Origins: Fourteen Billion Years of Cosmic Evolution (9 page)

Not everyone favors the anthropic approach. Some cosmologists attack it as defeatist, ahistorical (since this approach contradicts numerous examples of the success of physics in explaining, sooner or later, a host of once mysterious phenomena), and dangerous, because the anthropic approach smacks of intelligent-design arguments. Furthermore, many cosmologists find unacceptable, as grounds for a theory of the universe, the assumption that we live in a multiverse that contains a multitude of universes with which we can never interact, even in theory.

The debate over the anthropic principle highlights the skepticism that underlies the scientific approach to understanding the cosmos. A theory that appeals to one scientist, typically the one who thought it up, may seem ridiculous, or just plain wrong, to another. Both know that theories survive and thrive when other scientists find them best at explaining most of the observational data. (As a famous scientist once remarked, Beware of a theory that explains
all
the data—some of it will quite likely turn out to be wrong.)

The future may not produce a quick resolution to this debate, but it will surely bring forth other attempts to explain what we see in the universe. For example, Paul Steinhardt of Princeton University, who could use some tutoring in creating catchy names, has produced a theoretical “ekpyrotic model” of the cosmos in collaboration with Neil Turok of Cambridge University. Motivated by the section of particle physics called string theory, Steinhardt envisions a universe with eleven dimensions, most of which are “compactified,” more or less rolled up like a sock, so that they occupy only infinitesimal amounts of space. But some of the additional dimensions have real size and significance, except that we can’t perceive them because we remain locked into our familiar four. If you pretend that all of space in our universe fills an infinite thin sheet (this model reduces the three dimensions of space to two), you can imagine another, parallel sheet, and then picture the two sheets approaching and colliding. The collision produces the big bang, and as the sheets rebound from one another, each sheet’s history proceeds along familiar lines, giving birth to galaxies and stars. Eventually, the two sheets cease to separate and start to approach one another again, producing another collision and another big bang in each sheet. The universe thus has a cyclical history, repeating itself, at least in its broadest outlines, at intervals of hundreds of billions of years. Since “ekpyrosis” means “conflagration” in Greek (recall the more familiar word “pyromaniac”), the “ekpyrotic universe” reminds all those with Greek at the tips of their brains of the great fire that gave birth to the cosmos that we know.

This ekpyrotic model of the universe has emotional and intellectual appeal, though not enough to win the hearts and minds of many of Steinhardt’s fellow cosmologists. Not yet, anyhow. Something vaguely like the ekpyrotic model, if not this model itself, may someday offer the breakthrough that cosmologists now await in their attempts to explain the dark energy. Even those who favor the anthropic approach would hardly dig in their heels to resist a new theory that could provide a good explanation for the cosmological constant without invoking an infinite number of universes, of which ours happens to be one of the lucky ones. As one of R. Crumb’s cartoon characters once said: “What a wonderful, wacky world we live in! Wooey!”

Part II

The Origin of Galaxies and
Cosmic Structure

CHAPTER 7

Discovering Galaxies

T
wo and a half centuries ago, shortly before the English astronomer Sir William Herschel built the world’s first seriously large telescope, the known universe consisted of little more than the stars, the Sun and Moon, the planets, a few moons of Jupiter and Saturn, some fuzzy objects, and the galaxy that forms a milky band across the night sky. Indeed, the word “galaxy” derives from the Greek
galaktos
, or “milk.” The sky also held the fuzzy objects, scientifically named nebulae after the Latin word for clouds—objects of indeterminate shape such as the Crab nebula in the constellation Taurus, and the Andromeda nebula, which appears to live among the stars of the constellation Andromeda.

Herschel’s telescope had a mirror forty-eight inches across, an unprecedented size for 1789, the year it was built. A complex array of trusses to support and point this telescope made it an ungainly instrument, but when he aimed it at the heavens, Herschel could readily see the countless stars that compose the Milky Way. Using his forty-eight-incher, as well as a smaller, more nimble telescope, Herschel and his sister Caroline compiled the first extensive “deep sky” catalogue of northern nebulae. Sir John—Herschel’s son—continued this family tradition, adding to his father’s and aunt’s list of northern objects and, during an extended stay at the Cape of Good Hope at the southern tip of Africa, cataloguing some 1,700 fuzzy objects visible from the Southern Hemisphere. In 1864, Sir John produced a synthesis of the known deep sky objects:
A General Catalogue of Nebulae and
Clusters of Stars,
which included more than five thousand entries.

In spite of that large body of data, nobody at the time knew the true identity of the nebulae, their distances from Earth, or the differences among them. Nevertheless, the 1864 catalogue made it possible to classify the nebulae morphologically—that is, according to their shapes. In the “we call ’em as we see ’em” tradition of baseball umpires (who came into their own just about the time that Herschel’s
General Catalogue
was published), astronomers named the spiral-shaped nebulae “spiral nebulae,” those with a vaguely elliptical shape “elliptical nebulae,” and the various irregularly shaped nebulae—neither spiral nor elliptical—“irregular nebulae.” Finally, they called the nebulae that looked small and round, like a telescopic image of a planet, “planetary nebulae,” an act that has permanently confused newcomers to astronomy.

