A Ray of Light in a Sea of Dark Matter

By Charles Keeton

 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 Matter offers 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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• Language: English
• Publisher: Rutgers University Press
• Release date: Apr 30, 2014
• ISBN: 9780813572123

Book preview

A Ray of Light in a Sea of Dark Matter

Pinpoints is a series of concise books created to explore complex topics by explaining key theories, current scholarship, and important concepts in a brief, accessible style. Each Pinpoints book, in under 100 pages, enables readers to gain a working knowledge of essential topics quickly.

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Deborah Carr, Worried Sick: How Stress Hurts Us and How to Bounce Back

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Toby C. Jones, Running Dry: Essays on Water and Environmental Crisis

Charles Keeton, A Ray of Light in Sea of Dark Matter

A Ray of Light in a Sea of Dark Matter

Charles Keeton

Rutgers University Press

New Brunswick, New Jersey, and London

Library of Congress Control Number: 2014930070

ISBN: 978-0-8135-6532-3 (web PDF)

ISBN: 978-0-8135-7212-3 (ePub)

Copyright © 2014 by Charles Keeton

All rights reserved

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After they’ve clicked the light off

And everybody’s said goodnight,

What’s in the dark?

—Carl Memling

There are more things in heaven and earth, Horatio,

Than are dreamt of in your philosophy.

—William Shakespeare’s Hamlet

Contents

Preface

1. What’s in the Dark?

Astronomers use a variety of intellectual tools to study objects they cannot touch or even see.

2. When Mass Is Like Glass

Gravitational lensing can be understood through an analogy with a glass lens shaped like the base of a wine glass.

3. How Do You Weigh a Galaxy?

The motions of stars and light rays probe a galaxy’s gravitational field, and hence its mass.

4. Is Dark Matter MACHO or WIMPy?

Gravitational lensing within our galaxy helped demonstrate that dark matter is not just normal matter that is hard to see. It must be something exotic, which physicists are now trying to catch.

5. Finding What’s Missing

Conventional dark matter theory predicts that galaxies are surrounded by invisible clumps of dark matter. Lensing can help find them.

6. A Long Time Ago in a Galaxy Far, Far Away

Clusters of galaxies act as cosmic telescopes, helping astronomers study distant objects that would otherwise be too small and faint to see.

Glossary

Notes

Notes on Sources

Preface

The story of cosmology is one of hubris and humility. Hubris to think that we humans can peer out from our perch on Earth and comprehend the totality of the universe. Humility to discover how insignificant we are in the cosmic scheme. When Copernicus displaced the Earth from the center of the universe, that was just the beginning. Now we understand that our star is one of hundreds of billions in our galaxy, which itself is one of hundreds of billions of galaxies in the universe. Worse, the substance of which we are made—and the subject of all scientific inquiry until the twentieth century—turns out to be as exceptional as dewdrops on a spider’s web. The underlying fabric of the universe is something altogether different, unknown and exotic.

And yet . . . the humility is surely suffused with awe for what we have learned. In our galaxy, the stars and gas that we see are embedded in an enormous halo of matter that we do not see. The dark matter surrounds us and penetrates us; pieces of it are flying through you at this moment. It also binds our galaxy together; the Milky Way, like most spiral galaxies, is spinning so fast that it would fling itself apart if not for the gravitational pull of dark matter. Throughout the universe, dark matter outweighs the familiar visible matter by a ratio of 5:1, and it creates the cosmic web on which visible galaxies hang. Truly there is far more in the heavens than we ever dreamed.

Our knowledge of dark matter seems quite remarkable given that we have never seen the stuff. It derives from that special blend of curiosity, commitment, creativity, and critical thinking known as scientific inquiry. To many scientists, research has an air of childlike wonder; it begins with such eternal questions as, What’s this? Why did that happen? What’s over here? What would happen if . . . ? Playful curiosity is not enough, however. Scientists need to be creative in developing hypotheses and designing experiments to test them. They need to be rigorous in refining ideas that pass the test and discarding those that don’t. Last, but certainly not least, they need to be able to sort through messy, incomplete, and even contradictory evidence to extract appropriate conclusions.

The study of dark matter, in other words, requires intellectual skills that transcend cosmology and speak to the broad capabilities of the human mind. Our ability to marshal such skills to discover what’s in the dark suggests that our cosmic hubris is, perhaps, not entirely mistaken. We can in fact peer into the depths and understand what the universe is made of, when we put our minds to it.

This book recounts some of the intellectual history of dark matter. We focus on how astronomers use light rays as a diagnostic tool to study material that would otherwise remain hidden. The story begins a century ago, when Albert Einstein predicted that gravity bends light. He did not anticipate the scope of that simple idea; a half century passed before the Norwegian astrophysicist Sjur Refsdal recognized the cosmological potential of light bending, and another three decades before the Hubble Space Telescope and other instruments turned gravitational lensing into a tool for cosmology. Today, lensing plays a key role in the quest to understand dark matter.

This book draws on my research in gravitational lensing, and my teaching in astrophysics more broadly. My colleagues at Rutgers University and around the world have contributed a great deal to my ongoing education. They are too many to name individually, but some of their work is cited in the Notes on Sources. This is not a textbook but it does contain some specialized terminology that may not be familiar, so terms in boldface are defined in the glossary. I am grateful to Charles Bergquist, Art Congdon, Charley Keeton, Allan Moser, Leonidas Moustakas, Kelly Wieand, and my editor Leslie Mitchner for valuable comments on the manuscript. The custom lens shown in chapter 2 was built by William Schneider and the machine shop in the Department of Physics and Astronomy, with help from Jack Hughes, Dave Maiullo, and Kathleen Sindoni. My work has been supported by two grants from the National Science Foundation (AST-0747311 and AST-1211385).

Chapter 1: What’s in the Dark?

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