The Cosmic Village

Saul Perlmutter

by Jeffery Kahn | Within esoteric scientific circles, they call it "The Search for Omega." Aptly, the quest takes its name from the last letter of the Greek alphabet. Omega is ultimate knowledge, fateful knowledge. It resolves a question that has been on the mind of humanity for thousands of years: Will the universe last forever, or will it too die?

Undeniably, this is an audacious scientific pursuit. People still cannot predict next week's weather. To know where the universe is heading, humanity must be able to see billions and billions of years into the future. Who then are these dreamers who think they can foresee the fate of the universe?

Like us, they happen to live at an epic time, a moment when technology has ripened. Like us, they are the inheritors of centuries of accumulated knowledge. Lemaitre, Hubble, and Einstein: These venerables laid down the groundwork -- the Big Bang theory, Hubble's law, and the theory of Relativity. Primed with this legacy of knowledge and technology, a number of research teams now are engaged in a race to learn the fate of the universe.

Berkeley Lab astrophysicist Saul Perlmutter heads a team that may be leading the hunt. Says Perlmutter, "The question of the ultimate destiny of the universe goes way back. Yet, no answer really has been possible, not until this century and the advent of new scientific concepts. Even with these insights, we had to await the technology. About 10 years ago, it came of age. Thanks to advances in light detectors, computers, and the Internet, now we can do experiments that attempt to actually answer the question."

To resolve the eternal puzzle, scientists have chosen several different paths. The Supernova Cosmology Project, which Perlmutter heads, takes an ingenious and deceptively simple approach. Hubble taught us that the universe is expanding. This ongoing expansion began at the time of the Big Bang itself. Gravity, the pull of all the matter in the universe, acts like a brake on this expansion. Focusing on these paired phenomena, the Supernova Cosmology Project is measuring the universe's deceleration, the precise rate at which the expansion is slowing down. Once that is determined, humans will know the fate of their universe.

In our expanding universe, three alternative scenarios are possible. If the universe is slowing enough, eventually it will halt, contract, and collapse in the ultimate demolition derby. If this is our fate, the "Big Crunch" will take place some 50 billion years from now. About 1-5 billion years before the end, when all creation converges, galaxies will be ripped asunder. The last stars will be destroyed about one million years before the Big Crunch. In the other extreme, the universe may be destined to expand forever. While this scenario does have the advantage of providing a longer run for the collective cast of the universe, it is not a kinder fate. Expanding eternally, the universe would go dark when the last of the stars dies out in about 10 trillion years. Or, there's a third scenario, one favored by many scientists for reasons that involve mathematical elegance and perhaps human preference. In this outcome, delicately balanced between the Big Crunch and Eternal Darkness, the universe slows down ever so gradually.   Forever slowing, it never quite comes to a rest.

As befits a research project of this magnitude, the Supernova Cosmology Project is a global collaboration based at Berkeley Lab that involves researchers in Sweden, England, France, and Germany, and observatories that include the Keck and Hubble telescopes. The project began after Perlmutter devised a technique that uses a stellar beacon to reveal the universe's deceleration rate. The answer is being beamed to Earth, concealed within the dying light of exploding stars (supernovae).

Recent supernovae -- stars that exploded millions of years ago -- have been studied by a number of groups. Perlmutter came up with a way to find and analyze the ancients. His Supernova Cosmology Project uses telescopes to look deep into space, detecting and characterizing supernovae that exploded billions of years ago. Researchers can ascertain the amount the universe has expanded since the time of each explosion. If they can determine the age (date) of each supernova, then the team can deduce the rate at which the universe is slowing down.

It ought to be simple -- light travels at a uniform rate, so if you know the distance of a star you know how long it took for the light to reach Earth and the age of the supernova. But therein lies the rub. As Berkeley Lab's Gerson Goldhaber explains, "Distance has proved to be one of the most difficult things to measure in the universe. When a bright object is observed in the sky, it is hard to tell whether it is intrinsically bright or simply closer to us than a dimmer object."

It turns out there is a way to resolve this. Distant supernovae are the universe's natural yardsticks -- and hence provide a natural timeline.

Supernova 1987a Supernovae are rare or commonplace, depending upon your point of view. Within a single galaxy, stars explode only a few times per millennium, but among the billions and billions of galaxies in the universe as a whole, a supernova occurs every few seconds. Not many people have seen an exploding star, which for a month can shine almost as brightly as the light from an entire galaxy. Astronomers Kepler and Galileo are among the fortunates who had this once in a lifetime experience. Seeking out supernovae at observatories in Arizona, Hawaii, Chile, and the Canary Islands, Perlmutter and his team observe tens of thousands of galaxies at a time. They can spot a dozen supernovae in a single night.

Just as stars vary in the heavens, so too do exploding stars. The Supernova Cosmology Project is collecting the starlight from a unique class, Type Ia supernovae, that always gives off about the same amount of light. Type Ia supernovae occur when a white dwarf, an aging star about the size of the Earth but with roughly the same mass as the sun, accumulates too much matter from a companion star. At that bloated stage, intense gravitational pressure triggers a runaway thermonuclear explosion. Since all Type Ia supernovae have about the same amount of mass at the time of explosion, they give off about the same amount of light.

These supernovae -- the research team's "standard candles" -- can be dated. Says Perlmutter, "Type Ia supernovae serve as a measurement of distance because they can be calibrated as a standard candle with a known brightness. By measuring the light from a distant Type Ia supernova and comparing its observed brightness to the known brightness from similar supernovae in nearby galaxies, we can precisely calculate a supernova's distance from Earth. Keep in mind that the speed of light is constant. So, once we know distance, we also have a relative measure of how long ago each supernova occurred."

Perlmutter's team has captured the light of some 50 supernovae. If the team can determine the amount of expansion since the time of each of these events, then they will know the extent to which our expanding universe is running out of momentum. How much had the universe expanded at the time of each supernova? Spectroscopy provides the answer. Using powerful telescopes, researchers focus on a supernova and separate its spectrum of light into different wavelengths. They then determine the degree to which the supernova's spectral lines are shifted towards the red end of the spectrum. As Hubble taught us, the greater the "redshift," the faster the supernova is apparently receding and the greater the total expansion since the explosion. The Supernova Cosmology Project is comparing the amount of expansion over billions of years to that over shorter more recent periods of time, pinpointing the change in the rate of expansion.

The preliminary results are in and they indicate that the universe appears to be slowing down. Additional data will reveal a more precise picture of the future. Perlmutter says when his team has data on 100 supernovae, then they will be able to accurately project the future rate of slowdown. This will bring the "Search for Omega" to its end.

To understand Omega, you must enter the world of mathematics. Omega is the average density of mass in the entire universe. Long a matter of study, it remains the unknown value in an equation for the fate of the universe. Learn the value of Omega and you will know whether or not the universe lasts forever. For scientists, that has been an absolute stumper: no one knows the answer. However, the rate at which the universe is slowing down is directly related to the average mass density of the universe. Thus, the Supernova Cosmology Project is on the verge of deducing Omega.

As Perlmutter explains, matter is the source of gravity. If there is enough matter, then the pull of gravity will be powerful enough to stop the expansion of the universe. If there is not, then the universe will expand forever. Unless somehow, Omega equals the number 1. This is the favored value, a denouement where the universe is delicately balanced between the fates of contraction and expansion. Forever and ever, the universe slows, but never reaches its end.

Perlmutter realizes the magnitude of this moment in human history. "The question we are pursuing is one of history's oldest. Several other scientific teams also are in hot pursuit. Our team will get its initial answer in the next few years. By the millennium," says the scientist, "we may have an answer that can be believed.