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Cosmologists can now say with some confidence that the universe started out in a very hot and very dense state somewhere between 8 billion and 25 billion years ago, and that it has been expanding outward ever since-the Big Bang in a nutshell. They believe galaxies are strewn around the cosmos not randomly but according to a pattern that includes some patches with lots of galaxies and others with very few. They believe the universe is pervaded by mysterious dark matter, whose gravity has dominated cosmic history from the start.

But beyond that, things get murky. The experts don't know for sure how old or how big the universe is. They don't know what most of it is made of. They don't know in any detail how it began or how it will end. And, beyond the local cosmic neighborhood, they don't know much about what it looks like. Each of these questions is now under study; each bears directly on the others; and each could yield within the next few years to the intellectual and instrumental firepower now being brought to bear on it. Assuming, that is, that the universe cooperates.

THE AGE CRISIS: "You can't be older than your ma," quips Christopher Impey of the University of Arizona's Steward Observatory. Sounds obvious, maybe, but if Freedman and her colleagues are right about their space-telescope observations, it would seem that the universe hasn't caught on to this bit of common sense. The most straightforward interpretation of their data implies that the cosmos is 12 billion years old, max. But experts insist that the oldest stars in the Milky Way have been around for at least 14 billion years. "They could quite easily be several billion years older than that," says Yale's Pierre Demarque.

Demarque and his fellow stellar astronomers make a good case. The life and death of stars is something the scientists think they understand pretty well. They know about the nuclear reactions that power starshine; they know about what chemical elements the stars contain, and in what proportions; and they have created detailed, accurate computer simulations of stellar life cycles. When they say 14 billion years, it probably pays to listen.

But it also pays to listen to Freedman. She's a highly respected observational astronomer, and so are the 13 others on her space-telescope team. Moreover, theirs is only the latest in a series of measurements that point to a relatively young universe. Just a month before these results appeared in the journal Nature, two other sets of astronomers came out with their own young-universe observations. And while a handful of studies have emerged over the past few years arguing instead for an older cosmos, many more have converged on a younger age. The Freedman team's observations are considered by far the most definitive because they are based on the Hubble's extraordinarily clear vision.

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