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How Old Is the Universe?

How Old Is the Universe?

Copyright Date: 2011
Pages: 380
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  • Book Info
    How Old Is the Universe?
    Book Description:

    Astronomers have determined that our universe is 13.7 billion years old. How exactly did they come to this precise conclusion?How Old Is the Universe?tells the incredible story of how astronomers solved one of the most compelling mysteries in science and, along the way, introduces readers to fundamental concepts and cutting-edge advances in modern astronomy.

    The age of our universe poses a deceptively simple question, and its answer carries profound implications for science, religion, and philosophy. David Weintraub traces the centuries-old quest by astronomers to fathom the secrets of the nighttime sky. Describing the achievements of the visionaries whose discoveries collectively unveiled a fundamental mystery, he shows how many independent lines of inquiry and much painstakingly gathered evidence, when fitted together like pieces in a cosmic puzzle, led to the long-sought answer. Astronomers don't believe the universe is 13.7 billion years old--they know it. You will too after reading this book. By focusing on one of the most crucial questions about the universe and challenging readers to understand the answer, Weintraub familiarizes readers with the ideas and phenomena at the heart of modern astronomy, including red giants and white dwarfs, cepheid variable stars and supernovae, clusters of galaxies, gravitational lensing, dark matter, dark energy and the accelerating universe--and much more. Offering a unique historical approach to astronomy,How Old Is the Universe?sheds light on the inner workings of scientific inquiry and reveals how astronomers grapple with deep questions about the physical nature of our universe.

    eISBN: 978-1-4008-3613-0
    Subjects: Astronomy

Table of Contents

  1. Front Matter
    (pp. [i]-[vi])
  2. Table of Contents
    (pp. [vii]-[x])
  3. CHAPTER 1 Introduction: 13.7 Billion Years
    (pp. 1-6)

    Astronomers are at an enormous disadvantage, compared with other scientists. A biologist can bring a collection of fruit flies into his laboratory, encourage a particular behavior among those flies, and apply all the tools of his trade to studying that behavior. A chemist can mix chemicals together, heat them up or cool them down, and study how they react in the controlled environment of her laboratory. A geologist can hike up a mountain, collect rocks from a particular outcrop, and return these samples to his laboratory for analysis. A physicist can power up a laser and test the mechanical properties...

  4. I. The Age of Objects in Our Solar System

    • CHAPTER 2 4004 bce
      (pp. 9-15)

      How old is the Earth? Clearly it cannot be older than the rest of the universe, so if we could determine the age of the Earth we would have a minimum age for the universe. And that would be an excellent starting point for investigating its total age.

      We live on the Earth. By virtue of that location, we are able to learn more about the Earth than about other places in our solar system, let alone our universe. So let us begin attempting to measure the age of the Earth by observing the world around us. From this beginning,...

    • CHAPTER 3 Moon Rocks and Meteorites
      (pp. 16-26)

      During the seventeenth century, just when an apparent consensus was developing on the age of the Earth, the entire method of biblical chronology came into question. Since chronologists using the three different textual traditions of the Bible (the Hebrew, the Greek, and the Samaritan) obtained time spans since Adam that differed by nearly 2,000 years, the issue of which tradition was most accurate became important. Other scholars asked unanswerable questions that further called into question the method: Was Adam the first man or just the first biblical man? Was the Bible accurate in recording that Methuselah lived for 969 years?...

    • CHAPTER 4 Defying Gravity
      (pp. 27-40)

      Over the course of the twentieth century, geologists and geochemists slowly teased the secrets of the ages from Earth rocks, lunar samples, and meteorites. But we should keep in mind that even our very best estimate for the age of the Earth is only a lower limit. Perhaps no rocks survived the first 1 or 5 or 30 billion years of Earth history. Perhaps the Moon formed long after the Earth. Perhaps no ancient Moon rocks have survived, or perhaps the Apollo astronauts did not visit the most ancient lunar rock formations and thus did not bring back the oldest...

  5. II. The Ages of the Oldest Stars

    • CHAPTER 5 Stepping Out
      (pp. 43-54)

      To find out if we can learn more about the age of the universe from stars, we first need to know something about the stars themselves. What are they? We know of course that they are sources of light. So it stands to reason that if we come to understand more about the nature of light, we might use that knowledge to expand our understanding of the stars themselves. Among the most fundamental questions we might then ask about the stars (which we will indeed ask in Chapters Five and Six) are: How much light does each star emit? and...

