PROBING THE COSMIC FIREBALL

What makes it still more convincing is that an entirely different kind of observation-the long-standing search for lumpiness in the cosmic background radiation-now suggests independently that dark energy is real. The lumps themselves were first detected about a decade ago, thanks to the Cosmic Background Explorer satellite. At the time, astrophysicist and cobe spokesman George Smoot declared that "if you're religious, it's like seeing God."

But it was more like seeing God through dirty Coke-bottle glasses: the satellite saw lumps but couldn't determine much about them. In April, though, scientists offered up much sharper images from a balloon-borne experiment called boomerang (Balloon Observations of Millimetric Extragalactic Radiation and Geophysics), which lofted instruments into the Antarctic stratosphere; from another named maxima (Millimeter Anisotropy Experiment Imaging Array, which did the same over the U.S.); and from a microwave telescope on the ground at the South Pole, called dasi (Degree Angular Scale Interferometer).

All these measurements pretty much agreed with one another, confirming that the lumps scientists saw were real, not some malfunction in the telescopes. And just two weeks ago, astronomers from the Sloan Digital Sky Survey confirmed that this primordial lumpiness has carried over into modern times. The five-year mission of the survey, to make a 3-D map of the cosmos, is far from complete, but scientists reported at the American Astronomical Society's spring meeting in Pasadena, Calif., that it is clearer than ever that galaxies cluster together into huge clumps that reflect conditions that existed soon after the Big Bang.

To the unaided eye, the images are meaningless. A statistical analysis, however, shows that the early lumps-actually patches of slightly warmer or cooler radiation-don't come at random but rather at certain fixed sizes. "It's as though you're studying dogs," says University of Pennsylvania astrophysicist Max Tegmark, "and you find out that they come in just three types: Labrador, toy poodle and Chihuahua."

That turns out to be enormously important. Knowing the characteristic sizes and also the temperatures, to a millionth of a degree, of these warm and cool regions gives theoretical physicists all sorts of information about the newborn cosmos. They were already pretty sure, from the equations of nuclear physics and from measurements of the relative amounts of hydrogen, helium and lithium in the universe, that protons, neutrons and electrons (the building blocks of every atom in the cosmos) add up to only about 5% of the so-called critical density-what it would take to bring the cosmic expansion essentially to a halt by means of gravity.

But when you add Tegmark's "dogs," plus the more esoteric equations of sub-nuclear physics, it turns out that an additional 30% of the needed matter most likely comes in the form of mysterious particles that have been identified only in theory, never directly observed-particles with quirky names like neutralino and axion. These are the mysterious dark matter, or most of it anyway. The cosmic background radiation itself began to shine when the universe was 300,000 years old, but the temperature fluctuations were set in place when it was just a split-second old. "It's pretty cool," says Tegmark, "to be able to look back that far."

THE FLAT UNIVERSE

The dogs also yield another key bit of information: they tell theorists how the universe is curved, in the Einsteinian sense. There's no way to convey this concept to a nonphysicist except by two-dimensional analogy (see How Does the Universe Curve? diagram). The surface of a sphere has what's called positive curvature; if you go far enough in one direction, you will never get to the edge but you will eventually return to your starting point. An infinitely large sheet of paper is flat and, because it's infinite, also edgeless. And a saddle that extends forever is considered edgeless and negatively curved. It also turns out that any triangle you draw on the paper has angles that add up to 180º, but the sphere's angles are always greater than 180º, and the saddle's always less.

Same goes for the universe, but with one more dimension. According to Einstein, the whole thing could be positively or negatively curved or flat (but don't try to imagine in what direction it might be curved; it's quite impossible to visualize). "What the new measurements tell us," says Turner, "is that the universe is in fact flat. Draw a triangle that reaches all the way across the cosmos, and the angles will always add up to 180º."

According to Einstein, the universe's curvature is determined by the amount of matter and energy it contains. The universe we evidently live in could have been flattened purely by matter-but the new discoveries prove that ordinary matter and exotic particles add up to only about 35% of what you would need. Ergo, the extra curvature must come from some unseen energy-just about the amount, it turns out, suggested by the supernova observations. "I was highly dubious about dark energy based only on supernovas," says Princeton astrophysicist Edwin Turner (no relation to Michael, though the two often refer to each other as "my evil twin"). "This makes me take dark energy more seriously."

