Origins: Fourteen Billion Years of Cosmic Evolution by Donald Goldsmith, Neil deGrasse Tyson Read Free Book Online Page B

accumulated. From that we can then deduce how much ordinary matter, dark matter, and dark energy the universe comprises (the percentages are 4, 23, and 73, respectively). From there, it’s easy to tell whether or not the universe will expand forever, and whether or not the expansion will slow down or speed up as time passes.
    Ordinary matter is what everyone is made of. It exerts gravity and can absorb, emit, and otherwise interact with light. Dark matter, as we’ll see in Chapter 4, is a substance of unknown nature that produces gravity but does not interact with light in any known way. And dark energy, as we’ll see in Chapter 5, induces an acceleration of the cosmic expansion, forcing the universe to expand more rapidly than it otherwise would. The phrenology exam now says that cosmologists understand how the early universe behaved, but that most of the universe, then and now, consists of stuff they’re clueless about.
    Profound areas of ignorance notwithstanding, today, as never before, cosmology has an anchor. The CBR carries the imprint of a portal through which all of us once passed.
    The discovery of the cosmic microwave background added new precision to cosmology by verifying the conclusion, originally derived from observations of distant galaxies, that the universe has been expanding for billions of years. It was the accurate and detailed map of the CBR—a map first made for small patches of the sky using balloon-borne instruments and a telescope at the South Pole, and then for the entire sky by a satellite called the Wilkinson Microwave Anisotropy Probe (WMAP)—that secured cosmology’s place at the table of experimental science. We shall hear much more from WMAP, whose first results appeared in 2003, before our cosmological tale is done.
    Cosmologists have plenty of ego: how else could they have the audacity to deduce what brought the universe into being? But the new era of observational cosmology may call for a more modest, less freewheeling stance among its practitioners. Each new observation, each morsel of data, can be good or bad for your theories. On the one hand, the observations provide a basic foundation for cosmology, a foundation that so many other sciences can take for granted because they achieve rich streams of laboratory observations. On the other hand, new data will almost certainly dispatch some of the tall tales that theorists dreamed up when they lacked the observations that would give them thumbs up or down.
    No science achieves maturity without precision data. Cosmology has now become precision science.

CHAPTER 4
    Let There Be Dark
    G ravity, the most familiar of nature’s forces, offers us simultaneously the best and the least understood phenomena in nature. It took the mind of Isaac Newton, the millennium’s most brilliant and influential, to realize that gravity’s mysterious “action at a distance” arises from the natural effects of every bit of matter, and that the attractive forces between any two objects can be described by a simple algebraic equation. It took the mind of Albert Einstein, the twentieth century’s most brilliant and influential, to show that we can more accurately describe gravity’s action-at-a-distance as a warp in the fabric of space-time, produced by any combination of matter and energy. Einstein demonstrated that Newton’s theory requires some modification to describe gravity accurately—in predicting, for example, the amount by which light rays will bend when they pass by a massive object. Although Einstein’s equations are fancier than Newton’s, they nicely accommodate the matter that we have come to know and love. Matter that we can see, touch, feel, and occasionally taste.
    Don’t know who’s next in the genius sequence, but we’ve now been waiting well over half a century for somebody to tell us why the bulk of all the gravitational forces that we’ve measured in the universe arises from substances that we have neither seen, nor touched,