Cosmos - Carl Sagan (1980)
Chapter 10. THE EDGE OF FOREVER
There is a way on high, conspicuous in the clear heavens, called the Milky Way, brilliant with its own brightness. By it the gods go to the dwelling of the great Thunderer and his royal abode … Here the famous and mighty inhabitants of heaven have their homes. This is the region which I might make bold to call the Palatine [Way] of the Great Sky.
—Ovid, Metamorphoses (Rome, first century)
Some foolish men declare that a Creator made the world. The doctrine that the world was created is ill-advised, and should be rejected.
If God created the world, where was He before creation?… How could God have made the world without any raw material? If you say He made this first, and then the world, you are faced with an endless regression … Know that the world is uncreated, as time itself is, without beginning and end.
And it is based on the principles …
—The Mahapurana (The Great Legend),
Jinasena (India, ninth century)
Ten or twenty billion years ago, something happened—the Big Bang, the event that began our universe. Why it happened is the greatest mystery we know. That it happened is reasonably clear. All the matter and energy now in the universe was concentrated at extremely high density—a kind of cosmic egg, reminiscent of the creation myths of many cultures—perhaps into a mathematical point with no dimensions at all. It was not that all the matter and energy were squeezed into a minor corner of the present universe; rather, the entire universe, matter and energy and the space they fill, occupied a very small volume. There was not much room for events to happen in.
In that titanic cosmic explosion, the universe began an expansion which has never ceased. It is misleading to describe the expansion of the universe as a sort of distending bubble viewed from the outside. By definition, nothing we can ever know about was outside. It is better to think of it from the inside, perhaps with grid lines—imagined to adhere to the moving fabric of space—expanding uniformly in all directions. As space stretched, the matter and energy in the universe expanded with it and rapidly cooled. The radiation of the cosmic fireball, which, then as now, filled the universe, moved through the spectrum—from gamma rays to X-rays to ultraviolet light; through the rainbow colors of the visible spectrum; into the infrared and radio regions. The remnants of that fireball, the cosmic background radiation, emanating from all parts of the sky can be detected by radio telescopes today. In the early universe, space was brilliantly illuminated. As time passed, the fabric of space continued to expand, the radiation cooled and, in ordinary visible light, for the first time space became dark, as it is today.
The early universe was filled with radiation and a plenum of matter, originally hydrogen and helium, formed from elementary particles in the dense primeval fireball. There was very little to see, if there had been anyone around to do the seeing. Then little pockets of gas, small nonuniformities, began to grow. Tendrils of vast gossamer gas clouds formed, colonies of great lumbering, slowly spinning things, steadily brightening, each a kind of beast eventually to contain a hundred billion shining points. The largest recognizable structures in the universe had formed. We see them today. We ourselves inhabit some lost corner of one. We call them galaxies.
About a billion years after the Big Bang, the distribution of matter in the universe had become a little lumpy, perhaps because the Big Bang itself had not been perfectly uniform. Matter was more densely compacted in these lumps than elsewhere. Their gravity drew to them substantial quantities of nearby gas, growing clouds of hydrogen and helium that were destined to become clusters of galaxies. A very small initial nonuniformity suffices to produce substantial condensations of matter later on.
As the gravitational collapse continued, the primordial galaxies spun increasingly faster, because of the conservation of angular momentum. Some flattened, squashing themselves along the axis of rotation where gravity is not balanced by centrifugal force. These became the first spiral galaxies, great rotating pinwheels of matter in open space. Other protogalaxies with weaker gravity or less initial rotation flattened very little and became the first elliptical galaxies. There are similar galaxies, as if stamped from the same mold, all over the Cosmos because these simple laws of nature—gravity and the conservation of angular momentum—are the same all over the universe. The physics that works for falling bodies and pirouetting ice skaters down here in the microcosm of the Earth makes galaxies up there in the macrocosm of the universe.
Within the nascent galaxies, much smaller clouds were also experiencing gravitational collapse; interior temperatures became very high, thermonuclear reactions were initiated, and the first stars turned on. The hot, massive young stars evolved rapidly, profligates carelessly spending their capital of hydrogen fuel, soon ending their lives in brilliant supernova explosions, returning thermonuclear ash—helium, carbon, oxygen and heavier elements—to the interstellar gas for subsequent generations of star formation. Supernova explosions of massive early stars produced successive overlapping shock waves in the adjacent gas, compressing the intergalactic medium and accelerating the generation of clusters of galaxies. Gravity is opportunistic, amplifying even small condensations of matter. Supernova shock waves may have contributed to accretions of matter at every scale. The epic of cosmic evolution had begun, a hierarchy in the condensation of matter from the gas of the Big Bang—clusters of galaxies, galaxies, stars, planets, and, eventually, life and an intelligence able to understand a little of the elegant process responsible for its origin.