The flatness of the universe also means the theory of inflation has passed a key test. Originally conceived around 1980 (in the course of elementary-particle, not astronomical, research), the theory says the entire visible universe grew from a speck far smaller than a proton to a nugget the size of a grapefruit, almost instantaneously, when the whole thing was .000000000000000000000000000000000001 sec. old. This turbo-expansion was driven by something like dark energy but a whole lot stronger. What we call the universe, in short, came from almost nowhere in next to no time. Says M.I.T.'s Alan Guth, a pioneer of inflation theory: "I call the universe the ultimate free lunch." One of the consequences of inflation, predicted 20 years ago, was that the universe must be flat-as it now turns out to be.

If these observations continue to hold up, astrophysicists can be pretty sure they have assembled the full parts list for the cosmos at last: 5% ordinary matter, 35% exotic dark matter and about 60% dark energy. They also have a pretty good idea of the universe's future. All the matter put together doesn't have enough gravity to stop the expansion; beyond that, the antigravity effect of dark energy is actually speeding up the expansion. And because the amount of dark energy will grow as space gets bigger, its effect will only increase.

THE FATE OF THE COSMOS

That means that the 100 billion or so galaxies we can now see though our telescopes will zip out of range, one by one. Tens of billions of years from now, the Milky Way will be the only galaxy we're directly aware of (other nearby galaxies, including the Large Magellanic Cloud and the Andromeda galaxy, will have drifted into, and merged with, the Milky Way).

By then the sun will have shrunk to a white dwarf, giving little light and even less heat to whatever is left of Earth, and entered a long, lingering death that could last 100 trillion years-or a thousand times longer than the cosmos has existed to date. The same will happen to most other stars, although a few will end their lives as blazing supernovas. Finally, though, all that will be left in the cosmos will be black holes, the burnt-out cinders of stars and the dead husks of planets. The universe will be cold and black.

But that's not the end, according to University of Michigan astrophysicist Fred Adams. An expert on the fate of the cosmos and co-author with Greg Laughlin of The Five Ages of the Universe (Touchstone Books; 2000), Adams predicts that all this dead matter will eventually collapse into black holes. By the time the universe is 1 trillion trillion trillion trillion trillion trillion years old, the black holes themselves will disintegrate into stray particles, which will bind loosely to form individual "atoms" larger than the size of today's universe. Eventually, even these will decay, leaving a featureless, infinitely large void. And that will be that-unless, of course, whatever inconceivable event that launched the original Big Bang should recur, and the ultimate free lunch is served once more.

Astronomers and physicists are a cautious crew, and they insist that the mind-bending discoveries about dark matter, dark energy and the flatness of space-time must be confirmed before they are accepted without reservation. "We're really living dangerously," says Chicago's Turner. "We've got this absurd, wonderful picture of the universe, and now we've got to test it." There could be surprises to come: an Einstein-style cosmological constant, for example, is the leading candidate for dark energy, but it could in principle be something subtly different-a force that could even change directions someday, to reinforce rather than oppose gravity.

In any case, new tests of these bizarre ideas will not be too long in coming. Next week a satellite will launch from Cape Canaveral to make the most sensitive observations ever of the cosmic background radiation. Supernova watchers, meanwhile, are lobbying nasa for their own dedicated telescope so they won't have to queue up for time on the badly oversubscribed Hubble. And lower-tech telescopes and microwave detectors, both on the ground and lofted into the air aboard balloons, will continue to refine their own measurements. If the latest results do hold up, some of the most important questions in cosmology-how old the universe is, what it's made of and how it will end-will have been answered, only about 70 years after they were first posed. By the time the final chapter of cosmic history is written-further in the future than our minds can grasp-humanity, and perhaps even biology, will long since have vanished. Yet it's conceivable that consciousness will survive, perhaps in the form of a disembodied digital intelligence. If so, then someone may still be around to note that the universe, once ablaze with the light of uncountable stars, has become an unimaginably vast, cold, dark and profoundly lonely place.