Preface

Galileo once said: “Philosophy [nature] is written in that great book which ever is before our eyes—I mean the universe… . The book is written in mathematical language, and the symbols are triangles, circles and other geometrical figures.” So it proved to be with the arrangement of galaxies in the universe. To understand it would require geometrical language.

When I was 18 years old, I discovered a group of intricate, spongelike structures made of triangles, squares, pentagons, or hexagons—some of which neatly divided space into two equal and completely interlocking regions. These were regular spongelike polyhedrons—figures composed of regular polygons whose arrangement around each vertex was identical. Being a teenager, when confronted with the ancient Greek wisdom that there were five, and only five, regular polyhedrons (the tetrahedron, cube, octahedron, dodecahedron, and icosahedron)—and that this had been proven long ago—I said, “Well, maybe not.” I made this my highschool science project and took it to my local science fair in Louisville, Kentucky. Surprisingly, this would later play a role in my own path to understanding the arrangement of galaxies in the universe.

Johannes Kepler was my inspiration. He had also questioned the ancient wisdom of the five regular polyhedrons. Kepler thought that the three regular polygonal tilings of the plane should be counted as polyhedrons also: the checkerboard, the hexagonal chicken-wire pattern, and triangles, six around a point, filling the Euclidean plane. Both the checkerboard and the cube were equally regular arrangements of polygons (even though one turned out flat and the other, three-dimensional). Kepler thought a checkerboard, for example, could be considered a new regular polyhedron—with an infinite number of faces. But Kepler didn’t stop there; he also recognized two new regular starred polyhedrons. One has faces that are five-pointed stars like those on the American flag. Isn’t a star just as regular as a pentagon? It has five points, just like the pentagon, and is likewise made by drawing five equal-length lines connecting them. The only difference is that the lines are allowed to cross through each other! You just have to expand your mind a little to see five-pointed stars as regular. Kepler would take five-pointed stars as the faces of his new regular polyhedron. He had them cross through each other to form a three-dimensional star. Kepler understood that you could find new things by breaking the rules just a little. (See Color Plate 1.)

Kepler was also fascinated with how one might use polyhedrons in astronomy. There were six known planets in his day. If you built a set of spheres whose radii marked the distances of each from the Sun, you would have six nested spheres. He thought that you might fit the five previously known regular polyhedrons between each of these spheres to explain the geometry of the solar system. In this he was wrong. And when more planets were discovered, the idea broke down completely. But when Kepler was told planets must have circular orbits, he thought to use elliptical orbits instead, and in this he was famously right.

But would my spongelike polyhedrons—which had geometries like a marine sponge, with many holes percolating through them—remain a mathematical fantasy, or would they ever have any practical application in real-world astronomy? It turned out they had an application in understanding galaxy clustering.

Edwin Hubble discovered that our Milky Way galaxy containing 300 billion stars was not alone in space. There were countless other galaxies just as big as ours. Furthermore, this whole assembly of galaxies was expanding, as I describe in Chapter 1. But how exactly are these galaxies arranged in space? It was a puzzle that confronted astronomers. Galaxies congregated in clusters. Chapter 2 tells how Fritz Zwicky famously studied this at Caltech. His work led American cosmologists during the Cold War to adopt a meatball model in which the high-density clusters floated in a low-density sea, as described in Chapter 3. But the Russian school of cosmology favored a model where galaxies traced a giant honeycomb in space with large empty isolated voids. This was a Swiss cheese universe (Chapter 4). I found that the new theory of inflation¹ (Chapter 5) was inconsistent with either of these pictures and required a spongelike structure in which great clusters of galaxies were connected by filaments of galaxies and great voids were connected to each other by low-density tunnels (Chapter 6).

Considering the theory of inflation and remembering those polyhedrons from my youth, I wrote a paper with Adrian Melott (University of Kansas) and Mark Dickinson (Princeton University) predicting that galaxies must be arranged on a giant cosmic sponge. The efforts we made to verify this prediction became part of the larger story of how teams of observers embarked on heroic efforts to map the universe, as described in Chapters 7, 8, and 9. These studies would give us vital insight into how the universe began. Astronomers began to chart the distribution of galaxies in space. Just as cartographers of the past mapped Earth, these cosmic cartographers began mapping our universe. Starting with surveys of a thousand galaxies, major surveys have now grown to encompass well over a million galaxies. Three-dimensional maps of the galaxies’ distribution have now been made, and the structure they reveal has indeed proved to be spongelike. Great clusters of galaxies are connected by filaments, or chains of galaxies, in a spongelike geometry, while the low-density voids are connected to each other by low-density tunnels; this entire structure is now called the cosmic web. Fantastic filamentary chains of galaxies connecting great clusters have been found stretching over a billion light-years in length. These are the largest structures in the universe. Measuring one of them, called the Sloan Great Wall, landed Mario Jurić and me in the Guinness Book of Records—and we didn’t even have to collect the world’s largest ball of twine! I will explain how these largest structures in the universe arose as the greatly expanded fossil remnants of microscopic random quantum fluctuations in the early universe produced by inflation in the universe’s first 10⁻³⁵ seconds. This is supported by study of the fluctuations in the cosmic microwave background radiation left over from the universe’s first moments (Chapter 10).

Not only do these structures illuminate the early universe, but they can also be used to forecast our future, as described in the final chapter. Will the universe keep expanding exponentially forever, as some models suggest, or will it ultimately coast along in a slower fashion? Or, will the universe end catastrophically with a Big Rip singularity in the next 150 billion years? A careful study of the cosmic web can help answer these questions. Distinguishing among these possible alternative futures is one of the highest-priority areas of research in astronomy today.

Ranging from a humble high school science project to mapping projects involving hundreds of astronomers, this book will give you a window on how scientific research is done. It is a story of how unexpected connections can lead to new insights and how computer simulations combined with giant telescopic surveys have transformed our understanding of the universe in which we live. This is a semiautobiographical account focusing on my adventures but also emphasizing many of the people whose seminal ideas have influenced the field. I have had the good fortune to work with some of the greatest astronomers of our generation, investigating many of the aspects of this story in one way or another, from galaxy clustering, gravitational lensing, computer simulations, and mapping large-scale structures to inflation and dark energy. This book is told from my personal perspective as I meandered through the complicated web of talented people who fought for and finally won an understanding of how the universe on large scales is arranged. A cosmic web, if you will.

J. Richard Gott

Princeton, New Jersey