Next week, the venerable American Museum of Natural History in New York City will unveil an exhibit on the history of the universe. Descending a spiral ramp, visitors will journey through the cosmos beginning at its fiery birth some 13 billion years ago.
Few will notice the small metal plaque at the entrance to the gallery, let alone the mathematical symbols engraved upon it. But these symbols speak volumes about the size and fate of the cosmos—and the rapidity with which astronomers have come to embrace one of the most bizarre discoveries ever made.
Just 2 short years ago, two teams of astronomers presented the first evidence that we live in a runaway universe, driven to expand at a faster and faster rate.
That finding is in direct conflict with the simplest version of the Big Bang. According to that theory, the universe has expanded ever since its explosive birth, but gravity has gradually slowed the expansion. Even if the universe grows forever, the theory predicts that it should do so at a steadily decreasing rate.
Recent observations of exploded stars, however, suggest that the universe's rate of expansion is in fact increasing. Over the past year, new data appear to corroborate those findings.
To be sure, no one is yet claiming that the notion has been proved. "I don't think it's yet definitive, but it's certainly our current best model," says cosmologist David N. Spergel of Princeton University. It was good enough for Spergel, along with several other eminent astronomers, to recommend that the museum inscribe the parameter for an accelerating universe on its plaque.
Although the model may not be cast in concrete, it's now been engraved in bronze.
So far, astronomers have found no serious objections to the acceleration model. Nonetheless, "it's a big puzzle," says Scott Dodelson of the Fermi National Accelerator Laboratory in Batavia, Ill. Typically, he notes, theory leaps ahead of observations in cosmology. In this case, however, "people are struggling to understand the data. It's a crazy time," he says.
Several studies promise to tie up loose ends—or overturn the idea—over the next 2 years. Many of the tests have their roots in 1998 findings on the exploded stars called type 1a supernovas.
To determine whether or not the universe is revving up its rate of expansion, astronomers several years ago began comparing type 1a supernovas in distant regions of the universe with those nearby. Not only can these brilliant beacons be seen from far away—more than halfway to the edge of the observable universe—they also appear to have the same intrinsic brightness in both nearby and distant galaxies, like light bulbs of the same wattage.
Because light from a distant galaxy takes several billion years to reach Earth, astronomers observe that galaxy as it appeared when the universe was several billion years younger. If gravity were steadily slowing cosmic expansion, the distance between Earth and that remote galaxy would be less, and the galaxy would thus appear brighter, than if the expansion had proceeded at a constant rate. By the same token, a supernova in a remote galaxy would look brighter in a decelerating universe than it would in a universe where expansion has been constant.
In early 1998, two teams startled astronomers by finding exactly the opposite effect. Distant supernovas appeared 20 percent dimmer than expected for constant expansion, indicating that over the past few billion years, the universe's growth has sped up (SN: 12/19&26/98, p. 392).
Cosmologists have come to attribute the acceleration to an unusual form of energy that distributes itself uniformly throughout the cosmos rather than breaking into galaxies, galaxy clusters, superclusters, or other clumps. At present, this energy has a higher density than matter, and its gravitational influence dominates the cosmos.
Some call this energy the cosmological constant, a term first invoked by Albert Einstein in 1917 when he realized that his theory of gravity predicted a universe that was either expanding or contracting. Because standard wisdom at the time held that the universe is static, Einstein added the cosmological constant so that his equations would allow a stationary solution. He later abandoned the idea, calling it "my greatest blunder."
To explain the new observations, cosmologists have resurrected the cosmological constant. Many associate its energy with the sea of particles and antiparticles that, according to quantum mechanics, populates empty space. Others call it "funny energy" and propose that it relates to the quantum nature of gravity. In either case, it's exotic stuff and poorly understood.
In general relativity, the strength of gravity depends on pressure, energy, and matter. Funny energy acts as a negative pressure, pushing on the fabric of space-time. If the universe contains a large enough component of this exotic energy, the net effect of gravity becomes repulsive rather than attractive. Cosmic expansion speeds up instead of slowing down.