Clusters of galaxies All the universe today. Some are insignificant, paltry collections of a few dozen galaxies. The affectionately titled “Local Group” contains only two large galaxies of any size, both spirals: the Milky Way and M31. Other clusters run to immense hordes of thousands of galaxies in mutual gravitational embrace. There is some hint that the Virgo cluster contains tens of thousands of galaxies.
On the largest scale, we inhabit a universe of galaxies, perhaps a hundred billion exquisite examples of cosmic architecture and decay, with order and disorder equally evident: normal spirals, turned at various angles to our earthly line of sight (face-on we see the spiral arms, edge-on, the central lanes of gas and dust in which the arms are formed); barred spirals with a river of gas and dust and stars running through the center, connecting the spiral arms on opposite sides; stately giant elliptical galaxies containing more than a trillion stars which have grown so large because they have swallowed and merged with other galaxies; a plethora of dwarf ellipticals, the galactic midges, each containing some paltry millions of suns; an immense variety of mysterious irregulars, indications that in the world of galaxies there are places where something has gone ominously wrong; and galaxies orbiting each other so closely that their edges are bent by the gravity of their companions and in some cases streamers of gas and stars are drawn out gravitationally, a bridge between the galaxies.
Some clusters have their galaxies arranged in an unambiguously spherical geometry; they are composed chiefly of ellipticals, often dominated by one giant elliptical, the presumptive galactic cannibal. Other clusters with a far more disordered geometry have, comparatively, many more spirals and irregulars. Galactic collisions distort the shape of an originally spherical cluster and may also contribute to the genesis of spirals and irregulars from ellipticals. The form and abundance of the galaxies have a story to tell us of ancient events on the largest possible scale, a story we are just beginning to read.
The development of high-speed computers makes possible numerical experiments on the collective motion of thousands or tens of thousands of points, each representing a star, each under the gravitational influence of all the other points. In some cases, spiral arms form all by themselves in a galaxy that has already flattened to a disk. Occasionally a spiral arm may be produced by the close gravitational encounter of two galaxies, each of course composed of billions of stars. The gas and dust diffusely spread through such galaxies will collide and become warmed. But when two galaxies collide, the stars pass effortlessly by one another, like bullets through a swarm of bees, because a galaxy is made mostly of nothing and the spaces between the stars are vast. Nevertheless, the configuration of the galaxies can be distorted severely. A direct impact on one galaxy by another can send the constituent stars pouring and careening through intergalactic space, a galaxy wasted. When a small galaxy runs into a larger one face-on it can produce one of the loveliest of the rare irregulars, a ring galaxy thousands of light-years across, set against the velvet of intergalactic space. It is a splash in the galactic pond, a temporary configuration of disrupted stars, a galaxy with a central piece torn out.
The unstructured blobs of irregular galaxies, the arms of spiral galaxies and the torus of ring galaxies exist for only a few frames in the cosmic motion picture, then dissipate, often to be reformed again. Our sense of galaxies as ponderous rigid bodies is mistaken. They are fluid structures with 100 billion stellar components. Just as a human being, a collection of 100 trillion cells, is typically in a steady state between synthesis and decay and is more than the sum of its parts, so also is a galaxy.
The suicide rate among galaxies is high. Some nearby examples, tens or hundreds of millions of light-years away, are powerful sources of X-rays, infrared radiation and radio waves, have extremely luminous cores and fluctuate in brightness on time scales of weeks. Some display jets of radiation, thousand-light-year-long plumes, and disks of dust in substantial disarray. These galaxies are blowing themselves up. Black holes ranging from millions to billions of times more massive than the Sun are suspected in the cores of giant elliptical galaxies such as NGC 6251 and M87. There is something very massive, very dense, and very small ticking and purring inside M87—from a region smaller than the solar system. A black hole is implicated. Billions of light-years away are still more tumultuous objects, the quasars, which may be the colossal explosions of young galaxies, the mightiest events in the history of the universe since the Big Bang itself.
