The End

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An equally unsettling implication is that the universe is pervaded with a strange sort of "antigravity," a concept originally proposed by and later abandoned by Einstein as the greatest blunder of his life. This force, which has lately been dubbed "dark energy," isn't just keeping the expansion from slowing down, it's making the universe fly apart faster and faster all the time, like a rocket ship with the throttle wide open.

It gets stranger still. Not only does dark energy swamp ordinary gravity but an invisible substance known to scientists as "dark matter" also seems to outweigh the ordinary stuff of stars, planets and people by a factor of 10 to 1. "Not only are we not at the center of the universe," University of California, Santa Cruz, astrophysical theorist Joel Primack has commented, "we aren't even made of the same stuff the universe is."

These discoveries raise more questions than they answer. For example, just because scientists know dark matter is there doesn't mean they understand what it really is. Same goes for dark energy. "If you thought the universe was hard to comprehend before," says University of Chicago astrophysicist Michael Turner, "then you'd better take some smart pills, because it's only going to get worse."

ECHO OF THE BIG BANG

Things seemed a lot simpler back in 1965 when two astronomers at Bell Labs in Holmdel, N.J., provided a resounding confirmation of the Big Bang theory, at the time merely one of several ideas floating around on how the cosmos began. The discovery happened purely by accident: Arno Penzias and Robert Wilson were trying to get an annoying hiss out of a communications antenna, and after ruling out every other explanation--including the residue of bird droppings--they decided the hiss was coming from outer space.

Unbeknownst to the duo, physicists at nearby Princeton University were about to turn their antenna on the heavens to look for that same signal. Astronomers had known since the 1920s that the galaxies were flying apart. But theorists had belatedly realized a key implication: the whole cosmos must at one point have been much smaller and hotter. About 300,000 years after the instant of the Big Bang, the entire visible universe would have been a cloud of hot, incredibly dense gas, not much bigger than the Milky Way is now, glowing white hot like a blast furnace or the surface of a star. Because this cosmic glow had no place to go, it must still be there, albeit so attenuated that it took the form of feeble microwaves. Penzias and Wilson later won the Nobel Prize for the accidental discovery of this radio hiss from the dawn of time.

The discovery of the cosmic-microwave background radiation convinced scientists that the universe really had sprung from an initial Big Bang some 15 billion years ago. They immediately set out to learn more. For one thing, they began trying to probe this cosmic afterglow for subtle variations in intensity. It's clear through ordinary telescopes that matter isn't spread evenly throughout the modern universe. Galaxies tend to huddle relatively close to one another, dozens or even hundreds of them in clumps known as clusters and superclusters. In between, there is essentially nothing at all.