But are the supernova studies correct? "The cosmological constant is such an exotic and strange idea . . . there's no really good conceptual understanding of what it is," says Adam G. Riess, a member of the High-Z Supernova discovery team at the Space Telescope Science Institute in Baltimore.
"I think it really requires extraordinary proof to convince people that there's this whole other kind of energy that makes up most of the energy in the universe. The only way to give that extra proof is to really exhaust every other possibility," he says.
Among the confounding effects that might muddy the issue, two have taken center stage. Cosmic dust could make the supernovas look dimmer, or the more distant ones might have a composition different from the nearby ones, making them look fainter.
The first explanation calls for a special type of dust. If the culprit had been the ordinary form, researchers would already have detected it, says Riess.
Ordinary dust, composed of particles smaller than 0.1 micrometer, preferentially absorbs blue wavelengths of light, allowing red wavelengths to pass through relatively unimpeded. Observing the same supernovas at slightly different wavelengths—some redder, some bluer—astronomers have found no evidence that such dust significantly blocks the light.
That doesn't rule out a hypothetical type of intergalactic dust composed of particles 0.1 µm or larger in size. Dubbed gray dust by theorist Anthony N. Aguirre of Harvard University, such material would absorb red light almost as well as it does blue.
Gray dust would only betray its subtle presence through observation of a single supernova at two widely separated wavelengths, a test Riess and his colleagues performed last year. They found no difference in brightness when they observed a supernova at 400 nanometers, or blue light, with the Hubble Space Telescope, and at 900 nm, or near-infrared light, with the Keck I Telescope atop Hawaii's Mauna Kea.
"We don't see any evidence for gray dust," Riess says. Without similar studies of other supernovas, "we can't say categorically that it isn't out there, but this kind of observation disfavors it at a 95 to 98 percent confidence level," he notes. Riess reported the finding last month at a meeting of the American Astronomical Society in Atlanta.
Nor have astronomers so far found any significant differences in composition between nearby and distant supernovas. A new project is looking for differences between old and young supernovas in nearby galaxies.
Using telescopes with unusually large fields of view, some of them designed to hunt near-Earth asteroids, Saul Perlmutter of Lawrence Berkeley (Calif.) National Laboratory and his colleagues have begun a program to find large numbers of these supernovas. The project won't swing into full gear until next year.
To test the runaway-universe model directly, astronomers need to find supernovas at still greater distances than they have so far observed. Theory suggests that before cosmic expansion sped up, it had slowed down. That's because the youthful cosmos was much smaller and denser than it is today, and the gravitational tug exerted by ordinary matter in the universe's early days would have dwarfed any repulsive force associated with the cosmological constant (SN: 11/27/99, p. 341).
Measuring the brightness of extremely remote supernovas can reveal whether the universe had indeed undergone a period of deceleration. If expansion had slowed, supernovas from long ago would appear brighter than would be expected if expansion were constant.
The transition between slowing down and speeding up would have occurred when the universe was about one-third its current age, or about 9 billion years ago, astronomers calculate. To date, the supernova teams have examined two supernovas that hail from about this time. Over the next 2 years, Perlmutter says, researchers are likely to find enough of the extremely distant supernovas to determine whether there was a deceleration.
"This is a unique signature of a cosmological constant—namely that today the universe is accelerating, yesterday it was decelerating," says Riess. Neither dust nor differences in composition could mimic such behavior, he says.
"Nature would be cruel if it came up with a [different mechanism] that makes things look dimmer then brighter in just that way," says Spergel. "If we see both the slowing down and the speeding up, then it becomes a really compelling case."
A flying observatory devoted to studying type 1a supernovas could gather such data, Perlmutter says. Such a satellite, known as the Supernova Acceleration Probe (SNAP), may be launched in 2006. It could make measurements that would be precise enough to not only document funny energy but also distinguish between different theories of its nature, he says.
For instance, a theory known as quintessence suggests that the cosmological constant is not a constant at all but varies over time (SN: 2/28/98, p. 139). In this model, cosmic expansion would not have sped up as rapidly.
A completely independent line of evidence also leads to a runaway universe and a mysterious form of energy.
