Will the universe eventually collapse in the "big crunch," expand forever in the "big loneliness," or be torn to bits in the "big rip"?
The key to answering those questions appears to lie in a mysterious form of energy that has been cracking the cosmic whip on the universe for the past 6 billion to 7 billion years. This "dark energy" appears to be causing the universe to grow at an accelerated rate rather than a rate scientists previously thought would slow forever.
Six years after astronomers first stunned the scientific world with this discovery, researchers say dark energy still baffles them. Yet several studies reported over the past year have strengthened the evidence for dark energy's role as cosmic gas pedal, researchers say.
The results "point to the promise of improving our understanding of dark energy," says Michael Turner, head of the astronomy and astrophysics department at the University of Chicago. That understanding, he says, is critical to answering fundamental questions about the origin and future of the universe and the nature of matter and space-time.
Last week, researchers from Britain, Germany, and the United States announced results from studies of hot gas surrounding vast clusters of galaxies. The effort, using NASA's Chandra X-ray Observatory, was designed to determine the amount of dark energy the universe holds, compared with other forms of matter and energy. In addition, the team took a tentative stab at trying to see if the amount of dark energy changes with time - key to determining its nature.
Anyone reading their results might be excused for feeling a bit special. The team found that 4 percent of the universe is made of ordinary matter. Another 21 percent consists of so-called dark matter, inferred from its gravitational effects on matter. The remaining 75 percent consists of dark energy, which exerts a form of pressure that makes it act like gravity thrown into reverse. These figures are consistent with results reported last year from satellite measurements of the big bang's afterglow - the cosmic microwave background.
If the quantity of dark energy is constant, astronomers say, the universe will continue to expand at an increasing rate. In about 20 billion years or so, only about 100 galaxies might be visible from Earth. Think of it as the "big lonely." If dark energy were to change with time, it could relax to let gravity once again dominate, prompting the universe to collapse in the "big crunch." Or if the pace speeds up, it could lead to the "big rip," in which the fabric of space-time stretches so rapidly that even atoms get torn apart.
Based on the team's observations, dark energy is holding steady and "behaves much like the cosmological constant in Einstein's theories" about the evolution of the universe, says Steve Allen, an astronomer at Cambridge University in England and the team leader.
Essentially, this means that the amount of energy per volume of space remains constant. If this observation holds up under more rigorous programs, it would substantially narrow the range of explanations for what dark energy really is.
That's not bad for a "constant" that Albert Einstein dubbed his greatest blunder. In 1917, when he pondered the implications of his general relativity theory for the universe, most astronomers believed that the size of the universe didn't change. Other galaxies appeared on astronomers' photographic plates, but many thought the fuzzy images were nebulae or clusters of stars in the Milky Way.
When Einstein applied his equations to the observed universe, his numbers led him to an unsettling conclusion. Given the way his equations showed gravity distorting the shape of space-time, and given the amount of matter and energy in the universe to exert gravity, the universe could not remain static. It would have to collapse through gravitational attraction.
Observers saw no evidence for change, so he reasoned that there must be some form of "negative energy" offsetting gravity's tug. By tweaking his numbers, he could get a static universe.
In the 1920s, Edwin Hubble burst Einstein's bubble. Using the most powerful telescopes of the day, Hubble showed that the fuzzy patches were galaxies, and that the galaxies appeared to be speeding away. The universe was expanding.
For nearly 60 more years, astronomers would trot out Einstein's blunder to explain one new phenomenon or another, only to find later that more conventional explanations were correct.
Meanwhile, particle physicists in the 1960s were working on ideas in quantum mechanics in which a vacuum could exhibit a form of energy. And when they applied Einstein's theory of general relativity to this vacuum energy, thought to permeate the cosmos, it produced the gravitational repulsion that mimicked Einstein's cosmological constant. The work gave the feature an underpinning in physics it had lacked.
The only task left was to observe it in nature. That came in 1998, when two teams working independently reported observations that showed space expanding at a more rapid rate than it should be.
This time, invoking a gravitationally repulsive dark energy appears to be the correct answer. But how it relates to physicists' discoveries about the forces of nature and the subatomic particles associated with them remains a mystery. Dark energy could indeed be Einstein's cosmological constant. It could be a quantum field dubbed "quintessence." Or it could be a new aspect of gravity itself.
"Keep in mind that we call this dark energy, but that gives a false impression that we understand what it is. We really don't," says Adam Riess, astronomer with the Space Telescope Science Institute in Baltimore, Md. Early this year, Dr. Riess and colleagues added what many astronomers call a significant advance in observing dark energy.
In February, the team published results of Hubble Space Telescope observations that spanned a range of distances and periods in the universe's history. They found the period, some 6 billion years ago, when the shift occurred from a slowdown in the rate of expansion to an acceleration - a turning point that has become known as the "big jerk." The team used the light from a powerful "standard candle" - a type of exploding star, or supernova - to gauge distance. Then they used spectrographic data on these objects to determine the speed at which the galaxies containing the supernovae were receding.
A third major contribution came last year from a set of studies involving the cosmic microwave background measurements from a NASA satellite and observations from the Sloan Digital Sky Survey. Both pointed to dark energy as the dominant ingredient in the universe's recipe. And by combining data from the two, four teams working independently found evidence for the action of dark energy on the scale of galaxy clusters, which cover a huge expanse of space and embrace from 50 to 1,000 galaxies.
Taken together, the X-ray, supernova, and microwave-survey studies represent "extraordinary evidence" for dark energy, the University of Chicago's Turner said at a briefing last week.
The next step is to learn whether dark energy varies with time. Already, Riess and his colleagues have been awarded a generous amount of observing time with the Hubble Space Telescope next year to get a more detailed look at dark energy's workings. Ground- and space-based optical telescopes are being planned to study it. Ground-based radio telescopes are getting into the act. And Chandra will be surveying more clusters at a wider range of distances.
The discovery of dark energy has handed researchers "the most profound problem in all of science," Turner says. Solving it "will require a full-court press."