The Cosmic Web

Mysterious Architecture of the Universe

J. Richard Gott

• Publisher: Princeton University Press
• Year: 2016
• ISBN:
  9781400873289
• Topic: Physical Sciences
• Subtopic: Astronomy & Astrophysics

A gripping first-person account of how scientists came to understand our universe's mysterious structure J. Richard Gott was among the first cosmologists to propose that the structure of our universe is like a sponge made up of clusters of galaxies intricately connected by filaments of galaxies—a magnificent structure now called the "cosmic web" and mapped extensively by teams of astronomers. Here is his gripping insider's account of how a generation of undaunted theorists and observers solved the mystery of the architecture of our cosmos. The Cosmic Web begins with modern pioneers of extragalactic astronomy, such as Edwin Hubble and Fritz Zwicky. It goes on to describe how, during the Cold War, the American school of cosmology favored a model of the universe where galaxies resided in isolated clusters, whereas the Soviet school favored a honeycomb pattern of galaxies punctuated by giant, isolated voids. Gott tells the stories of how his own path to a solution began with a high-school science project when he was eighteen, and how he and astronomer Mario Juri? measured the Sloan Great Wall of Galaxies, a filament of galaxies that, at 1.37 billion light-years in length, is one of the largest structures in the universe.Drawing on Gott's own experiences working at the frontiers of science with many of today's leading cosmologists, The Cosmic Web shows how ambitious telescope surveys such as the Sloan Digital Sky Survey are transforming our understanding of the cosmos, and how the cosmic web holds vital clues to the origins of the universe and the next trillion years that lie ahead.

Chapter 1

Hubble Discovers the Universe

It is fair to say that Edwin Hubble discovered the universe. Leeuwenhoek peered into his microscope and discovered the microscopic world; Hubble used the great 100-inch-diameter telescope on Mount Wilson in California to discover the macroscopic universe.

Before Hubble, we knew that we lived in an ensemble of stars, which we now call the Milky Way Galaxy. This is a rotating disk of 300 billion stars. The stars you see at night are all members of the Milky Way. The nearest one, Proxima Centauri, is about 4 light-years away. That means that it takes light traveling at 300,000 kilometers per second about 4 years to get from it to us. The distances between the stars are enormous—about 30 million stellar diameters. The space between the stars is very empty, better than a laboratory vacuum on Earth. Sirius, the brightest star in the sky, is about 9 light-years away.

The Milky Way is shaped like a dinner plate, 100,000 light-years across. We are located in this thin plate. When we look perpendicular to the plate, we see only those stars that are our next-door neighbors in the plate; most of the stars in these directions are less than a few hundred light-years away. We see about 8,000 naked-eye stars scattered over the entire sky; these are all our nearby neighbors in the plate, a tiny sphere of stars nestled within the thin width of the plate. But when we look out through the plane of the plate we see the soft glow of stars that are much farther from us but still within the plane of the plate. They trace a great circle 360° around the sky. Here we are seeing the circumference of the giant plate itself, as we look around the sky in the plane of the plate. We call this band of light the Milky Way. When Galileo looked at this band of light in his telescope in 1610, he found its faint glow was due to a myriad of faint stars—faint because they are so distant. With the naked eye we can see only their combined faint glow; we cannot resolve that glow into individual stars. It took a telescope to do that. For a long time, this constituted the known universe. Our galaxy appeared to be sitting alone in space—an island universe.

In 1918 our idea of our place in the universe started to change. Harlow Shapley discovered that the Sun was not at the center of the Milky Way but instead was about halfway out toward the edge. We were off center. Shapley felt like the new Copernicus. Just as Copernicus had moved Earth from the center of the solar system and properly placed the Sun at its center, Shapley moved the solar system from the center of the Milky Way to a place in its suburbs. Our position in the universe was looking less and less special. Shapley’s monumental work did revolutionize our thinking about our place in the universe. He had a right to suppose that he had made what would be the most important discovery in astronomy in the twentieth century. Time would later put Shapley on its cover, on July 29, 1935. Shapley was the dean of American astronomers. But his great discovery of 1918 was soon to be eclipsed—twice—by Hubble.

Hubble studied the Andromeda Nebula, which had been thought by many, including Shapley, to be a gas cloud within the Milky Way. The word nebula comes from the Latin nubes, or “cloud,” denoting the fuzzy appearance of these objects. By careful observations with the new 100-inch telescope, Hubble discovered that Andromeda was actually an entire galaxy roughly the size of the Milky Way and very far away. Furthermore there were many other similar spiral-shaped nebulae seen in the sky, and these were all galaxies like our Milky Way! He classified galaxies by their shapes—elliptical, spiral, and irregular—like some botanist classifying microbes. He observed in different directions and counted the number of galaxies he found. There seemed to be an equal number in different directions. On the largest scales the universe was homogeneous. There were fainter galaxies further and further away. We were just one galaxy in a vast universe of galaxies. This would have been discovery enough, but Hubble was not finished. He measured the distances to these galaxies. From spectra of these galaxies he could measure their velocities. He found that the further away a galaxy was, the faster it was moving away from us. The whole universe was expanding! This was astonishing. Isaac Newton had a static universe. Even Einstein, genius of curved spacetime, thought the universe must be static. The discovery that the universe was expanding was quite simply, astounding. It caused Einstein to revise his ideas about his field equations of general relativity—to backtrack on the changes he had made in them to produce a static cosmology. The expansion of the universe has profound implications.