The word “quasar” is an acronym for “quasi-stellar radio source.” After it became clear that not all of them were powerful radio sources, they were called QSO’s (“quasi-stellar objects”). Because they are starlike in appearance, they were naturally thought to be stars within our own galaxy. But spectroscopic observations of their red shift (see below) show them likely to be immense distances away. They seem to partake vigorously in the expansion of the universe, some receding from us at more than 90 percent the speed of light. If they are very far, they must be intrinsically extremely bright to be visible over such distances; some are as bright as a thousand supernovae exploding at once. Just as for Cyg X-1, their rapid fluctuations show their enormous brightness to be confined to a very small volume, in this case less then the size of the solar system. Some remarkable process must be responsible for the vast outpouring of energy in a quasar. Among the proposed explanations are: (1) quasars are monster versions of pulsars, with a rapidly rotating supermassive core connected to a strong magnetic field; (2) quasars are due to multiple collisions of millions of stars densely packed into the galactic core, tearing away the outer layers and exposing to full view the billion-degree temperatures of the interiors of massive stars; (3) a related idea, quasars are galaxies in which the stars are so densely packed that a supernova explosion in one will rip away the outer layers of another and make it a supernova, producing a stellar chain reaction; (4) quasars are powered by the violent mutual annihilation of matter and antimatter, somehow preserved in the quasar until now; (5) a quasar is the energy released when gas and dust and stars fall into an immense black hole in the core of such a galaxy, perhaps itself the product of ages of collision and coalescence of smaller black holes; and (6) quasars are “white holes,” the other side of black holes, a funneling and eventual emergence into view of matter pouring into a multitude of black holes in other parts of the universe, or even in other universes.
In considering the quasars, we confront profound mysteries. Whatever the cause of a quasar explosion, one thing seems clear: such a violent event must produce untold havoc. In every quasar explosion millions of worlds—some with life and the intelligence to understand what is happening—may be utterly destroyed. The study of the galaxies reveals a universal order and beauty. It also shows us chaotic violence on a scale hitherto undreamed of. That we live in a universe which permits life is remarkable. That we live in one which destroys galaxies and stars and worlds is also remarkable. The universe seems neither benign nor hostile, merely indifferent to the concerns of such puny creatures as we.
Even a galaxy so seemingly well-mannered as the Milky Way has its stirrings and its dances. Radio observations show two enormous clouds of hydrogen gas, enough to make millions of suns, plummeting out from the galactic core, as if a mild explosion happened there every now and then. A high-energy astronomical observatory in Earth orbit has found the galactic core to be a strong source of a particular gamma ray spectral line, consistent with the idea that a massive black hole is hidden there. Galaxies like the Milky Way may represent the staid middle age in a continuous evolutionary sequence, which encompasses, in their violent adolescence, quasars and exploding galaxies: because the quasars are so distant, we see them in their youth, as they were billions of years ago.
The stars of the Milky Way move with systematic grace. Globular clusters plunge through the galactic plane and out the other side, where they slow, reverse and hurtle back again. If we could follow the motion of individual stars bobbing about the galactic plane, they would resemble a froth of popcorn. We have never seen a galaxy change its form significantly only because it takes so long to move. The Milky Way rotates once every quarter billion years. If we were to speed the rotation, we would see that the Galaxy is a dynamic, almost organic entity, in some ways resembling a multi-cellular organism. Any astronomical photograph of a galaxy is merely a snapshot of one stage in its ponderous motion and evolution.* The inner region of a galaxy rotates as a solid body. But, beyond that, like the planets around the Sun following Kepler’s third law, the outer provinces rotate progressively more slowly. The arms have a tendency to wind up around the core in an ever-tightening spiral, and gas and dust accumulate in spiral patterns of greater density, which are in turn the locales for the formation of young, hot, bright stars, the stars that outline the spiral arms. These stars shine for ten million years or so, a period corresponding to only 5 percent of a galactic rotation. But as the stars that outline a spiral arm burn out, new stars and their associated nebulae are formed just behind them, and the spiral pattern persists. The stars that outline the arms do not survive even a single galactic rotation; only the spiral pattern remains.
The speed of any given star around the center of the Galaxy is generally not the same as that of the spiral pattern. The Sun has been in and out of spiral arms often in the twenty times it has gone around the Milky Way at 200 kilometers per second (roughly half a million miles per hour). On the average, the Sun and the planets spend forty million years in a spiral arm, eighty million outside, another forty million in, and so on. Spiral arms outline the region where the latest crop of newly hatched stars is being formed, but not necessarily where such middle-aged stars as the Sun happen to be. In this epoch, we live between spiral arms.