For most of its history, astronomy has remained plainspoken, using descriptive methods of inquiry that greatly resembled those used in botany. Using their lengthening compendia of stars and fuzzy things, astronomers searched for patterns and sorted objects according to them. Quite a sensible step, too. Most people, beginning in childhood, arrange things according to appearance and shape without even being told to do so. But this approach can carry you only so far. The Herschels always assumed, because many of their fuzzy objects span a patch of about the same size on the night sky, that all the nebulae lay at about the same distance from Earth. So to them it was simply good, evenhanded science to subject all the nebulae to the same rules of sorting.

Trouble is, the assumption that all nebulae lay at similar distances turned out to be badly mistaken. Nature can be elusive, even devious. Some of the nebulae classified by the Herschels are no farther away than the stars, so they are relatively small (if a trillion miles across can be called “relatively small”). Others turned out to be much more distant, so they must be much larger than the fuzzy objects relatively close to us if they are to appear the same size on the sky.

The take-home lesson is that at some point you’ve got to stop fixating on what something looks like and start asking what it is. Fortunately, by the late nineteenth century, advances in science and technology had empowered astronomers to do just that, to move beyond merely classifying the contents of the universe. That shift led to the birth of astrophysics, the useful application of the laws of physics to astronomical situations.

During the same
era when Sir John Herschel published his vast catalogue of nebulae, a new scientific instrument, the spectroscope, joined the search for nebulae. The sole job of a spectroscope is to break light into a rainbow of its component colors. Those colors, and features embedded within them, reveal not only fine details about the chemical composition of the light source but also, because of a phenomenon called the Doppler effect, the motion of the light source toward or away from Earth.

Spectroscopy revealed something remarkable: the spiral nebulae, which happen to predominate outside the swath of the Milky Way, are nearly all moving away from Earth, and at extremely high speeds. In contrast, all the planetary nebulae, as well as most irregular nebulae, are traveling at relatively low speeds—some toward us and some away from us. Had some catastrophic explosion taken place in the center of the Milky Way, booting out only the spiral nebulae? If so, why weren’t any of them falling back? Were we catching the catastrophe at a special time? In spite of advances in photography that brought forth faster emulsions, enabling astronomers to measure the spectra of ever dimmer nebulae, the exodus continued and these questions remained unanswered.

Most advances in astronomy, as in other sciences, have been driven by the introduction of better technology. As the 1920s opened, another key instrument appeared on the scene: the formidable 100-inch Hooker Telescope at the Mount Wilson Observatory near Pasadena, California. In 1923, the American astronomer Edwin P. Hubble used this telescope—the largest in the world at that time—to find a special breed of star, a Cepheid variable star, in the Andromeda nebula. Variable stars of any type vary in brightness according to well-known patterns; Cepheid variables, named for the prototype of the class, a star in the constellation Cepheus, are all extremely luminous and therefore visible over vast distances. Because their brightness varies in recognizable cycles, patience and persistence will yield an increasing number to the careful observer. Hubble had found a few of these Cepheid variables within the Milky Way and estimated their distances; yet, to his astonishment, the Cepheid he found in Andromeda was much dimmer than any of those.

The most likely explanation for this dimness was that the new Cepheid variable, and the Andromeda nebula in which it lives, sits at a distance much greater than those to the Cepheids in the Milky Way. Hubble realized that this placed the Andromeda nebula at so great a distance that it could not possibly lie among the stars in the constellation Andromeda, nor anywhere within the Milky Way—and could not have been kicked out, along with all its spiral sisters, during a catastrophe milk spill.

The implications were breathtaking. Hubble’s discovery showed that spiral nebulae are entire systems of stars in their own right, as huge and as packed with stars as our own Milky Way. In the phrase of the philosopher Immanuel Kant, Hubble had demonstrated that “island universes” by the dozens must lie outside our own star system, for the object in Andromeda merely led the list of well-known spiral nebulae. The Andromeda nebula was, in fact, the Andromeda
galaxy
.

By 1936, enough
island universes had been identified and photographed through the Hooker and other large telescopes that Hubble, too, decided to try his hand at morphology. His analysis of galaxy types rested upon the untested assumption that variations from one shape to another among galaxies signify evolutionary steps from birth to death. In his 1936 book
Realm of the Nebulae,
Hubble classified galaxies by placing the different types along a diagram shaped like a musical tuning fork, whose handle represents the elliptical galaxies, with rounded ellipticals at the far end of the handle and flattened ellipticals near the point where the two tines join. Along one tine lie the ordinary spiral galaxies: those nearest the handle have their spiral arms wound extremely tightly, while those toward the tine’s end have increasingly loosely wound spiral arms. Along the other tine are spiral galaxies whose central region displays a straight “bar,” but are otherwise similar to ordinary spirals.

