SCIENCE REVIEW BY CARL ZIMMER

Known unknowns beyond the night sky

Astronomers using the Hubble Space Telescope discovered this ghostly ring of dark matter, which was formed during a collision between two massive galaxy clusters.
Astronomers using the Hubble Space Telescope discovered this ghostly ring of dark matter, which was formed during a collision between two massive galaxy clusters.
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Sunday, January 30, 2011

In 1969, an astronomer named Jeremiah Ostriker realized that the Milky Way was spinning too fast. That may sound odd, given that it takes the sun 230 million years to make a full orbit. But when Ostriker tried to simulate the Milky Way on a computer, he found that it was spinning so quickly that it should have ripped itself apart long ago. There weren't enough stars to hold it together.

Ostriker went to his fellow Princeton scientist James Peebles to share his puzzle. "There's something wrong here," Ostriker said to Peebles. The two scientists decided there could only be one solution: The stars we can see in the Milky Way are just a small fraction of the actual galaxy. They are embedded in a vast, unseen halo, made of an unknown stuff that has come to be known as dark matter. When Ostriker and Peebles looked to other galaxies, they found hints of dark matter there as well.

Other astronomers didn't want to believe it. After all, they had spent the past four centuries learning about the universe by collecting its light in their telescopes. Now it seemed they were missing most of the cosmic show. But as Richard Panek chronicles in his fascinating new book, "The 4 Percent Universe," it turned out that there was a lot more wrong with the universe than even Ostriker had realized and that his and Peebles's work was only the beginning of an enormous undertaking by many scientists.

The latest surveys of the universe indicate that only 4 percent of it is made of ordinary matter. Nearly 23 percent is made of dark matter, which some physicists suspect consists of wispy subatomic particles that may someday be caught in a detector. And the remaining 73 percent is made up of something far more baffling: an energy that is causing the universe to expand at an ever-increasing rate. Scientists call it "dark energy," and they have no idea what it is. "Get rid of us and of everything else we've ever thought of as the universe," writes Panek, "and very little would change."

In "The 4 Percent Universe," Panek reconstructs the five decades of research that led to this humbling realization. It is one of the most important stories in the history of science, but also one of the hardest to tell. The science is fiendishly tricky, and the human history is also arcane. The discoveries of dark matter and dark energy were not the work of some lone genius. It took a huge network of scientists. In 2007, Cambridge University awarded the Gruber Prize in Cosmology, the highest honor a cosmologist can win, to the discoverers of dark energy. Fifty-two winners showed up.

Inevitably, Panek has to plunge into the bureaucratic depths of modern science. His pages are splattered with acronyms - NSF, CARA, COBE, DASI - for funding agencies and assorted astronomy projects. From time to time, I had the same trouble keeping track of the scientists that I have with the characters in big Russian novels. Some chapters have only tenuous links to the rest of the book, reading more like magazine articles than vital episodes in a greater story.

But Panek's passion for the mysteries of dark matter and dark energy wins the day. He succeeds because he recognizes that he's writing not just about red shifts and supernovae, but about people. They're unusual people to say the least, happy to stay up all night to take pictures of stars or write software that compares those pictures to find the pop of supernovae exploding in distant galaxies. And they're far from perfect. The astronomer Vera Rubin gets to pay a visit to the legendary physicist George Gamow to talk about the nature of the universe, only to discover he's an oaf who yells at his wife. "Where were his papers? What had she done with his papers? Why was she always going through his papers?" Panek writes. "Whether Gamow's wife was ever actually there, Rubin couldn't be sure."

The success of "The 4 Percent Universe" also stems from Panek's wisdom about how science works. It's easy to think that the discovery of dark matter and dark energy - the realization that we have no idea what most of the universe is made of - is a story of failure. Actually, scientists are delighted to have learned that they have blasted beyond any ready explanation that physics can offer for the universe. Now they have the opportunity to build a new physics that can make sense of how the universe, both light and dark, really works. "What greater legacy could a scientist leave a universe?" Panek asks.

Carl Zimmer is the author of "The Tangled Bank: An Introduction to Evolution."

The 4 Percent Universe

Dark Matter, Dark Energy, and the Race to Discover the Rest of Reality

By Richard Panek

Houghton Mifflin Harcourt. 297 pp. $26


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