A New Chapter in the History of the Universe
With advances in satellite technology, scientists have a better sense of the origin of the universe. But why has it been such an exhilarating time for cosmologists?
Published November 1, 2003
By Robert Irion
Academy Contributor

A small satellite recently generated big headlines by staring at empty space. In February, scientists announced that a modest explorer orbiting a million miles from Earth revealed some of the most basic properties of the universe. We learned that our cosmos exploded into existence about 13.7 billion years ago, that the first stars started shining when the universe was just 200 million years old, and that space will expand faster and faster as time goes on. The discoveries came from stunning images of the most ancient light we will ever see.
Cosmologists are still buzzing about the importance of this faint light, called the “cosmic microwave background,” or CMB. Quite literally, the radiation is the fading heat left over from the birth of the cosmos in the Big Bang. It fills space with a nearly uniform glow of microwaves. However, the microwaves are so cold that only the world’s most sensitive detectors can see them. Subtle patterns in the glow — blobs of slightly warmer and slightly colder space — contain key information about what happened in the Big Bang’s chaotic aftermath. “It’s the cleanest probe we have of the early universe,” says astrophysicist Amber Miller of Columbia University.
The satellite results were striking, but researchers aren’t stopping there. For instance, Miller will help erect a 20-foot telescope dish high in the desert of northern Chile to study small patches of the radiation in greater detail. This should let astronomers trace how giant clusters of galaxies have evolved. Meanwhile, Miller’s colleague at Columbia, astrophysicist Zoltan Haiman, is examining other imprints seared into the microwave glow by the first stars. Those signatures may solve the puzzle of how stars formed so quickly after the Big Bang.
An Exhilarating Time for Cosmologists
It all adds up to an exhilarating time for cosmologists. “This is a completely new chapter in our study of the universe,” says Max Tegmark of the University of Pennsylvania. “The CMB is attracting a lot of the best young physicists and astronomers,” adds Marc Kamionkowski of the California Institute of Technology. “They want to work in this field.” That group includes Haiman and Miller, both of whom joined Columbia’s faculty in the fall of 2002.
The field’s current star is the little satellite called WMAP, for Wilkinson Microwave Anisotropy Probe. NASA launched the $95 million mission in June 2001 and sent it to deep space with a unique flight plan. Instead of orbiting around Earth, the satellite meanders around a special place called the L2 Lagrange point. At that spot, almost 1 million miles from Earth in a straight line away from the sun, the combined gravitational pulls of Earth and the sun counteract the force required for the satellite to revolve in tandem with Earth. WMAP thus drifts lazily around L2, needing little fuel. “We’re our own little planet out there,” says WMAP team member Lyman Page of Princeton University.
The calm setting in Earth’s shadow is crucial. The satellite’s sensors must register microwaves at a frigid -455°F – only about 5 degrees above absolute zero, the temperature at which all motion stops. Actually, the challenge is far harder: two pairs of detectors must spot differences of a few millionths of a degree as the satellite peers at two separate patches of the sky. By combining millions of those readings for a year, the WMAP team charted minuscule temperature fluctuations across the entire sky.
A Thing of Beauty
The pattern looks complex, but to scientists it’s a thing of beauty. “In some sense, the CMB is very simple to interpret,” Haiman says. Physicists think the warmer and colder ripples started out as quantum fluctuations in the intensely hot fireball of the big bang. The fluctuations made parts of the hot plasma slightly more dense than others. Those dense spots kept growing, because their stronger gravity pulled more material inward. At the same time, intense radiation from the hotter spots tried to blast them apart. This push-and-pull created oscillations that reverberated through the infant universe.
For a long time after the Big Bang, Haiman explains, this plasma was too hot for atoms to form. Instead, atomic nuclei and electrons dashed freely through space. The electrons absorbed light, trapping all light within the oscillating plasma. Then, when the universe was about 380,000 years old, temperatures were cool enough for electrons to bind to the nuclei and make hydrogen and helium atoms. Space suddenly became transparent. Light escaped for the first time.
However, the light didn’t stream equally in all directions. Rather, it preserved a record of the oscillations: slightly hotter radiation in some spots, slightly colder in others. The denser patches formed the seeds for stars and galaxies to grow, as their gravity continued to attract more gas. More than 12 billion years later, it seems miraculous that today’s instruments can still perceive this ancient light and its ripples.
Main Conclusions
After analyzing the temperature ebbs and flows, the WMAP scientists announced their main conclusions in February:
– The physical sizes of the hot and cold spots, which are related to how long the radiation has traveled toward us, show the universe is 13.7 billion years old (plus or minus 200 million years). That’s a far more accurate age than previously known.
– A tendency for the microwaves to align in preferred directions, a property called “polarization,” suggests that the first luminous objects burst forth with light about 200 million years after the Big Bang. Before WMAP, most astronomers assumed that star formation took several hundred million years longer than that.
– The magnitudes of the temperature ripples, when compared to physical theories of how they formed, reveal the overall ingredients of the universe. Our strange cosmos consists of just 4% ordinary matter, the stuff of stars, planets, and people. Another 23% is unidentified “dark matter,” and a whopping 73% is “dark energy.” This mysterious component, which acts as a kind of antigravity, arises from some unknown property of the fabric of space itself. It is growing more powerful as the universe gets bigger, so the universe now expands ever faster as time goes on.