The periodic passage of the solar system through spiral arms may conceivably have had important consequences for us. About ten million years ago, the Sun emerged from the Gould Belt complex of the Orion Spiral Arm, which is now a little less than a thousand light-years away. (Interior to the Orion arm is the Sagittarius arm; beyond the Orion arm is the Perseus arm.) When the Sun passes through a spiral arm it is more likely than it is at present to enter into gaseous nebulae and interstellar dust clouds and to encounter objects of substellar mass. It has been suggested that the major ice ages on our planet, which recur every hundred million years or so, may be due to the interposition of interstellar matter between the Sun and the Earth. W. Napier and S. Clube have proposed that a number of the moons, asteroids, comets and circumplanetary rings in the solar system once freely wandered in interstellar space until they were captured as the Sun plunged through the Orion spiral arm. This is an intriguing idea, although perhaps not very likely. But it is testable. All we need do is procure a sample of, say, Phobos or a comet and examine its magnesium isotopes. The relative abundance of magnesium isotopes (all sharing the same number of protons, but having differing numbers of neutrons) depends on the precise sequence of stellar nucleo-synthetic events, including the timing of nearby supernova explosions, that produced any particular sample of magnesium. In a different corner of the Galaxy, a different sequence of events should have occurred and a different ratio of magnesium isotopes should prevail.
The discovery of the Big Bang and the recession of the galaxies came from a commonplace of nature called the Doppler effect. We are used to it in the physics of sound. An automobile driver speeding by us blows his horn. Inside the car, the driver hears a steady blare at a fixed pitch. But outside the car, we hear a characteristic change in pitch. To us, the sound of the horn elides from high frequencies to low. A racing car traveling at 200 kilometers per hour (120 miles per hour) is going almost one-fifth the speed of sound. Sound is a succession of waves in air, a crest and a trough, a crest and a trough. The closer together the waves are, the higher the frequency or pitch; the farther apart the waves are, the lower the pitch. If the car is racing away from us, it stretches out the sound waves, moving them, from our point of view, to a lower pitch and producing the characteristic sound with which we are all familiar. If the car were racing toward us, the sound waves would be squashed together, the frequency would be increased, and we would hear a high-pitched wail. If we knew what the ordinary pitch of the horn was when the car was at rest, we could deduce its speed blindfolded, from the change in pitch.
The Doppler effect. A stationary source of sound or light emits a set of spherical waves. If the source is in motion from right to left, it emits spherical waves progressively centered on points 1 through 6, as shown. But an observer at B sees the waves as stretched out, while an observer at A sees them as compressed. A receding source is seen as red-shifted (the wavelengths made longer); an approaching source is seen as blue-shifted (the wavelengths made shorter). The Doppler effect is the key to cosmology.
Light is also a wave. Unlike sound, it travels perfectly well through a vacuum. The Doppler effect works here as well. If instead of sound the automobile were for some reason emitting, front and back, a beam of pure yellow light, the frequency of the light would increase slightly as the car approached and decrease slightly as the car receded. At ordinary speeds the effect would be imperceptible. If, however, the car were somehow traveling at a good fraction of the speed of light, we would be able to observe the color of the light changing toward higher frequency, that is, toward blue, as the car approached us; and toward lower frequencies, that is, toward red, as the car receded from us. An object approaching us at very high velocities is perceived to have the color of its spectral lines blue-shifted. An object receding from us at very high velocities has its spectral lines red-shifted.* This red shift, observed in the spectral lines of distant galaxies and interpreted as a Doppler effect, is the key to cosmology.
During the early years of this century, the world’s largest telescope, destined to discover the red shift of remote galaxies, was being built on Mount Wilson, overlooking what were then the clear skies of Los Angeles. Large pieces of the telescope had to be hauled to the top of the mountain, a job for mule teams. A young mule skinner named Milton Humason helped to transport mechanical and optical equipment, scientists, engineers and dignitaries up the mountain. Humason would lead the column of mules on horseback, his white terrier standing just behind the saddle, its front paws on Humason’s shoulders. He was a tobacco-chewing roustabout, a superb gambler and pool player and what was then called a ladies’ man. In his formal education, he had never gone beyond the eighth grade. But he was bright and curious and naturally inquisitive about the equipment he had laboriously carted to the heights. Humason was keeping company with the daughter of one of the observatory engineers, a man who harbored reservations about his daughter seeing a young man who had no higher ambition than to be a mule skinner. So Humason took odd jobs at the observatory—electrician’s assistant, janitor, swabbing the floors of the telescope he had helped to build. One evening, so the story goes, the night telescope assistant fell ill and Humason was asked if he might fill in. He displayed such skill and care with the instruments that he soon became a permanent telescope operator and observing aide.
