An Illuminating Journey.

Reading cosmic history from the Big Bang's radiation

Holding his 3-month-old son, Joshua, in his arms, cosmologist David N. Spergel will proudly watch the launch next week of a NASA satellite that he helped father. The satellite will record the remnant glow from the infant universe in greater detail than ever before. Even before the Microwave Anisotropy Probe satellite gets off the ground, however, Spergel and his colleagues have hatched a plan to examine that early light in a dramatically different way.

The satellite will continue the practice of treating the cosmic microwave background (CMB)--the glow left over from the Big Bang--as a snapshot of the early universe. Taking a new perspective, Spergel and his colleagues propose to use that radiation as a flashlight to illuminate the evolution of structure in the universe over its 13-billion-year history. That evolution began with the condensation of gas clouds into fledgling galaxies and continued with production of the first stars and the assembly of galaxies into mammoth clusters.

To realize this ambitious idea, Spergel and several other astronomers intend to examine the subtle markings acquired by the CMB as it traversed billions of light-years to reach Earth. Like a weary traveler who has picked up dust from each country he's visited, the microwave background has been marked by all the cosmic architecture that it has encountered.

As it streams across the universe, the CMB "is lighting up the past between here and there," says Spergel, who is based at Princeton University. The photons in the CMB have been traveling freely through space ever since the universe was about 300,000 years old. "And a lot of things happened to them along the way," Spergel notes "You have lots of imprints on the microwave background from the emergence of structure."

Those imprints show up on a much finer scale than that of the primordial features painted onto the CMB by the Big Bang itself. Those relatively large hot and cold spots in the CMB, which the spaceborne Microwave Anisotropy Probe will detect, represent the seeds from which galaxies and galaxy clusters ultimately arose.

Galaxies and galaxy clusters buffet the CMB photons coursing through them. These interactions generate hot and cold spots that differ by only a millionth of a degree or so from the average temperature of the microwave background, a chilly 2.76 kelvins. That's one-tenth as small as the temperature fluctuations imprinted on the CMB by the tumultuous conditions in the nascent universe.

The tinier, post-Big Bang variations also occur on spatial scales only one-third the size of the primordial hot and cold spots that the Microwave Anisotropy Probe can discern.

The sensitive detectors required to study these tiny variations, which theorists first described in the late 1960s and 1970s, are now available, says Spergel. Moreover, these instruments can work in observatories on Earth and don't require a costly launch into space. At a meeting of the American Physical Society in Washington, D.C., last April, Spergel described these devices and the theory driving their development.

One of the devices, proposed by Lyman A. Page of Princeton, along with Spergel, Mark J. Devlin of the University of Pennsylvania in Philadelphia, and their collaborators, is a single, 6-meter radio telescope that would be built in the desert of northern Chile. If funded, it could begin operating in 2004, he estimates.

John E. Carlstrom of the University of Chicago and his colleagues are targeting the same goal with a two-instrument approach. The first instrument, now under development, consists of an array of six 3.5-m radio dishes at the Owens Valley Radio Observatory near Big Pine, Calif. It will ultimately be used in tandem with an existing array at the observatory and with a group of radio telescopes known as the Berkeley Illinois Maryland Association array, now located in Hat Creek, Calif. The team expects to find thousands of previously unseen clusters through their imprint on the CMB.

Carlstrom's second instrument, which would be installed at the South Pole, would be an even more sensitive CMB telescope. Together, the two instruments would be akin to a rough and a fine focus on the small-scale fluctuations in the CMB

The Owens Valley array would work because galaxy clusters are bathed in hot gas that affects photons. When photons from the CMB strike the gas, they scatter and gain energy. Because the photons are kicked up to higher energies, there are fewer at lower energies than expected. It's this deficit, known as the Sunyaev-Zel'dovich effect, that Carlstrom and his colleagues have already begun to detect with existing instruments.

Using the information that they plan to get from the new array in California, says Carlstrom, astronomers can determine how the density of clusters has changed over cosmic time. That in turn can provide important clues about the kind of universe we live in. For instance, if the universe is expanding at a constant rate and the density of matter is relatively high, "then clusters would still be forming at a pretty good clip today," says Carlstrom. "Matter would still be winning the battle against the expansion of the universe."

On the other hand, suppose the cosmos has recently revved up its rate of expansion, as recent studies indicate (SN: 3/31/01, p. 196). Then, clusters would had to have formed relatively early in the history of the universe. At later times, gravity would not have been powerful enough to resist the accelerated expansion, and galaxies could not have congregated into clusters.

A coincidence of timing makes the study of clusters particularly intriguing, says Wayne Hu of the University of Chicago. He and other astronomers estimate that galaxies began gathering into clusters when the universe was roughly half its current age. That's about the same time that the cosmos began accelerating its expansion rate, according to recent observations. Whatever bizarre entity caused that acceleration--cosmologists call it dark energy, for want of a better term--may be revealed by closely examining when and how rapidly clusters assembled, notes Hu.