‘Precision Science’
Other projects had concluded that the universe has similar constituents, but WMAP nailed it with authority, says astrophysicist John Bahcall of the Institute of Advanced Study in Princeton: “This announcement represents a rite of passage for cosmology from speculation to a precision science.”
WMAP’s successes have pushed scientists to probe even more deeply into the intricacies of the CMB. For instance, Haiman thinks the polarization patterns — only seen indirectly by WMAP — will teach us how the first stars coalesced. This conundrum has long fascinated Haiman. “We know for sure from the CMB itself that the universe started from a smooth state,” he says. “But today we look around us, and the universe is full of structures. How did this happen?”
The answers lie not in the moment when light began to stream freely, but in later events that altered the microwaves. “We usually think of the cosmic microwave background as a snapshot of the baby universe at an age of 380,000 years, when the radiation emanated,” Haiman says. “The naïve picture is that it comes to us completely unaffected, 13.7 billion years later. But as this radiation has traveled, everything that has happened on its way to us potentially leaves an imprint. It probes the intervening years. And the most conspicuous signature is this epoch of the first structure formation.”
Reionization
As Haiman explains, the infant universe was hot and ionized, or full of electrical charge, because no electrons were bound to atomic nuclei. After 380,000 years, electrons joined with atoms. The universe became electrically neutral. Thus began a long epoch dubbed the “dark ages,” because nothing in the universe shone. When the first stars finally did spark into life, their fierce ultraviolet light stripped those electrons away from hydrogen atoms. Parts of the universe once again became electrically charged, inside bubbles of space around baby stars. Astronomers call that critical process “reionization.”
Theorists had thought it would take much longer than 200 million years before conditions were calm enough for gas clouds to collapse into tight balls. Even more puzzling, computer simulations suggest that the first stars were colossal objects: 100 times the mass of our sun or more.
Current theories have a hard time accounting for such monstrous stars so soon. Some process must have cooled off the hydrogen gas more efficiently than astronomers thought possible, Haiman says. Only then could the cold gas get dense and compact enough to ignite stars. Starlight’s debut in the young universe probably wasn’t smooth, Haiman adds. Massive stars radiated so violently that they would have interfered with other stars trying to form nearby. “I see this process as sputtering to life, with lots of starts and stops initially,” he notes.
Further analysis of the CMB should help him and his colleagues trace this key epoch in cosmic history. They will get a raft of new data after 2007, when the European Space Agency launches an exquisitely sensitive satellite called Planck. That mission will probe the microwave polarization patterns in far greater detail.
Space Mission Not Required
Not all CMB research requires a space mission. Indeed, Columbia’s Amber Miller is among scores of astrophysicists who have their feet — and their experiments — firmly on the ground. “You can never touch an instrument again once it’s in space, but we can tinker as much as we need to on the ground,” she says. What’s more, research on the ground is much cheaper. But the disadvantage is steep: The ground is a much warmer and noisier place than deep space. Spotting temperature ripples of millionths of a degree isn’t practical from most spots on Earth. For that reason, CMB scientists usually work at the coldest, driest, highest sites they can find.
For their next major telescope, Miller and her colleagues chose a spot 17,000 feet high in northern Chile’s Atacama Desert. “It’s one of the driest spots in the world,” Miller says. “Almost nothing grows, and the rocks are bright reds and purples. At night, you can see more stars than you could ever imagine.” The location is intoxicating in another sense: “Your brain doesn’t work too well at 17,000 feet. We really have to work together and watch each other.”
The project, called the Atacama Cosmology Telescope (ACT), is led by Lyman Page at Princeton. Starting in 2006, a microwave-collecting dish about 20 feet across will steer signals from deep space onto a large array of super-sensitive electronic detectors. The team’s goal is to examine the microwave patterns at much smaller scales than WMAP — down to fractions of an angular degree. Other experiments at the South Pole call for a similar approach.
Holes in the CMB
Page’s team will search for “holes in the CMB,” in Miller’s words. According to theory, giant clusters of galaxies should distort the microwaves as they stream past. Hot gas embedded in a massive cluster should boost some of the colder, low-frequency microwaves to higher frequencies. So, seeing a low-frequency “hole” in the background should indicate that ACT is looking toward a huge galaxy cluster. Pointing the telescope in many different directions will reveal how commonly such clusters arose. Furthermore, astronomers will use large optical telescopes to spot the clusters visually and gauge their distances.
The hole phenomenon, called the “Sunyaev-Zel’dovich effect,” is not just a galactic census. “This is an extremely powerful way to probe the growth of the largest structures in the universe,” Miller says. Just as it is hard to understand how stars formed within 200 million years of the Big Bang, astronomers wonder how the universe managed to spawn gigantic clusters of thousands of galaxies. Large masses of hidden dark matter clearly played the key role. However, astronomers don’t have a handle on the early stages of that process. It’s all encoded in the CMB, and powerful telescopes like ACT should expose what transpired.
Telling the story of how mass assembled in the universe is but one of many ways that astrophysicists plan to use the CMB as a tool. Other challenges await, such as an explanation of the bizarre dark energy. We can expect scientists to poke and prod at the oldest light in the universe for the next decade and beyond, searching for answers. As John Bahcall says, “We have to learn how to understand this unattractive universe, because we have no other choice.”
Also read: The Anthropic View of the Universe and What Caused the ‘Bang’ of the Big Bang?
About the Author
Robert Irion, a freelance science journalist in Santa Cruz, California, and a contributing correspondent for Science, is coauthor of One Universe: At Home in the Cosmos, published by Joseph Henry Press.