Just as ancient explorers mapped the shape of Earth, cosmic cartographers are charting the shape and density of the universe. Listening to the cosmic-microwave background, the whisper of radiation left over from the Big Bang, researchers have confirmed a long-standing prediction that the universe is flat. It has just enough matter and energy so that the fabric of space-time is not curved and parallel lines never meet.
A telescope perched high in the Chilean Andes and a balloon-borne detector on a test flight over Palestine, Texas, have closely examined subtle temperature variations—tiny hot spots and cold spots—in the microwave background. These variations, the imprint left on the infant universe from the Big Bang, were first glimpsed 8 years ago by a NASA satellite.
That satellite, however, could only view the hot and cold spots as broad brushstrokes, averaged over large chunks of the sky.
The newer experiments reveal the temperature variations in much finer detail. Both have found that variations in temperature reach their peak over patches of sky that are 1° across. This size, twice the apparent diameter of the full moon, fits the model for a flat universe. Other experiments had hinted at the same finding, but these newer data are the most convincing.
The temperature fluctuations recorded by the Chilean telescope, known as the Mobile Anisotropy Telescope (MAT), and the balloon-borne detector, BOOMERANG, correspond to microscopic fluctuations in the density of the universe when it was about 300,000 years old. At that time, the cosmos had cooled to the temperature where matter and light cease to interact strongly and radiation could stream freely into space.
Amber D. Miller of Princeton University and her colleagues reported the MAT results in the Oct. 10, 1999 Astrophysical Journal Letters. Brendan P. Crill of the California Institute of Technology in Pasadena, a member of the Italian-U.S. BOOMERANG team, presented the balloon findings last month at the Atlanta meeting.
"These are hard measurements to get right, but I think now one can feel some confidence that the fluctuations do really peak on the 1° scale," says Spergel.
In a flat universe, the total density of energy and matter must equal the so-called critical density. A slew of other studies shows, however, that the universe is seriously underweight. Analysis of the clustering of galaxies across the sky, for example, indicates that matter provides only about one-third of the critical density.
To balance the cosmic ledger, some form of energy must make up the missing 70 percent of the critical density. The amount of exotic energy suggested by the supernova studies fills the bill.
"Taken together, the cosmic-microwave background and the supernova data are very powerful because the two are completely unrelated to each other," says Riess.
Although each set of experiments is susceptible to its own errors, "the experiments don't have the same Achilles heel," he adds. "It's pretty suggestive that [the microwave-background studies] fit together very nicely with the supernova data," adds Spergel.
After carefully combining the results of the supernova studies with those of the microwave background and other observations, two astronomers say that the findings require a cosmological constant. Max Tegmark of the University of Pennsylvania in Philadelphia and Matias Zaldarriaga of the Institute for Advanced Study in Princeton reported their analysis at last month's astronomy conference.
Spergel worries, however, that other cosmological models could be consistent with the single peak observed in the microwave-background fluctuations. Compelling proof that the universe is flat, he says, will come if researchers can detect a predicted second and third peak.
In the meantime, the American Museum of Natural History wants to make sure it won't get caught flatfooted. "Our expectation is that we will reevaluate the prevailing winds of cosmology every 5 years and adjust both the plaque and the exhibit accordingly," says curator Frank Summers. The plaque, he notes, is attached by screws and can easily be replaced.
Some astronomers are confident, however, that definitive data may already be in hand. BOOMERANG's test flight in 1997 examined the microwave background for just 4.5 hours. A year later, it logged 190 hours during a flight over Antarctica. BOOMERANG researcher Andrew E. Lange of Caltech says that in theory, the 1998 experiment is capable of finding a second peak if it's there.
Crill says the team hopes to announce the results of the 1998 flight in a month or two. Several astronomers told Science News that they have heard rumors that the team has already found the second and third peaks.
Another satellite, the Microwave Anisotropy Probe, is set for launch this November. Much more sensitive than BOOMERANG and capable of viewing the entire sky, it should find several peaks if they exist, Spergel says.
"For both the cosmic-microwave-background and the supernova studies, there are data in the next 2 to 3 years that will make this go from a very suggestive case, a best-bet case, to a really compelling one," Spergel predicts.
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