After World War I, there came to Mount Wilson the soon-to-be famous Edwin Hubble—brilliant, polished, gregarious outside the astronomical community, with an English accent acquired during a single year as Rhodes scholar at Oxford. It was Hubble who provided the final demonstration that the spiral nebulae were in fact “island universes,” distant aggregations of enormous numbers of stars, like our own Milky Way Galaxy; he had figured out the stellar standard candle required to measure the distances to the galaxies. Hubble and Humason hit it off splendidly, a perhaps unlikely pair who worked together at the telescope harmoniously. Following a lead by the astronomer V. M. Slipher at Lowell Observatory, they began measuring the spectra of distant galaxies. It soon became clear that Humason was better able to obtain high-quality spectra of distant galaxies than any professional astronomer in the world. He became a full staff member of the Mount Wilson Observatory, learned many of the scientific underpinnings of his work and died rich in the respect of the astronomical community.
The light from a galaxy is the sum of the light emitted by the billions of stars within it. As the light leaves these stars, certain frequencies or colors are absorbed by the atoms in the stars’ outermost layers. The resulting lines permit us to tell that stars millions of light-years away contain the same chemical elements as our Sun and the nearby stars. Humason and Hubble found, to their amazement, that the spectra of all the distant galaxies are red-shifted and, still more startling, that the more distant the galaxy was, the more red-shifted were its spectral lines.
The most obvious explanation of the red shift was in terms of the Doppler effect: the galaxies were receding from us; the more distant the galaxy the greater its speed of recession. But why should the galaxies be fleeing us? Could there be something special about our location in the universe, as if the Milky Way had performed some inadvertent but offensive act in the social life of galaxies? It seemed much more likely that the universe itself was expanding, carrying the galaxies with it. Humason and Hubble, it gradually became clear, had discovered the Big Bang—if not the origin of the universe then at least its most recent incarnation.
Almost all of modern cosmology—and especially the idea of an expanding universe and a Big Bang—is based on the idea that the red shift of distant galaxies is a Doppler effect and arises from their speed of recession. But there are other kinds of red shifts in nature. There is, for example, the gravitational red shift, in which the light leaving an intense gravitational field has to do so much work to escape that it loses energy during the journey, the process perceived by a distant observer as a shift of the escaping light to longer wavelengths and redder colors. Since we think there may be massive black holes at the centers of some galaxies, this is a conceivable explanation of their red shifts. However, the particular spectral lines observed are often characteristic of very thin, diffuse gas, and not the astonishingly high density that must prevail near black holes. Or the red shift might be a Doppler effect due not to the general expansion of the universe but rather to a more modest and local galactic explosion. But then we should expect as many explosion fragments traveling toward us as away from us, as many blue shifts as red shifts. What we actually see, however, is almost exclusively red shifts no matter what distant objects beyond the Local Group we point our telescopes to.
There is nevertheless a nagging suspicion among some astronomers that all may not be right with the deduction, from the red shifts of galaxies via the Doppler effect, that the universe is expanding. The astronomer Halton Arp has found enigmatic and disturbing cases where a galaxy and a quasar, or a pair of galaxies, that are in apparent physical association have very different red shifts. Occasionally there seems to be a bridge of gas and dust and stars connecting them. If the red shift is due to the expansion of the universe, very different red shifts imply very different distances. But two galaxies that are physically connected can hardly also be greatly separated from each other—in some cases by a billion light-years. Skeptics say that the association is purely statistical: that, for example, a nearby bright galaxy and a much more distant quasar, each having very different red shifts and very different speeds of recession, are merely accidentally aligned along the line of sight; that they have no real physical association. Such statistical alignments must happen by chance every now and then. The debate centers on whether the number of coincidences is more than would be expected by chance. Arp points to other cases in which a galaxy with a small red shift is flanked by two quasars of large and almost identical red shift. He believes the quasars are not at cosmological distances but instead are being ejected, left and right, by the “foreground” galaxy; and that the red shifts are the result of some as-yet-unfathomed mechanism. Skeptics argue coincidental alignment and the conventional Hubble-Humason interpretation of the red shift. If Arp is right, the exotic mechanisms proposed to explain the energy source of distant quasars—supernova chain reactions, supermassive black holes and the like—would prove unnecessary. Quasars need not then be very distant. But some other exotic mechanism will be required to explain the red shift. In either case, something very strange is going on in the depths of space.
