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Welcome to the Dark Side

What is the universe made of? Look around: Everything we see with our eyes and with our most powerful instruments—all the stars, planets, galaxies, dust, and gas—everything that we think of as atom-based matter is only 5 percent of what we now know exists. The rest is what Dr. Carroll calls the "dark sector," consisting of:

 Dark matter: First proposed in the 1930s, the idea that there is missing mass influencing the behavior of galaxies began to look more and more likely from the 1970s on. We know that it is matter because we can detect its gravitational influence on visible matter. But we can't see it. An inventory of the distribution of dark matter throughout space shows that it constitutes 25 percent of the energy density of the universe.
 Dark energy: The greatest discoveries are those that are completely unexpected. That was the case in the late 1990s, when two teams of astronomers competing to measure the rate at which the expansion of the universe is slowing down (as virtually everyone thought it must be) discovered that it is speeding up instead. A previously unknown, all-pervasive dark energy must be at work, representing 70 percent of the energy density of the universe.

Together dark matter and dark energy account for all but a tiny fraction of everything there is; the ordinary matter that's left over is like the seasoning on the main dish. The story of how we arrived at this startling cosmic recipe is an absorbing scientific drama, which takes you through the breakthrough discoveries in astronomy and physics since the turn of the 20th century.

Concept by concept, Dr. Carroll gives you the tools to appreciate this subject in depth. He explains why scientists believe we live in a smooth, expanding universe that originated in a hot, dense state called the Big Bang. He describes the features of the infant universe that led to the large-scale structure we observe today. He takes you through the standard model of particle physics and shows how it provides the framework for understanding the interaction of all matter and radiation. And, he helps you understand why dark matter and dark energy are logical consequences of a wide range of scientific theories and observations and how together they complete a grand picture of the universe that has emerged over the last century.

Smoking Guns

Several significant clues disclose the existence of dark matter and dark energy. In the case of dark matter, we have the evidence of:

 Galaxy dynamics: The motions of stars in galaxies and galaxies within clusters indicate that there is far more matter than is implied by visible stars and gas.
 Gravitational lensing: The bending of light in a gravitational field, called gravitational lensing, shows the existence of gravitational fields—hence, matter—where there is essentially no ordinary matter.
 Super-hot gas: Hot x-ray-emitting gas that pervades clusters of galaxies provides a measure of the total gravitational field of the system, revealing that there is more than ordinary matter present.
 Echoes of the Big Bang: Variations in the leftover radiation from the Big Bang demonstrate that there must be dark matter pulling the ordinary matter we see.

Dark matter is as plain as day compared to dark energy, which reveals itself subtly but unmistakably through:

 Exploding stars: Type Ia supernovae provide a standard candle to measure the distances to faraway galaxies. By combining this information with redshift, which measures how fast a galaxy is receding, astronomers conclude that something is causing galaxies to recede at a faster and faster velocity.
 Geometry of space: Observations that space is very close to "flat," with neither positive nor negative curvature, imply a total energy density for the universe that is stunningly consistent with the dark-energy hypothesis.

What Are They?

For all their mystery, dark matter and dark energy fit like puzzle pieces into the existing framework of fundamental physics. They also have intriguing connections to the more speculative realms of the discipline, such as cosmic inflation and string theory. Inflation was designed to explain the early universe in a way that could clarify otherwise perplexing features about the geometry and temperature of the universe, but insights into dark energy provided a great boost to the scenario of an inflationary universe. And although it is even more speculative than inflation, string theory suggests ideas for explaining dark matter in terms of supersymmetric particles and quantum gravity, as well as a provocative scenario for explaining dark energy in terms of extra dimensions and other universes.

But the overriding question remains: What are dark matter and dark energy? We don't yet know, but physicists have come up with an array of creative ideas and ways to test them. Dr. Carroll covers the most promising proposals and looks ahead to experiments that will dramatically improve our understanding of the dark sector.

For now, we can only be humbled by this latest demonstration of our curious place in the grand scheme of things. "Not only are we not, like Aristotle would have it, sitting at the center of the cosmos," muses Dr. Carroll, "we are not even made of the same stuff as the cosmos. The kinds of things we're made of are only 5 percent of the energy density of the universe. This is a big deal!"

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