Hubble imagined that galaxies start their lives as round ellipticals and become flatter and flatter as they continue to take shape, ultimately revealing a spiral structure that slowly unfurls with the passage of time. Brilliant. Beautiful. Even elegant. But just plain wrong. Not only were entire classes of irregular galaxies omitted from this scheme, but astrophysicists would later learn that the oldest stars in every galaxy were about the same age, implying that all the galaxies were born during a single era in the history of the universe.

For three decades (with some research opportunities lost because of World War II), astronomers observed and catalogued galaxies in accordance with Hubble’s tuning-fork diagram as ellipticals, spirals, and barred spirals, with irregulars a minority subset, completely off the chart because of their strange shapes. Of elliptical galaxies one might say, as Ronald Reagan did about California’s redwoods, that when you’ve seen one, you’ve seen them all. Elliptical galaxies resemble one another in possessing neither the spiral-arm patterns that characterize spirals and barred spirals, nor the giant clouds of interstellar gas and dust that give birth to new stars. In these galaxies, star formation ended many billion years ago, leaving behind spherical or ellipsoidal groups of stars. The largest elliptical galaxies, like the largest spirals, each contain many hundred billion stars—perhaps even a trillion or more—and have diameters close to a hundred thousand light-years. With the exception of professional astronomers, no one has ever sighed over the fantastic patterns and complex star formation histories of an elliptical galaxy for the excellent reason that, at least in comparison with spirals, ellipticals have simple shapes and straightforward star formation: they all turned gas and dust into stars until they could do so no more.

Happily, spirals and barred spirals furnish the visual excitement so lacking in ellipticals. The most deeply resonant of all the galaxy images that we may ever see, a view of the entire Milky Way taken from outside it, will stir our hearts and minds, just as soon as we manage to send a camera several hundred thousand light-years above or below the central plane of our galaxy. Today, when our most far-flung space probes have traveled a billionth of that distance, this goal may seem unattainable, and indeed even a probe that could reach nearly the speed of light would require a long wait—far longer than the current span of recorded history—to yield the desired result. For the time being, astronomers must continue to map the Milky Way from inside, sketching the galactic forest by delineating its stellar and nebular trees. These efforts reveal that our galaxy closely resembles our closest large neighbor, the great spiral galaxy in Andromeda. Conveniently located about 2.4 million light-years away, the Andromeda galaxy has provided a wealth of information about the basic structural patterns of spiral galaxies, as well as about different types of stars and their evolution. Because all of the Andromeda galaxy’s stars have the same distance from us (plus or minus a few percent), astronomers know that the stars’ brightnesses correlate directly with their luminosities, that is, with the amounts of energy they emit each second. This fact, denied to astronomers when they study objects in the Milky Way but applicable to every galaxy beyond our own, has allowed them to draw key conclusions about stellar evolution with greater ease than would be true for stars in the Milky Way. Two elliptical satellite galaxies that orbit the Andromeda galaxy, each containing a few percent of the number of stars in the main galaxy, have likewise furnished important information about the lives of stars, and the overall galactic structure of elliptical galaxies. On a clear night far from city lights, a keen-eyed observer who knows where to look can spot the fuzzy outline of the Andromeda galaxy—the most distant object visible to the unaided eye, shining with light that left on its journey while our ancestors roamed the gorges of Africa in search of roots and berries.

Like the Milky Way, the Andromeda galaxy lies midway along one tine of Hubble’s tuning-fork diagram, because its spiral arms are neither particularly tightly nor loosely wound. If galaxies were animals in a zoo, there would be one cage devoted to ellipticals but several animal houses for the glorious spirals. To study a Hubble Telescope image of one of these beasts, typically (for the closer ones) seen from 10 or 20 million light-years, is to enter a world of sight so rich in possibility, so deep in separation from life on Earth, so complex in structure, that the unprepared mind may reel, or may provide a defense by reminding its owner that none of this can thin the thighs or heal the fractured bone.

Irregulars, the orphans of the galactic class system, comprise about 10 percent of all galaxies, with the rest split between spirals and ellipticals, strongly favoring spirals. In contrast to ellipticals, irregular galaxies typically contain a higher proportion of gas and dust than spirals do, and offer the liveliest sites of ongoing star formation. The Milky Way has two large satellite galaxies, both irregular, confusingly named the Magellanic Clouds because the first white men to notice them, sailors on Magellan’s circumnavigation of Earth in 1520, thought at first they were seeing wisps of clouds in the sky. This honor fell to Magellan’s expedition because the Magellanic Clouds lie so close to the south celestial pole (the point directly above Earth’s South Pole) that they never rise above the horizon for observers in the most populated Northern latitudes, including those in Europe and most of the United States. Each of the Magellanic Clouds contains many billion stars, though not the hundreds of billions that characterize the Milky Way and other large galaxies, and display immense star-forming regions, most notably the “Tarantula nebula” of the Large Magellanic Cloud. This galaxy also has the honor of having revealed the closest and brightest supernova to appear during the past three centuries, Supernova 1987A, which must have actually exploded about 160,000
B.C.
for its light to reach Earth in 1987.