The imprint that a cluster leaves on the microwave background provides no indication of where that cluster lies and therefore how far back in time it hails, Carlstrom notes. To obtain that crucial piece of information, he and his colleagues plan to use large visible-light or infrared telescopes to directly image each cluster whose presence they discern with their radio-telescope array.

Because faraway clusters of a given mass interact with the microwave background just as strongly as nearby ones do, it should be possible to find the most distant clusters in the universe using this technique, Carlstrom adds.

It also should be possible to infer a cluster's velocity, which leaves another fingerprint on the CMB, Spergel notes. If a cluster is moving toward Earth, the CMB photons that the cluster altered will gain energy and be shifted to shorter, more energetic wavelengths. If the cluster is moving away from Earth, the CMB photons lose energy and are shifted to longer, less energetic wavelengths. In this way, "you can actually measure the velocity of large-scale structures in the universe," says Spergel.

Studying yet another imprint on the cosmic microwave background, astronomers hope to learn about the amount and distribution of dark matter, the invisible material that is believed to account for more than 95 percent of the matter in the universe. Such hidden matter seems to be required because all the visible material in the cosmos can't provide enough gravitational glue to keep clusters of galaxies intact--or, for that matter, even individual galaxies.

According to Einstein's general theory of relativity, all massive objects, whether or not they can be seen, betray their presence by altering the passage of photons that travel nearby. For instance, a massive cluster of galaxies in the foreground will distort the image of a background galaxy by bending and brightening the light from that object. That effect, known as gravitational lensing, slightly but noticeably distorts the radiation journeying through space from the cosmic microwave background.

The degree to which the microwave background undergoes this distortion indicates the density of dark matter and how smoothly it's distributed, notes Hu. "Using the cosmic microwave background to map the dark matter is an exciting possibility, because it can map this invisible material at the largest distances and over the largest scales possible," he says.

The telescopes that Spergel, Carlstrom, and others are proposing also have a chance of determining when the first generation of stars formed. That's because the ultraviolet light from these stars probably reionized the universe, separating atoms into ions and electrons, says Spergel. Atoms were initially ionized for several hundred thousand years following the hot Big Bang, but electrons and ions recombined as the universe cooled.

Unlike atoms, electrons readily scatter CMB photons, bouncing them back and forth. That's why the photons weren't free to stream into space until ions and electrons combined into atoms, some 300,000 years after the Big Bang. Reionization creates clumps of free electrons, and when the microwave background encountered this patchy, electrically charged fog, some of the stream of CMB photons were scattered in another direction. Other photons, initially moving in a different direction, were scattered into the stream.

The scattering acts to slightly blur, or wash out, the primordial hot and cold spots--the large-scale temperature fluctuations imprinted on the microwave background by the Big Bang. In addition, the motion of the clumps of electrons leaves their small-scale imprints--much smaller hot and cold spots--on the radiation.

Once the universe became reionized, it stayed that way. Although the first objects to reionize the universe had the greatest impact on the microwave background, all the stars that ever existed also have left their marks, often along the same line of sight with respect to scientists' telescopes. This overlaying effect prevents researchers from precisely determining when the first stars emerged, notes Spergel.

Still, the overall intensity of the microwave background over small patches of sky could provide a clue about when the first stars were born, he notes. If it turns out that reionization occurred earlier rather than later, a greater number of CMB photons would have been scattered, thereby leaving a greater imprint on the primordial radiation. That's because at earlier times, the universe was densest, and newly created clumps of electrons presented the greatest obstacle to the CMB radiation traveling through the clumps.

Even though the CMB can be a useful tool, such studies can never precisely pinpoint the timing of reionization, cautions Abraham Loeb of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. The imprint on the CMB "is a cumulative effect. You need other [information] before you can say when reionization took place," he says.

There is another signature carried by the CMB that can provide information on the timing of these events, says Hu. Whenever radiation gets scattered by, say, a cloud of electrons, it tends to become more polarized. That means that the electric and magnetic fields that make up a light wave vibrate only in particular directions. If the reionization occurred earlier rather than later, CMB photons underwent more scattering and become more polarized. The amount of polarization can therefore pinpoint how long ago the original crop of stars arose.

Although no telescope has yet detected the polarization of the CMB, several more-sensitive telescopes are under development, including a European Space Agency satellite called Planck.

Even with Planck, says Loeb, researchers would still have to make indirect inferences about the presence of galaxies. Directly imaging the very first starlit galaxies that arose in the universe may require extraordinarily large infrared telescopes, some 10 times the size of the largest light detectors now on the ground. Or, it might take an instrument such as the Next Generation Space Telescope, the proposed follow-up mission to the Hubble Space Telescope, Loeb says.

Such instruments won't be available for another decade. Until then, says Spergel, studies of the tiniest wiggles in the cosmic microwave background may be the best bet for determining how the universe unfolded.

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