I am greatly relieved that the universe is finally explainable.
I was beginning to think it was me. As it turns out, physics, like a grating relative, has all the answers.
Woody Allen (The New Yorker, July 28, 2003)
A short time ago cosmology seemed settled in the comfy chair of complacency, confident in the apparent resolution of many of its major issues. All known data converged on a uniform chronology of cosmic history—an account so widely accepted that it had come to be known as the standard model.
Every student of astronomy could recite the then-known facts: The universe began in a fiery explosion called the Big Bang and then expanded for billions of years. Over time its rate of expansion has gradually slowed and its constituent particles have come together to form galaxies, planets … and us. Eventually, depending on its material density, it will either fizzle out in a “Big Whimper” or shrink back down to an infinitesimal point in a “Big Crunch.” These options were delineated by what are technically known as the Friedmann models: simple solutions discovered by Russian cosmologist Alexander Friedmann of the gravitational equations developed by Albert Einstein. The task of cosmology appeared relatively straightforward—to establish the precise age of the universe, firm up
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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos Introduction: The Quest for Cosmic Understanding I am greatly relieved that the universe is finally explainable. I was beginning to think it was me. As it turns out, physics, like a grating relative, has all the answers. Woody Allen (The New Yorker, July 28, 2003) A short time ago cosmology seemed settled in the comfy chair of complacency, confident in the apparent resolution of many of its major issues. All known data converged on a uniform chronology of cosmic history—an account so widely accepted that it had come to be known as the standard model. Every student of astronomy could recite the then-known facts: The universe began in a fiery explosion called the Big Bang and then expanded for billions of years. Over time its rate of expansion has gradually slowed and its constituent particles have come together to form galaxies, planets … and us. Eventually, depending on its material density, it will either fizzle out in a “Big Whimper” or shrink back down to an infinitesimal point in a “Big Crunch.” These options were delineated by what are technically known as the Friedmann models: simple solutions discovered by Russian cosmologist Alexander Friedmann of the gravitational equations developed by Albert Einstein. The task of cosmology appeared relatively straightforward—to establish the precise age of the universe, firm up
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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos the sequence of events and reduce the possible endgames down to one. Sure there were open questions, but mainstream cosmologists saw these as refinements. Most researchers believed in a clear-cut model of the universe that had little room for change after the first few moments of its history. Much debate was centered on pinning down what happened during the initial ticks of the cosmic clock. A few of us pondered alternatives to the canon—theories of the universe that strayed from the simplest version of the Big Bang. Like the standard model, these were legitimate mathematical solutions— albeit of variations, reinterpretations, or extensions—of Einstein’s equations. Mainstream cosmologists knew about such alternatives but tended to treat them as mere curiosities. In the absence of evidence to the contrary, these researchers advised, why reach beyond conventional approaches? The situation was akin, in some ways, to the state of affairs before the age of Johannes Kepler and Galileo Galilei. From the 2nd century until the 16th century AD, astronomy relied on the coarse measurements of planetary motion recorded by the Alexandrian thinker Claudius Ptolemy (born circa 85 AD). In his pivotal text, the Almagest, Ptolemy developed a clockwork model of the solar system that corresponded well to his data. Consisting of wheels within wheels ultimately turning around Earth, Ptolemy’s model showed how planets could follow distinct patterns as they moved across the sky. Because his scheme explained all known facts and fit in well with religious views, scholars found little reason to dispute it. True, it could be simplified, as the Polish astronomer Nicholas Copernicus pointed out, by placing the Sun at the center instead of Earth. But even Copernicus had no new data to back up his proposition. What changed matters at the turn of the 17th century—as well as at the turn of the 21st century—were substantial improvements in astronomical measuring techniques that led to an enhanced understanding of the movements of celestial bodies. Superior naked-eye
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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos measurements of the Martian orbit taken by the Danish astronomer Tycho Brahe led Kepler to conclude in 1609 that the planets follow elliptical paths around the Sun. At approximately that time, images from the first astronomical telescope inspired Galileo to propose that the planets are worlds in their own right and that the stars are distant suns. These findings, in turn, led to the Newtonian portrait of a vast, possibly infinite, universe—home to myriad celestial objects interacting with one another according to the law of gravity. Telescopes became larger and larger, revealing deeper layers of cosmic order. As they demonstrated, in the race across the celestial plains, stars are hardly lone rangers. Rather, they ride like horses on grand merry-go-rounds called galaxies. Galaxies belong to clusters— assembled, in turn, into even greater superclusters. In 1929, Edwin Hubble, using a colossal device on Mount Wilson in California, discovered that all distant galaxies are receding from each other. This finding led to the standard Big Bang model of an expanding universe—the crown jewel of 20th-century cosmology. Just as it was enhanced observations that led science to abandon the Ptolemaic model and usher