    • CHAPTER 6 Distances and Light
      (pp. 55-60)

      In the second century bce, the Greek astronomer Hipparchus compiled a catalog of about 850 stars and, like his mechanical successor the Hipparcos satellite, he noted the position and brightness of each star. Unlike Hipparcos, however, Hipparchus was unable to measure distances to stars; in fact he did not even try because he assumed, as did all astronomers of his day, that all stars were equally distant from the Earth. His brightness measurements, which he calledmagnitudes, were therefore comparisons of how bright the starsappearedin the night sky relative to each other.

      According to Hipparchus, first magnitude stars...

    • CHAPTER 7 All Stars Are Not the Same
      (pp. 61-77)

      Two millennia ago, Hipparchus made the assumption that all stars are the same distance from Earth and therefore differ in brightness because some are intrinsically brighter than others. It was a reasonable assumption for his time, but by the eighteenth century it was no longer tenable. Aristotle’s physics and his geocentric cosmology had been replaced by Newtonian physics and by the heliocentric cosmology of Copernicus. Astronomers would remain unable to measure the distances to any stars until the fourth decade of the nineteenth century, but already they were in universal agreement that the stars in the heavens were at many...

    • CHAPTER 8 Giant and Dwarf Stars
      (pp. 78-93)

      For four decades, from the 1820s through the 1850s, a broad spectrum of the scientific community—astronomers, physicists, chemists, glassmakers, and even photographers—turned their attention to spectra. They studied the spectra of celestial objects, of chemical elements, of enclosed vapors in laboratory settings, and of the Earth’s atmosphere. From their measurements and experiments, they generated the knowledge that would lead, by the end of the century, to an effective methodology for classifying the stars. By the early twentieth century, astronomers would go a step further, using these spectral classifications, in combination with other measurements of stars, to unveil the...

    • CHAPTER 9 Reading a Hertzsprung-Russell (H-R) Diagram
      (pp. 94-100)

      The Hertzsprung-Russell diagram is so fundamental to virtually everything astronomers have learned about stars, galaxies, and the universe that it’s worth taking some extra time to make sure we understand exactly how powerful a tool it is. Remember that Henry Norris Russell’s first diagram, published in 1914, plotted the absolute magnitude of a star on its vertical axis and its Harvard spectral type on the horizontal axis. A century later, the basic approach for how astronomers make such a diagram of luminosity versus temperature has not changed much; however, astronomers have found several other observational measurements that they often use...

    • CHAPTER 10 Mass
      (pp. 101-110)

      Why are some stars hotter than others, and why are the hotter stars more luminous than most of the cooler stars? At the beginning of the twentieth century, most astronomers answered this question by saying that stars are born hot and luminous. According to this explanation, at their birth all stars appear in the top, left corner of the H-R diagram. As they age, they lose energy and cool off, thereby moving downwards and to the right. But as tidy as this answer may have sounded, it failed to explain the simultaneous existence of both red dwarfs (stars moving downward...

    • CHAPTER 11 Star Clusters
      (pp. 111-125)

      Take the most casual glance at the nighttime sky and you will quickly notice that the stars are not distributed uniformly from horizon to horizon. Almost 3,000 years ago Homer, in hisIliad, and Hesiod, in hisWorks and Days,had already mentioned clusters of stars in their writings: the Pleiades and the Hyades. Any careful observer, equipped with even the smallest telescope, will immediately perceive, as did Galileo, that some objects that appear to the naked eye as stars are actually clusters of stars. Galileo identified a star cluster he called the Nebula of Orion (or Orion’s Head), with...

    • CHAPTER 12 Mass Matters
      (pp. 126-132)

      In the end, virtually every aspect of stellar astrophysics rests on a single property of stars—mass. In this chapter, we try to make clear exactly why this is so.

      Stars are born regularly in the Milky Way, coming together from fragments of giant clouds in interstellar space in regions like the Orion Nebula. Each interstellar cloud is a volume of space filled mostly with gas (single atoms and molecules) and is characterized by a temperature (or a narrow range of temperatures throughout the cloud), size, mass, composition, and speed of rotation. The temperature of the cloud describes the kinetic...

