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A Ray of Light in a Sea of Dark Matter

A Ray of Light in a Sea of Dark Matter

Charles Keeton
Series: Pinpoints
Copyright Date: 2014
Edition: DGO - Digital original
Published by: Rutgers University Press
Pages: 104
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    A Ray of Light in a Sea of Dark Matter
    Book Description:

    What's in the dark? Countless generations have gazed up at the night sky and asked this question-the same question that cosmologists ask themselves as they study the universe.The answer turns out to be surprising and rich. The space between stars is filled with an exotic substance called "dark matter" that exerts gravity but does not emit, absorb, or reflect light. The space between galaxies is rife with "dark energy" that creates a sort of cosmic antigravity causing the expansion of the universe to accelerate. Together, dark matter and dark energy account for 95 percent of the content of the universe. News reporters and science journalists routinely talk about these findings using terms that they assume we have a working knowledge of, but do you really understand how astronomers arrive at their findings or what it all means?Cosmologists face a conundrum: how can we study substances we cannot see, let alone manipulate? A powerful approach is to observe objects whose motion is influenced by gravity. Einstein predicted that gravity can act like a lens to bend light. Today we see hundreds of cases of this-instances where the gravity of a distant galaxy distorts our view of a more distant object, creating multiple images or spectacular arcs on the sky. Gravitational lensing is now a key part of the international quest to understand the invisible substance that surrounds us, penetrates us, and binds the universe together.A Ray of Light in a Sea of Dark Matteroffers readers a concise, accessible explanation of how astronomers probe dark matter. Readers quickly gain an understanding of what might be out there, how scientists arrive at their findings, and why this research is important to us. Engaging and insightful, Charles Keeton gives everyone an opportunity to be an active learner and listener in our ever-expanding universe.

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    eISBN: 978-0-8135-6532-3
    Subjects: Astronomy
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Table of Contents

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  1. Front Matter (pp. i-vi)
  2. Table of Contents (pp. vii-viii)
  3. Preface (pp. ix-xiv)
  4. Chapter 1 What’s in the Dark? (pp. 1-10)

    Astronomers have long been denizens of the dark, spending nighttime hours on remote mountaintops capturing tiny traces of light from afar. For decades, observers had to ride on telescopes all night, cramped in a cage opposite the mirror, to keep the instrument focused on the target during long-exposure photographs. Then they would carry the photographic plates to the darkroom to be developed. Later, teams of assistants would pore over the plates to measure planets, stars, and galaxies. Observational astronomy was painstaking, and every photon was precious.

    A practical change is under way, driven (as in so many other fields) by...

  5. Chapter 2 When Mass Is Like Glass (pp. 11-21)

    Before discussing how astronomers use light bending to investigate mass, it is good to understand the physical phenomenon itself. The term “gravitational lensing” is very apt, because mass actually acts like a certain kind of glass lens. Starting with traditional optics can help clarify what happens with gravity.

    When a light beam passes from air into glass, it slows down. (The same can be said of water, oil, or any other translucent substance that is denser than air.) If the beam hits the surface at an angle, one side slows down before the other, causing the beam to bend (see...

  6. Chapter 3 How Do You Weigh a Galaxy? (pp. 22-32)

    As mentioned in chapter 1, astronomy often begins with the identification of patterns. The patterns we see first are not always the ones that have the most physical significance, however. It is important to understand which patterns are coincidental and which offer deep insights into the structure of galaxies.

    Stars are scattered across the night sky, but our minds try to impose some order on the randomness. Human societies have identified two types of patterns in the stars. Asterisms are groupings of a few stars that seem to take on recognizable shapes: the Big Dipper, the Summer Triangle, and the...

  7. Chapter 4 Is Dark Matter MACHO or WIMPy? (pp. 33-45)

    What is the substance that enshrouds galaxies? Our first guess might be something familiar that is just hard to see. We can certainly imagine that dim things could be out there. Some of the brightest objects in our night sky (such as the Moon, Venus, and Jupiter) are vivid only because they are illuminated by a star. If these bodies were moved out of the solar system, they would become extremely hard to see. Amassing enough planets might be challenging; even giant Jupiter is 1,000 times less massive than the Sun, so it would take roughly 1,000 times more planets...

  8. Chapter 5 Finding What’s Missing (pp. 46-54)

    While the quest to understand dark matter has been taken up by physicists, it still has valuable contributions from astrophysics in general and gravitational lensing in particular. Here the story shifts back to lensing by distant galaxies, and it has two threads that merged at the turn of the twenty-first century.

    The first thread involves lens systems in which a foreground galaxy creates four images of a background quasar. In the 1990s, we began to gather a sizable sample of four-image systems and observe them with the Hubble Space Telescope and radio telescopes. The sharp pictures let us measure the...

  9. Chapter 6 “A Long Time Ago in a Galaxy Far, Far Away” (pp. 55-66)

    So far we have thought about using lensing the way a doctor uses X-rays, analyzing the way light travels through something to learn what’s inside. But we can shift our focus to the light sources themselves and use lensing to help us study objects that would otherwise be too small and/or faint. We can, in other words, use gravitational lenses as cosmic telescopes that boost our Earth-bound telescopes and let us peer further into the depths of the universe.

    Our telescopes already act as time machines. The speed of light is large but finite, so it takes a certain amount...

  10. Glossary (pp. 67-72)
  11. Notes (pp. 73-74)
  12. Notes on Sources (pp. 75-88)
  13. Back Matter (pp. 89-89)