in the modern age, it is the dramatically improved equipment and techniques that have resulted in a rethinking of the standard cosmological approach. In the 1990s and early 2000s astronomy leapt above the clouds with extraordinary new orbiting instruments. Circling high above Earth at distances ranging from hundreds to hundreds of thousands of miles, these telescopic satellites have spanned the spectrum with their light-gathering power. Joining the Hubble Space Telescope, equipped to collect optical light, are infrared instruments, X-ray probes, and several microwave detectors—including the Cosmic Background Explorer and, most recently, the Wilkinson Microwave Anisotropy Probe (WMAP). WMAP has yielded the most precise estimate to date for the age of the universe: 13.7 billion years. Space-based imaging has been accompanied by other astronomical breakthroughs. Digital cameras, able to absorb and record every single photon (particle of light) streaming down from space, have
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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos led to unprecedented precision and deeper-than-ever sky surveys. With these electronic spectacles, once-faint blurs have revealed themselves as extremely distant galaxies that can be analyzed and cataloged. Masterful computer algorithms piece together terabytes of photonic information into detailed three-dimensional images of space. Consequently, for the very first time, astronomy has added realistic depth to its spatial maps. A leading ground-based project, called the Sloan Digital Sky Survey, has employed these state-of-the-art techniques in a comprehensive three-dimensional scan of a large portion of the northern sky. Mapping more than 200,000 galaxies, the survey has dramatically increased our knowledge of vast segments of space. Paradoxically, though these instruments and programs have provided more information about the universe than ever before in scientific history, they have revealed how much we really do not know. In particular, they have confirmed a gnawing suspicion among cosmologists that the vast majority of the universe is composed of invisible materials and unidentified energies. As the telescopic results have indicated, only a small fraction of the mass of the cosmos constitutes ordinary matter. The rest is terra incognita! Not only do unseen powers appear to dominate space, they seem to govern its overall dynamics—causing the universe to expand at an ever-increasing rate. In short, we appear to live in an accelerating universe fueled by a hidden dynamo of mysterious origin. This extraordinary discovery sent shock waves through the world of cosmology, displacing a number of long-held conceptions. No longer can cosmologists focus on the simplest models with the most basic kinds of matter—the textbook examples of expanding universes. Rather, the new findings have revealed more unusual possibilities and solutions. Some of these novel proposals hypothesize strange new substances with properties unlike anything ever seen. Could, for instance, objects exist with negative mass? Could there be shadow worlds able to communicate with us only through the pull of gravity?
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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos Could there be particles so energetic they have yet to be produced in our particle accelerators? Perhaps the next generation of powerful detectors will reveal such unusual entities. Other revolutionary schemes involve modifying the law of gravity itself. Could it be that both Newton and Einstein—the greatest geniuses in physics—were wrong about the nature of the gravitational force? Perhaps their portraits of gravity, like unfinished masterpieces, require extra flourishes. Yet another option involves transforming one or more of nature’s constants into a variable. A group of physicists recently speculated that the speed of light could vary over time. Other “variable constant” ideas involve slowly changing values of the fine-structure constant (the parameter governing the strength of electromagnetic interactions), the gravitational constant, and even mass itself. Finally, some of the most promising approaches for explaining the cosmological mysteries postulate the existence of a fifth dimension beyond ordinary space and time. The fifth dimension arises as a means of unifying all known forces of nature into a single theory. Although its origins date back to the early days of Einstein’s general theory of relativity, it has recently been revived in methods for unification called supergravity, string theory, and M-theory. Traditionally, if a fifth dimension exists, physicists have imagined it to be so small that it could hardly be detected. However, many contemporary approaches envision a large extra dimension, one comparable in scale to conventional space and time. In such a case the fifth dimension could influence the dynamics of the universe and possibly explain why it is accelerating. Moreover, if celestial mechanics is truly five dimensional, the Big Bang need not have been the beginning of time. Rather, it could have been a transition between different cosmic eras. Perhaps the actual cosmos is eternal and its finite age only an illusion wrought by the limitations of our senses. Indeed, even with the best of all possible astronomical devices, much about the universe could well remain mysterious. Our place in
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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos the cosmos is incomparably small; our time in it is but a humming-bird’s beat. It would not be too surprising if there are aspects of reality for which we, like dwellers on a tiny desert island, have little knowledge. One of the greatest learning tools at our disposal is human intuition. Given our peripheral position in the oceans of space, we can use the power of logical deduction to infer much about what lies on distant shores. An outstanding example of the use of human intuition to extend our knowledge far beyond Earth involves the mystery of why the sky is dark at night. By applying some thought to this riddle, there is much we can learn about the universe at large.