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	Dark Cosmos: In Search of Our Universe's Missing Mass and Energy By: Hooper, Dan Published By: HarperCollins ePub for Digital Editions Price: $11.99 PDF for Digital Editions Price: $11.99 Mobipocket Reader Price: $11.99 Microsoft Reader Price: $11.99 PDF for Digital EditionsePub for Digital EditionsMobipocket ReaderMicrosoft Reader	EBOOK OPTIONS ADD TO CART ADD TO WISHLIST TELL A FRIEND RECEIVE ALERTS
	Everyone knows that there are things no one can see, for example, the air you're breathing or a black hole, to be more exotic. But not everyone knows that what we can see makes up only 5 percent of the Universe. The rest is totally invisible to us. The invisible stuff comes in two varieties—dark matter and dark energy. One holds the Universe together while the other tears it apart. What these forces really are has been a mystery for as long as anyone has suspected they were there, but the latest discoveries of experimental physics have brought us closer to that knowledge. Particle physicist Dan Hooper takes his readers, with wit, grace, and a keen knack for explaining the toughest ideas science has to offer, on a quest few would ever have expected: to discover what makes up our dark cosmos.

Excerpt

Chapter One

Our Dark Universe

The world is full of obvious things which nobody by any chance ever observes.

—Sherlock Holmes

Take a look around you. You see a world full of things. Tables, chairs, the floor, a cup of coffee, shoes, bicycles—things. Most of us casually think of the world as space filled with such things, the sort of stuff you can hold in your hand or stub your toe on. But how much of our world is really made up of objects that you can see? Think of the air you're breathing. It's invisible. Nevertheless, it is there, even if your experience of it is somewhat indirect as your chest expands and contracts, and your breath whistles through your nose. The visible world is not all there is to the Universe. Relying solely on our eyes to learn what's out there would cause us to overlook a great deal.

Although the point I'm making might seem obvious, it is one worth bearing in mind. Just as we cannot see the air, we cannot see most of the Universe. During the past several de-cades, several lines of evidence have led to the conclusion that about 95 percent of our Universe's mass and energy exists in some form that is invisible to us. Hidden. Evading our detection almost entirely. That might seem ridiculous, but just as the act of blowing up a balloon helps us see the air we breathe, our hidden Universe does leave clues that we can decipher to confirm its existence. Galaxies are seen rotating at much greater speeds than are possible without the presence of extra matter. And the large-scale structure and evolution of our Universe, from the Big Bang to the present-day expansion and acceleration, seem to require more mass and energy than we see—some twenty times more. This picture of the invisible gets weirder. Of this mysterious and subtle majority of our world, only about a third is thought to be matter. Appropriately, it is called dark matter. The other two-thirds is stranger yet, and is called dark energy.

Thousands of physicists, astronomers, and engineers are actively working toward the goal of understanding the nature of dark matter and dark energy. Many of these scientists are skilled experimenters, designing ultra-sensitive detectors in deep underground mines, constructing new kinds of telescopes capable of detecting much more than simply light, or operating particle colliders that smash matter together at incredible speeds. Others, such as I, are theoretical physicists, struggling to understand with pencil, paper, and powerful computers how dark matter and dark energy fit into our world as we currently understand it.

Although the scope of these collective efforts is staggering, the basic motivations are nothing new. For as long as people have pondered their world, they have tried to identify what it is made of. The philosophers of ancient civilizations speculated and hypothesized endlessly on such matters, if not always very successfully. Millennia later, but still in much the same spirit, the heirs to those philosophers discovered and codified the chemical elements of the periodic table that we are all taught in school. Twentieth-century physics has further revealed an incredible world of quantum particles. These particles are part of a beautiful and elegant theory that successfully describes nearly all of the phenomena observed in our Universe. But, alas, nearly all is not nearly enough.

Long before the advent of modern chemistry and physics, the peoples of early civilizations made countless attempts at understanding the composition of the things around them. The ancient Greek philosopher Empedocles provided one of the most enduring of those ideas when he hypothesized that each type of matter in the Universe arises from a specific combination of four fundamental elements: air, earth, fire, and water. Empedocles, followed by Plato and a long list of others, thought that it would be impossible to change one pure element into another, but by melding together different quantities of these pure elements, any substance could be formed.

The healthy system of discussion and debate among learned Greeks fostered further investigation. Elementalists, such as the philosopher Democritus, conjectured that all matter was made up of a finite number of individual, indivisible particles that he called atoms. Democritus believed, as did the other elementalists, that these fundamental particles could not be destroyed or created, but only arranged in different patterns or in different quantities to make different substances. A slippery substance, for example, would be made out of round, smooth atoms. An object made up of atoms with hooks or other such shapes could stick or lock together in dense groups to form heavy substances, such as gold. This basic idea of Democritus's turned out to be, very roughly, correct.

Modern chemists know that the qualities of a substance are not so simply determined by the superficial properties of atoms themselves, but instead largely result from the interactions among atoms. But despite the failure of the ancient elementalists to build an accurate atomic theory, the concepts at the foundation of their theory represented a major step forward in scientific thought. Many of the concepts are essentially the same as those taught in nearly every chemistry classroom today. The atoms of modern chemistry, however, are not the indivisible and fundamental objects envisioned by Democritus.

During the twentieth century, as experimenters probed deeper into the nature of the atom, they found that atoms are not indivisible. Experiments by physicists such as J. J. Thomson and Ernest Rutherford showed that atoms themselves are made up of constituent parts: protons, neutrons, and electrons. And in a further refutation of Democritus, physicists found that one element could be changed into another by adding or removing those parts. Modern-day alchemy—but without the appeal of gold. In the 1960s and 1970s it was learned that protons and neutrons themselves are made up of even smaller particles. It seems that the Greek concept of the atom applies more to these smaller particles than to the objects in the periodic table that we call atoms.

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