horizons

The Fate of the Universe Forecast

Blake Edgar

Sometimes the world can seem a vast and forlorn place. Now it appears as though our lonely planet occupies an increasingly bigger and emptier cosmos. From computer terminals high in the Berkeley Hills and from telescopes in Chile, Hawaii, Arizona, and the Canary Islands, astrophysicist Saul Perlmutter and colleagues have been steadily homing in on a solution to the ultimate riddle of the universe: how will it all end? That the universe is expanding is old news, but evidence from ancient light emitted by distant exploding stars, or supernovae, indicates that the universe may keep getting bigger and bigger.

As recently as two years ago, scientists on the Supernova Cosmology Project (SCP), based at Lawrence Berkeley National Laboratory, expected to confirm that the universe's growth was destined to gradually grind to a halt. What a difference a year makes. The January 1, 1998, issue of Nature contained SCP scientists' results from a study of 40 supernovae. In that paper, and at a meeting of the American Astronomical Society, Perlmutter's team foretold the apparent fate of the universe: endless expansion. Despite the encumbering ball and chain of gravity from all the matter in the universe, the expansion rate shows no sign of slowing; instead, its pace has quickened. Last December, the journal Science chose the discoveries made by the SCP, and corroborating research by a competing group, the High-Z Supernova Search Team, as the scientific "Breakthrough of the Year."

"Once upon a time we thought that all we were going to be measuring was the slowing of the rate of expansion," says Perlmutter. But now, he says, "it sure looks like we're living in a universe that will expand forever."

In the 1920s, renowned astronomer Edwin Hubble first determined that the universe was expanding based on his observations from southern California's Mount Wilson Observatory. Like Hubble, who studied a class of stars known as Cepheid variables, the supernovae teams measure the exploding stars' redshift–how much the wavelength of light has been stretched, from our Earth-bound vantage, toward the red end of the spectrum during the light's passage through space. A star's redshift reveals how much expansion the universe has undergone since the light left.

The two groups specifically target Type Ia (that's "one A") supernovae, which occur when a white dwarf star draws so much gas from a neighboring red giant star that it becomes a thermonuclear bomb. Because Type Ia's all shine with about the same peak intensity of brightness, the supernovae can serve as "standard candles," as if they bear stamps indicating their wattage. By comparing the known brightness from nearby supernovae with the observed brightness of far-off ones, the teams can calculate the distance to the latter. Since the speed of light does not change, knowing the distance tells scientists how long ago the supernovae exploded.

For instance, one supernova discovered two years ago by the SCP, named SN1997ap, had a redshift value of 0.83. It exploded when the universe was still a starry-eyed youngster, and the light from its fiery death took seven billion years to reach us. Until last fall, SN1997ap was the most ancient and distant supernova positively identified. But in October, the SCP detected a supernova with a redshift of 1.2, double that of most the Type Ia's sampled so far. Nicknamed Albinoni, its light has been traveling through space for ten billion years. (The official name of this supernova is SN1998ex, but SCP team members have taken to nicknaming their latest quarry after classical composers.) When Albinoni exploded, says Perlmutter, the star was about eight billion light-years away, but its present distance from Earth reveals that the universe has expanded twofold since the explosion.

Type Ia supernovae's shortcomings as research subjects boil down to the three R's: rare, random, and rapid. They go off only two or three times per millennium in a given galaxy and can happen in any corner of the sky. While Type Ia's burn almost as bright as the combined stars of an entire galaxy, their blaze of glory fades within a month. To overcome these obstacles, after the SCP formed in 1988, Perlmutter and LBL colleagues Carl Pennypacker and Gerson Goldhaber had to devise an innovative search strategy to guarantee finding them.

The team begins a telescope-based search just after a new moon, the best time to catch supernovae before they peak. While a telescope scans the sky, a charged-coupled device (CCD) camera photographs thousands of galaxies. Pictures of the same swath of sky taken a few weeks apart can be compared on high-speed computers using sensitive image-processing software specially developed for this task. The software subtracts the light patterns of the first image from the later image to highlight any new light sources since the first image was made. At such vast distances, detecting spectral explosions resembles a subtle game of "what's wrong with this picture?"

Despite initial skepticism from many experts, this strategy proved itself in 1992 and continues to get more refined. In 1995, eleven supernovae turned up in two nights, and last year the SCP found 17 supernovae during a two-night observing run. So far, the team has located more than 100 supernovae, followed 80 of those, and completed analysis on about half of them. The High-Z team, formed in 1995 by Brian Schmidt of Australia's Mount Stromlo and Siding Spring Observatories, adopted a similar search strategy and has now tracked 50 supernovae, of which 14 have been analyzed. Both teams have converged on the same exciting conclusion.

