Govert Schilling - The Elephant in the Universe : Our Hundred-Year Search for Dark Matter

OUR HUNDRED-YEAR SEARCH FOR DARK MATTER

Govert Schilling

The Belknap Press of Harvard University Press

Cambridge, Massachusetts

London, England

2022

Copyright 2022 by Govert Schilling

Foreword copyright 2022 by the President and Fellows of Harvard College

All rights reserved

Cover design: Henry Sene Yee

978-0-674-24899-1 (cloth)

978-0-674-27617-8 (EPUB)

978-0-674-27618-5 (PDF)

The Library of Congress has cataloged the printed edition as follows:

Names: Schilling, Govert, author.

Title: The elephant in the universe : our hundred-year search for dark matter / Govert Schilling.

Description: Cambridge, Massachusetts : The Belknap Press of Harvard University Press, 2022. | Includes bibliographical references and index.

Identifiers: LCCN 2021034391

Subjects: LCSH: Dark matter (Astronomy)History. | CosmologyHistory.

Classification: LCC QB791.3 .S32 2022 | DDC 523.1/126dc23

LC record available at https://lccn.loc.gov/2021034391

A Hindoo Fable

It was six men of Indostan

To learning much inclined,

Who went to see the Elephant

(Though all of them were blind),

That each by observation

Might satisfy his mind.

The First approached the Elephant,

And happening to fall

Against his broad and sturdy side,

At once began to bawl:

God bless me!but the Elephant

Is very like a wall!

The Second, feeling of the tusk,

Cried: Ho!what have we here

So very round and smooth and sharp?

To me t is mighty clear

This wonder of an Elephant

Is very like a spear!

The Third approached the animal,

And happening to take

The squirming trunk within his hands,

Thus boldly up and spake:

I see, quoth he, the Elephant

Is very like a snake!

The Fourth reached out his eager hand,

And felt about the knee.

What most this wondrous beast is like

Is mighty plain, quoth he;

T is clear enough the Elephant

Is very like a tree!

The Fifth, who chanced to touch the ear,

Said: Een the blindest man

Can tell what this resembles most;

Deny the fact who can,

This marvel of an Elephant

Is very like a fan!

The Sixth no sooner had begun

About the beast to grope,

Then, seizing on the swinging tail

That fell within his scope,

I see, quoth he, the Elephant

Is very like a rope!

And so these men of Indostan

Disputed loud and long,

Each in his own opinion

Exceeding stiff and strong,

Though each was partly in the right,

And all were in the wrong!

So, oft in theologic wars

The disputants, I ween,

Rail on in utter ignorance

Of what each other mean,

And prate about an Elephant

Not one of them has seen!

John Godfrey Saxe, 1872

The term dark matter is used to represent most of the matter in the universefive times more prevalent than ordinary matter, like the atoms that make up stars and planets. But, as the name suggests, we cannot see dark matter. We infer its existence only indirectly through its gravitational influence on visible matter. In this way, dark matter encapsulates our ignorance.

Like all good mysteries, the puzzle of dark matter is enduring. It has intrigued scientists for a hundred years. Observations and scientific theories suggest that dark matter could be made of any number of hypothetical building blocks: weakly interacting massive particles, so-called axions, even atoms that do not interact with ordinary matter or light. Today there is a scientific consensus that dark matter likely came out of the fiery soup during the origins of the universe, an ocean of invisible particles with initially small random motions. Although scientists have not detected any of these invisible particles yet, they have measured the imprint of the fluctuations. Today those dark matter fluctuations are evident in the slightly varied brightness of the cosmic microwave background, the relic radiation left over from the big bang.

Lord Kelvin was the first to offer a dynamical estimate of what we now think of as dark matter. In a talk given in 1884, Kelvin theorized that there might be dark bodies in the Milky Way. Almost fifty years and many ideas later, Swiss-American astronomer Fritz Zwicky estimated that there is more mass in galaxy clusters than is visually observable. In the 1970s evidence for invisible particles was revealed through the pathbreaking work of Vera Rubin, Kent Ford, and Kenneth Freeman. They showed that the dynamics of gas and stars in galaxies imply the existence of invisible mass in a halo that extends well outside the inner region, where ordinary matter concentrates. And in 1983 Moti Milgrom proposed a theory of modified Newtonian dynamics to explain the missing-mass problem. In this alternative hypothesis of gravity, Milgrom postulated that Newtons laws do not apply to galaxies.

Like most explorations in science, historical theories of dark matter found supporters and critics. Milgroms simple prescription for modified dynamics at low accelerations accounts for the nearly flat rotation curves in many galaxy halos extremely well, even after four decades of scrutiny. But the theory does not adequately account for Zwickys observed properties of galaxy clusters. Another possibility is that dark matter is strongly self-interacting and avoids galactic cores. And the hypotheses continue.

Throughout this book, Govert Schilling leads us on a captivating tour through the theories of dark matter and efforts to observe it, from early times to the present day. Well travel with him to astronomical observatories on the ground and in space and to particle detectors in underground caves and tunnels. As we circle the globe, we meet the scientists, the protagonists of the story, who have spent their careers searching for a solution to the puzzle. It is a wide-ranging cast of characters. There are towering figures in the field of dark matter research, like Jim Peebles and Jerry Ostriker. There are younger scientists, true believers, skeptics, and heretics. Through their stories, we garner an extraordinary view into the past, present, and future of one of the deepest enigmas in science.

As The Elephant in the Universe shows, the search for dark matter is a work in progress. Hence the abundance of scientific interpretations. But one day all of the pieces of the puzzle will fall into place. It is with Schillings stellar guidance that we join leading scientists in their crusade to understand this unknown gravitating matter, and, along the way, delight in the mysteries of our universe.

In 1995 astronomers announced that they had developed sensitive spectrometers that made it possible to precisely measure the velocities of stars. Within a few years, I reckoned, these tools would be used to discover extrasolar planets: if the spectrometer picked up tiny, periodic perturbations in the velocity of a star, then there might be a massive planet nearby, the gravity of which was disrupting the parent stars movement through space. I decided to start researching a new book on the hunt for exoplanets, in the hope that the breakthrough find could be described in the closing chapter.

In October of that same year, when Michel Mayor and Didier Queloz announced their discovery of 51 Pegasi bthe first confirmed planet beyond our solar system orbiting a Sun-like starI realized I had to hurry. For most of 1996, I worked on hardly anything else. My (Dutch) book,