Dark energy. Dark matter. These strange and invisible substances don't just sound mysterious: their unexpected appearance in the cosmic census is upending long-held notions about the nature of the Universe. Astronomers have long known that the Universe is expanding, but everything they could see indicated that gravity should be slowing this spread. Instead, it appears that the Universe is accelerating its expansion and that something stronger than gravity--dark energy--is at work. In Einstein's TelescopeEvalyn Gates, a University of Chicago astrophysicist, transports us to the edge of contemporary science to explore the revolutionary tool that unlocks the secrets of these little-understood cosmic constituents. Based on Einstein's theory of general relativity, gravitational lensing, or "Einstein's Telescope," is enabling new discoveries that are taking us toward the next revolution in scientific thinking--one that may change forever our notions of where the Universe came from and where it is going.

In recent years, astronomy went over to the dark side and has yet to return. Baffling entities called dark matter and dark energy are the two biggest mysteries facing 21st-century astronomers. Their arrival is enough to turn Darth Vader dark-green with envy.

It's been a jolt to learn that the universe we've become so familiar with — all those planets, stars, swirling galaxies and... Washington Post Book Review (read the entire Washington Post review) glowing gases — is just four percent of the overall content. Six times more consists of another type of matter altogether, possibly a subatomic particle yet to be discovered. The remaining three-quarters? A bizarre energy — an anti-gravitational pressure — that permeates space-time and has the potential to rip our cosmos apart in the distant future. We are mere flotsam within this covert cosmic realm.

What stands out in Evalyn Gates' cogent review of this intriguing topic is the sheer cleverness astronomers have demonstrated in fashioning tools to study the unseeable. As its name implies, dark matter emits no light. It can be detected only by its gravitational effects. So astronomers study it by closely observing how light, coursing through the universe, is gravitationally bent as it passes by the spheres of dark, invisible matter that surround galaxies and clusters of galaxies. This is, in effect, the ingenious "telescope" that Gates refers to in her title. Following Einstein's rules of gravity, a dark-matter mass can serve as a gigantic gravitational lens that magnifies more distant celestial objects lying behind it, such as primordial galaxies too dim to see otherwise. "Seen through an optical telescope," Gates writes, "a cluster is a large collection of galaxies ... Observed with an X-ray telescope, a cluster looks like a big blob of hot gas. Viewed through Einstein's Telescope, a cluster appears to be a giant dent in space-time caused by far more matter than is visible in gas and galaxies." In this way, dark matter's gravitational fingerprints are being found all over the cosmos, enabling astronomers to map where it is and assess what it might consist of to act the way it does.

In the cosmic scheme of things, the notion of dark energy is far more startling. Astronomers assumed for decades that our expanding universe was slowing down, only to find out that it's actually accelerating, somehow boosted by the presence of an odd, cosmos-wide pressure. As Gates writes so engagingly, its recent detection was "like finding an elephant on top of a table impeccably set with the finest china and silver ... We stare in shock at the uninvited guest and demand to know where the elephant came from — and how it got into (the) room." To find out, astronomers and physicists are exploring many avenues, from carrying out computer simulations to tracing dark energy's effects on the distribution of galaxies throughout the universe. Google is even getting involved, developing a Web portal that will allow astronomers, students and amateurs alike to assess data gathered by a special telescope, poised next decade to regularly scan the entire hemisphere from the Chilean Andes.

Gates aims to write for both professional scientists and laypeople, though she openly concedes that to newcomers some of these concepts will be "difficult to digest the first time through." In places her book does read like a textbook, but at least a textbook with style. A dry tome wouldn't ask you to look through the end of an empty wineglass to learn how dark matter can bend light due to the warps it imprints on space-time.

So much more has been revealed since I wrote a book on dark matter 16 years ago. A reviewer at that time was dismayed that I offered no final answers to the mystery. There's still no happy ending, but as Gates so aptly demonstrates, describing how science works toward a solution provides 99 percent of the enjoyment for any reader.

Marcia Bartusiak teaches in the MIT Graduate Program in Science Writing. Her latest book, "The Day We Found the Universe," will be published this spring.

Reviewed by Marcia Bartusiak, Washington Post Book World (Copyright 2006 Washington Post Book World Service/Washington Post Writers Group)
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There is far more to the universe than meets the eye: invisible dark matter and dark energy constitute the vast bulk of the cosmos and are responsible for its accelerating expansion. Gates, assistant director of the Kavli Institute for Cosmological Physics, explores the science of these invisible phenomena and the questions they raise about the universe's origins, its present and its future. Gates explains how scientists discovered the existence of dark matter and their theories about the nature of the particles (with named like WIMPs) that form it. Astrophysicists have found tools to measure the invisible mass: the stars themselves. Drawing on Einstein's theory of general relativity, scientists can see dark matter using gravitational lensing'"by measuring the deflection of light around a cosmic object, they can measure the object's mass. Presenting complicated topics concisely and clearly, Gates explains what we know about the universe, what scientists wish they knew, and what's at stake'"the fate of the universe itself.

Based on Einstein's theory of general relativity, gravitational lensing--known as Einstein's Telescope--is enabling new discoveries that are taking researchers toward the next revolution in scientific thinking--one that may change forever the notions of where the Universe is headed. Illustrated.

Evalyn Gatesis the assistant director of the Kavli Institute for Cosmological Physics, a senior research associate at the University of Chicago, and the former astronomy director of the Adler Planetarium. Her writing has appeared in Physics Todayand the Chicago Tribune.