If the universe were static, as Newton and Einstein had supposed, then it could be infinitely old. It would always have been here. This avoided Aristotle’s problem of first cause. If the universe had a finite age, however, then something must have caused it. But what caused that? Unless one is willing to accept an infinite regression of causes, there must be a first cause—but the question remains: what caused the first cause? An expanding universe brought this question back into play. If you played the tape of history backward, you would see all the galaxies crashing together in the past. There must have been something to start all this expansion—a Big Bang—that began the universe. We now know this occurred 13.8 billion years ago. What caused this Big Bang? Astronomers following Hubble would work on that.

Hubble was the most important astronomer in the twentieth century. Time magazine put him on its cover on February 9, 1948. Behind him was a picture of the Palomar Observatory, whose new 200-inch-diameter telescope could extend Hubble’s observations. He was the first person to observe with that telescope. Later Time would select Hubble as one of the 100 most influential people in the twentieth century (the only astronomer so honored). Despite the acknowledged importance of his discoveries, Hubble failed to get the American Astronomical Society’s highest award, the Russell Lectureship, given each year to an outstanding American astronomer for lifetime achievement. It reminds one of the Nobel Prize committee’s failure to award the Nobel Prize in Literature to Leo Tolstoy, even though they had several chances to do so before he died. The greatest people are often controversial. As with most groundbreaking discoveries, the whole story is more complicated, and interesting, than just the simple outline I have given so far. So let’s look into the story in more detail.

Shapley Blazes the Trail

Harlow Shapley had measured the position of the Sun in the Milky Way by using globular clusters. He measured their distances using RR Lyrae variable stars as objects of standard luminosity—standard candles. RR Lyrae stars are 40 to 50 times as luminous as the Sun and so can be seen out to fairly large distances. They all have about the same intrinsic luminosity, the same wattage as lightbulbs, if you will. (The Sun, for example, has a luminosity of 4 × 10²⁶ watts—equal to 4 trillion-trillion 100-watt lightbulbs.) If you saw an RR Lyrae star, you could figure out how far away it was by seeing how faint it appeared to be in the sky. It’s like seeing a row of standard street lights extending down a street. They all have the same intrinsic luminosity, but the most distant ones will be fainter than the nearby ones.

Light emitted from a star spreads out in all directions, creating an ever-expanding sphere of light. Let’s say you are 1,000 light-years from a star. The light that is passing you from that star is a spherical shell with a radius r of 1,000 light-years. The area of that sphere is 4πr², or about 12 million square light-years. If you were 2,000 light-years away, the light would be diluted over an area of 4πr² or 4π × (2,000 light-years)²—about 4 × 12 million square light-years. The new sphere is twice as big as the one before and has an area 4 times as great. This means that your detector—let’s say your 200-inch-diameter telescope—will intercept ¼ as much radiation from the star as it would if it were only 1,000 light-years away from the star. If you are twice as far away, the star appears ¼ as bright. Brightness is measured in watts per square meter falling on your detector. Brightness diminishes like one over the square of the distance, a fundamental relationship called, not surprisingly, the inverse-square law.

Shapley could take repeated pictures of globular clusters of stars. A globular star cluster orbiting within the Milky Way would contain over 100,000 stars orbiting about the cluster’s center of mass, like bees around a hive. Stars whose brightness varied from picture to picture could be identified as variable stars. Shapley could measure these stars’ brightnesses as a function of time. He could recognize RR Lyrae variables by their periods of oscillation (the length of time between peaks in brightness, characteristically less than a day) and their amplitude of oscillation (the factor by which their brightness changed from brightest to faintest). Shapley could look at a particular RR Lyrae star and know its intrinsic luminosity. This was invaluable. Knowing its intrinsic luminosity, he could measure its apparent brightness in the sky and calculate its distance. The fainter it was, the farther away it would be. By measuring the apparent brightness of the RR Lyrae variables in a globular cluster, Shapley could measure the distance to the globular cluster itself. For more distant globular clusters, he used the brightness of the brightest stars in the cluster as a distance indicator, and for the most distant globular clusters, he used the clusters’ angular sizes to estimate their distances: a cluster half the angular size was twice as far away.

Shapley measured the distances to many globular clusters, which orbit the center of the Milky Way galaxy in a nearly spherical distribution along paths that take them far above and below the flat “dinner plate” where most stars lie. Looking out above and below the galactic plane allowed him to find globular clusters at great distances, free of the confusing obscuring effects of interstellar dust in the plane itself. Shapley found that the 3D distribution of globular clusters in space was off-center relative to Earth. This result was puzzling: these globular clusters were orbiting the center of the Milky Way and should be centered on it, yet Shapley found more globular clusters (and ones that were further away) on one side of the sky than on the other. The distribution of globular clusters seemed centered on a point in the direction of the constellation of Sagittarius about 25,000 light-years away. This point marked the center of the galaxy. Shapley had shown that we were not at the center of the Milky Way—but rather our solar system was about halfway between the center and the outer edge. This showed that the Sun was not at a special location at the center of the galaxy.

