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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 is 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 180[degrees], but the sphere's angles are always greater than 180[degrees], 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 180[degrees]."

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