    • CHAPTER 13 White Dwarfs and the Age of the Universe
      (pp. 133-158)

      The finite main-sequence lifetimes of stars lead us directly toward two different methods for estimating the age of the universe. By the end of this chapter, we will have in hand one of these age estimates; namely, an estimate obtained from the temperatures and cooling times of white-dwarf stars. The second estimate, which derives from observations of large clusters of stars, will be the subject of the following chapter.

      Amazingly, it is the inert cores of old, dead stars—for that is what white dwarfs are—that provide the means to estimate their ages from their temperatures. In addition, white...

    • CHAPTER 14 Ages of Globular Clusters and the Age of the Universe
      (pp. 159-172)

      We know that stars are born out of giant interstellar clouds, that some of those clouds form clusters with only a few dozen or few hundred stars while others form clusters with hundreds of thousands of stars. We also know from observations of clusters of newborn stars that the time required for all of the stars in a cluster to form—from the birth of the first star to the birth of the last—is at most a few million years. Except in the very youngest clusters, the age of the cluster (hundreds of millions to many billions of years)...

  6. III. The Age of the Universe

    • CHAPTER 15 Cepheids
      (pp. 175-190)

      In Part II of this book, we followed the path astronomers took in unraveling the astrophysics of stars. We discovered that once we understood how stars generate energy and thus how they live and die, we could, by two independent methods learn the ages of white dwarfs and of globular clusters, some of the oldest objects in the Milky Way and perhaps in the universe. Using these methods we obtained ages for some of the white dwarfs and globular clusters in the Milky Way, which gave us independent but consistent figures for the lowest possible age of the universe (since...

    • CHAPTER 16 An Irregular System of Globular Clusters
      (pp. 191-201)

      Many great topics of debate in nineteenth-century astronomy were well on their way to resolution with the invention of spectroscopy, the creation of the Harvard spectral classes, and the discovery of the Hertzsprung-Russell diagram. But early in the twentieth century, perhaps the greatest of all nineteenth-century puzzles in astronomy remained unsolved: What is the nature of the greatspiral nebulae? Are they “island universes,” what we today would call galaxies, similar to the Milky Way, or are they nebulae embedded within and intrinsically part of the Milky Way. Stated more directly, is the Milky Way the entire universe or is...

    • CHAPTER 17 The Milky Way Demoted
      (pp. 202-208)

      In the third decade of the twentieth century, the golden boy Edwin Hubble, standing on the shoulders of Henrietta Leavitt and Harlow Shapley, peered further out into the universe than anyone believed possible. What he saw put an end once and for all to the great debate over the nature of spiral nebulae.

      As a high school student, Edwin Hubble was the 1906 Illinois state champion in the high jump, setting a state record in the event. As an undergraduate at the University of Chicago, he was on Big Ten championship teams in both track and basketball before he was...

    • CHAPTER 18 The Trouble with Gravity
      (pp. 209-216)

      Hubble’s universe of 1925 raised an astrophysical question of enormous interest: What happens to galaxies over time in a universe made of many galaxies, massive galaxies located hundreds of kiloparsecs apart? Newton’s law of gravity, formulated in 1687, had been improved upon by Albert Einstein in his 1915 general theory of relativity, but we can still use Newton’s simpler conception of gravity to understand the fundamental gravity issue.

      Newton’s law of gravity states that the magnitude of the force of attraction between any two objects in the universe depends on the masses of the two objects and the distance between...

    • CHAPTER 19 The Expanding Universe
      (pp. 217-234)

      Until very recently in human history, those who thought hard about the universe, whether they were theologians, natural philosophers, or scientists, could agree on one thing. The universe had a center. In the Aristotelian universe, which was viewed as consistent with and supportive of medieval biblical theology, the Earth occupied the privileged center of a small and finite universe, with the sphere of the stars just beyond the sphere of Saturn and, in the view of many cognoscenti, with the spheres of the angels and of God in the seventh heaven, just beyond the celestial sphere. In 1543, Nicholas Copernicus...