In 1920 Shapley had a famous debate with Heber Curtis about the nature of the spiral nebulae. In the period from 1771 to 1781 Charles Messier had made a catalog of nebulae. Through a small telescope they look like softly blurry patches of light and can be confused with comets. Messier was a comet hunter and wanted to make sure he didn’t mistake these objects for new comets, so he took special note of them and cataloged them. These blurry objects actually include a number of different types of things. Some Messier objects (labeled by an M followed by their number in the catalog) are supernova gas ejecta (like the Crab Nebula M1) and some, like the Dumbbell Nebula (M27), are gas shed during the process of a star collapsing to form a white dwarf. Some are globular clusters (like M13), some are loose star clusters like the Pleiades (M45), many are gas clouds (star-forming regions) in the Milky Way, like the Orion Nebula (M42), and many more are actually external galaxies, like Andromeda (M31), the Pinwheel (M101), the Whirlpool (M57), M81, M87, and so on. The spiral nebulae, such as M31, M57, M81, and M101, were the subjects of the Shapley-Curtis debate. Their spiral shapes made them look somewhat like hurricanes seen from space. They had spiral arms winding outward from the center—like a pinwheel. Sometimes they were seen face-on, where they showed off circular shapes, and sometimes they were seen nearly edge-on, looking like dinner plates seen from the side. Were these gas clouds within the Milky Way or were they external galaxies like our own seen at great distances? Shapley maintained that they were gas clouds within the Milky Way. Curtis maintained they were external galaxies just like our own.

The proposals of famous astronomers and philosophers of the past came into the mix. The ancient Greek philosopher Democritus proposed that the band of light known as the Milky Way could actually be the light of distant stars (right idea—and in about 400 BC!). This idea would be confirmed by Galileo when he turned a telescope to the heavens. In 1750 Thomas Wright speculated that the Milky Way was a thin sheet of stars (right) but thought this was really part of a large, thin spherical shell of stars orbiting a dark center (wrong). Thus from a great distance he thought our galaxy should resemble a sphere of stars, a round blurry blob. Then he proposed that many of the faint nebulae we saw were entire galaxies like our own (right!). In 1755 William Herschel (the discoverer of Uranus) designated a subclass of nebulae he called “spiral nebulae.” That same year the preeminent philosopher of his day, Immanuel Kant, proposed that the spiral nebulae were actually galaxies like our own seen at great distances—he called them “island universes.” Curtis had these ideas on his side.

Shapley spent most of the time defending his recent determination of the enormous size of the Milky Way; he thought this result would make the predicted distances to the spiral nebulae seem ridiculously large if they were to be objects comparable to the Milky Way in size. Some novae (stars that suddenly flare in brightness by a large factor without exploding) were observed in spiral nebulae, and these had brightnesses comparable to other novae in the Milky Way, placing them firmly within our galaxy. Curtis mentioned this point against himself. But in fact, these were supernovae, not novae at all but vastly more luminous stellar explosions that were actually just as far away as Curtis needed. Curtis’s best argument came from noticing that the spectra of the spiral nebulae looked like the spectra of star clusters, not those of gas clouds. The debate ended inconclusively. Most people in the audience probably left with the same views they had when they entered. In science, such questions are not settled by debates or by who scores more oratorical points. They are often settled by new and decisive data—which Hubble would soon be perfectly positioned to supply.

Hubble Changes the Game

Like most people who make important contributions, Hubble was blessed with both talent and luck. Born in Marshfield, Missouri, in 1889, Hubble held the high school high-jump record for the state of Illinois. He attended the University of Illinois and later went to Oxford as a Rhodes Scholar. Rhodes scholarships rewarded athletic as well as academic prowess. When he returned from England, he spent some time in my hometown of Louisville, Kentucky, living for part of that time in a quiet, genteel area of Louisville called the Highlands, where my mother and grandmother once lived. Hubble followed his father’s wishes that he study law, but after his father’s death, he turned to his true interests in science. He was a high school teacher for a while before going to graduate school at the University of Chicago, where he earned his PhD in astronomy; for his thesis research, he took photographs of faint nebulae. Here he had mastered the skill that would be needed to settle the Curtis-Shapley controversy. After a brief period of service in World War I, he returned to get a staff position at Mount Wilson. He was hired by George Ellery Hale. His good fortune was compounded. Yerkes Observatory, where he had done his doctoral work, possessed the largest refracting telescope in the world with a diameter of 40 inches. This was and still remains the largest refracting telescope ever built. It had a lens at the front, which brought light to a focus at the back, where an eyepiece was placed to view the image. Galileo’s first telescope was a refracting telescope whose lens had a diameter of 1.46 inches. With this he was able to resolve stars in the soft band of light called the Milky Way. The Yerkes telescope was...