Cosmologists have long wondered whether the universe would expand infinitely or ultimately collapse in a climactic "big crunch." The SCP's initial redshift data fell frustratingly right in the middle with regard to an answer. But when the team started employing the Cerro Tolo telescope in Chile for searching, the powerful Keck Telescope atop Mauna Kea for spectra and redshifts, and the orbiting Hubble Space Telescope for imaging of the more distant supernovae, the data became more precise, and more provocative. By late summer 1997, the data distribution narrowed toward supporting infinite expansion, inducing much head-scratching and second-guessing among the scientists.

Both teams have taken steps to rule out factors that could potentially be confounding their results. Could interstellar dust be making the supernovae look fainter, and therefore more distant? Comparisons of color between nearby and far-off supernovae suggest not. Could supernovae explode in different ways, making their brightness inconstant? Intricate analysis of day-by-day changes in brightness reveal that both ancient and more recent supernovae behave alike. Could clumps of matter obscuring the path of light bend it so that the supernovae appear fainter? Perlmutter acknowledges this, but even in a worst-case scenario, he says, the results would not change significantly.

"This is the first hard data for a century-old question," notes Perlmutter. "We shouldn't be surprised to be surprised [by the results]."

For some scientists, surprise turned to incredulity as the theoretical implications of the supernova work set in. Cosmologists now must revisit an idea called the cosmological constant, which Albert Einstein invented in 1917, only to retract later as the "biggest blunder" of his career. Einstein was struggling with how to reconcile his theory of general relativity with the prevailing concept of a static universe. To balance the attractive force of gravity generated by matter and keep the universe from collapsing, at least on paper, Einstein fudged his equation with a hypothetical repulsive energy. This is the cosmological constant, represented by the Greek letter lambda.

Whether or not lambda exists is of more than academic interest. Such an energy would determine the curvature of space and could play the trump card in deciding the destiny of the cosmos. If the supernovae results are correct, they imply that we reside at a critical moment in the universe's history, a few billion years after the cosmological constant overtook the mass density of matter as the most influential force around and began the final state of acceleration in which the universe will continue to expand forever. As Perlmutter says, "If you have any cosmological constant at all, it's the master of the universe."

Einstein dismissed the cosmological constant after Edwin Hubble determined from redshift measurements that the universe was expanding, and now more precise redshift measurements have revived the beleaguered enigma, suggesting that irony is another universal force. Lambda could resolve a few of cosmology's current conflicts, such as the universe's age, early history, and present topography, but a model for the universe in which most of the energy is unknown and unseen leaves many uncomfortable. Writes Timothy Ferris in The Whole Shebang, "Lambda is cosmology's batty aunt in the attic: Few scientists like it very much, but it keeps turning up."

Some theorists, though, seem to welcome lambda's revival. Says cosmologist Joel Primack of the University of California at Santa Cruz, "Einstein's greatest blunder wasn't a blunder at all. There seems to be something like lambda; we're just not sure it's a constant." Not long ago, cosmologists joked that their field had a nearly infinite ratio of theory to data. But given a recent flood of increasingly precise measurements from supernovae and other sources, says Primack, "If any theory can survive this onslaught of data, it is probably true."

The measurements made by the supernovae researchers rely on several recent technological advances. First, the light-detecting CCD cameras got bigger and more sensitive. So did the telescopes that collect light for the cameras. Computers grew faster and cheaper and enabled the team to develop essential image-processing software. And the rapid evolution of the Internet has allowed SCP members to forward data directly from a remote telescope to Berkeley, where team members can begin to identify a supernova within an hour of its first being observed.

Both teams are busily seeking and studying more supernovae in an effort to ascertain the true numerical values of lambda and a complementary term, omega, which describes the density of all matter. From these two figures, the fate of the universe can be derived. Perlmutter says that their results so far support the popular big bang and inflationary models for the origin of the universe and suggest that lambda–be it an elastic vacuum energy or some other as yet unknown form–might be responsible for two-thirds of all energy in the universe.

"It's an extraordinarily important result, and quite rightly called the scientific breakthrough of 1998," says cosmologist Michael Turner of the University of Chicago and Fermilab. "Cosmology is very exciting [now]. Some would even say we're entering a golden age."

Blake Edgar is an associate editor at California Wild.