    • CHAPTER 20 The Hubble Age of the Universe
      (pp. 235-244)

      We now have a model with which to understand the history of the universe: the universe began with all its matter and energy very close together, because space itself was vanishingly small. Over time, space has expanded. The galaxies, which formed within a few hundred million years after the birth of the universe, have remained fixed at their original locations in space. It is on account of the expansion of space that the galaxies appear to be flying apart even though they are not moving through space. The distances between galaxies are directly proportional to the speeds at which they...

    • CHAPTER 21 The Accelerating Universe
      (pp. 245-260)

      For decades, teams of astronomers competed to make the most precise measurements of the current value of the Hubble constant. Of course, since the universe is expanding we know that the Hubble “constant” is a misnomer: it should not have been constant over the entire age of the universe, and this is why it is best to identify it as thecurrentvalue of the Hubble constant. To make this point clear in their mathematics, astronomers use the notationH, without the subscripted zero, to denote a Hubble constant that changes with time andH₀to indicate the value of...

    • CHAPTER 22 Dark Matter
      (pp. 261-281)

      The concept of dark matter is one of the most exciting, compelling, mysterious, and perhaps disturbing ideas that has emerged from modern astrophysics. Besides being intriguing in its own right, dark matter has enormous implications for our understanding of how we fit into the universe. And it turns out to be crucial when we use the cosmic microwave background (Chapter Twenty-four) to determine the age of the universe (Chapter Twenty-six).

      The termdark mattermeans, quite simply, matter that produces so little light that we cannot see it. We can, however, detect the presence of dark matter even when it...

    • CHAPTER 23 Exotic Dark Matter
      (pp. 282-300)

      Astronomers had been finding signs pointing to the existence of dark matter for two hundred years, and by the mid-1980s the evidence was overwhelming. The nature of this invisible matter, however, remained a mystery. Those examples that had been found and identified—black holes, white dwarfs, planets, and hot gas—had turned out to be no different in their physics from the kind of matter that is found on Earth. It was widely believed, therefore, that the still missing matter responsible for the flat rotation curves of galaxies would prove no different.

      As we saw in the last chapter, to...

    • CHAPTER 24 Hot Stuff
      (pp. 301-318)

      Just before Hubble found evidence that the universe was expanding, the Belgian priest and physicist Georges Lemaître and, completely independently, the Russian physicist Alexander Friedmann suggested that a universe governed by Einstein’s law of gravity, general relativity, had to be either expanding or contracting. Friedmann’s contribution was to show that Einstein’s static spherical universe was dynamically unstable; any small disturbance would cause it to expand or contract. While his work was almost purely mathematical, Lemaître’s research was directly related to physical cosmology. He noted that the evidence shows that the universe we live in is not contracting and therefore must...

    • CHAPTER 25 Two Kinds of Trouble
      (pp. 319-332)

      The cosmic microwave background together with the expansion of the universe and the relative amounts of the elements remains one of the fundamental observational pillars of modern Big Bang cosmology. To the limits of experimental accuracy, as measured first by Penzias and Wilson and over the next two decades by all those who worked to improve the accuracy of cosmic microwave background observations, the temperature of the CMB was absolutely identical in every direction (isotropic), with a single exception.

      This exception, which cosmologists call thedipole anisotropy, has nothing to do with the CMB and everything to do with the...

    • CHAPTER 26 The WMAP Map of the CMB and the Age of the Universe
      (pp. 333-359)

      We’re zeroing in on our goal now. In fact, with the WMAP map in our toolkit, we have just about everything we need to calculate an age for the universe. But first we must learn how to read the map. We need also to keep in mind that any age estimate based on the WMAP map will be model-dependent; that is, it will only be as reliable as cosmologists’ understanding of dark energy and their calculations of the amounts of dark and normal matter that exist in the universe. What we want to find out is whether the age arrived...

    • CHAPTER 27 A Consistent Answer
      (pp. 360-364)

      How old is the universe?For several centuries, astronomers have been making observations of the nighttime sky, and gradually we have made progress toward solving the riddles of the heavens. By combining observations with the laws of physics, we have teased enormous amounts of information from the light we collect with our telescopes. From this accumulated effort of a dozen generations of scientists, what have we learned about the age of the universe?

      Logically, we know that the universe must be older than the Earth, the Moon, and our solar system. We know that the oldest rocks on all the...

  7. INDEX
    (pp. 365-370)