the formation of galaxies, the building blocks of the universe. At the same time, they connect so many different pieces of the cosmological puzzle."