The apparent recession of the galaxies, with the red shift interpreted through the Doppler effect, is not the only evidence for the Big Bang. Independent and quite persuasive evidence derives from the cosmic black body background radiation, the faint static of radio waves coming quite uniformly from all directions in the Cosmos at just the intensity expected in our epoch from the now substantially cooled radiation of the Big Bang. But here also there is something puzzling. Observations with a sensitive radio antenna carried near the top of the Earth’s atmosphere in a U-2 aircraft have shown that the background radiation is, to first approximation, just as intense in all directions—as if the fireball of the Big Bang expanded quite uniformly, an origin of the universe with a very precise symmetry. But the background radiation, when examined to finer precision, proves to be imperfectly symmetrical. There is a small systematic effect that could be understood if the entire Milky Way Galaxy (and presumably other members of the Local Group) were streaking toward the Virgo cluster of galaxies at more than a million miles an hour (600 kilometers per second). At such a rate, we will reach it in ten billion years, and extra-galactic astronomy will then be a great deal easier. The Virgo cluster is already the richest collection of galaxies known, replete with spirals and ellipticals and irregulars, a jewel box in the sky. But why should we be rushing toward it? George Smoot and his colleagues, who made these high-altitude observations, suggest that the Milky Way is being gravitationally dragged toward the center of the Virgo cluster; that the cluster has many more galaxies than have been detected heretofore; and, most startling, that the cluster is of immense proportions, stretching across one or two billion light-years of space.
The observable universe itself is only a few tens of billions of light-years across and, if there is a vast supercluster in the Virgo group, perhaps there are other such superclusters at much greater distances, which are correspondingly more difficult to detect. In the lifetime of the universe there has apparently not been enough time for an initial gravitational nonuniformity to collect the amount of mass that seems to reside in the Virgo supercluster. Thus Smoot is tempted to conclude that the Big Bang was much less uniform than his other observations suggest, that the original distribution of matter in the universe was very lumpy. (Some little lumpiness is to be expected, and indeed even needed to understand the condensation of galaxies; but a lumpiness on this scale is a surprise.) Perhaps the paradox can be resolved by imagining two or more nearly simultaneous Big Bangs.
If the general picture of an expanding universe and a Big Bang is correct, we must then confront still more difficult questions. What were conditions like at the time of the Big Bang? What happened before that? Was there a tiny universe, devoid of all matter, and then the matter suddenly created from nothing? How does that happen? In many cultures it is customary to answer that God created the universe out of nothing. But this is mere temporizing. If we wish courageously to pursue the question, we must of course ask next where God comes from. And if we decide this to be unanswerable, why not save a step and decide that the origin of the universe is an unanswerable question. Or, if we say that God has always existed, why not save a step and conclude that the universe has always existed?
Every culture has a myth of the world before creation, and of the creation of the world, often by the mating of the gods or the hatching of a cosmic egg. Commonly, the universe is naively imagined to follow human or animal precedent. Here, for example, are five small extracts from such myths, at different levels of sophistication, from the Pacific Basin:
In the very beginning everything was resting in perpetual darkness: night ppressed everything like an impenetrable thicket.
—The Great Father myth of
the Aranda people of
Central Australia
All was in suspense, all calm, all in silence; all motionless and still; and the expanse of the sky was empty.
—The Popol Vuh of the
Quiché Maya
Na Arean sat alone in space as a cloud that floats in nothingness. He slept not, for there was no sleep; he hungered not, for as yet there was no hunger. So he remained for a great while, until a thought came to his mind. He said to himself, “I will make a thing.”
—A myth from Maiana,
Gilbert Islands
First there was the great cosmic egg. Inside the egg was chaos, and floating in chaos was P’an Ku, the Undeveloped, the divine Embryo. And P’an Ku burst out of the egg, four times larger than any man today, with a hammer and chisel in his hand with which he fashioned the world.
—The P’an Ku myths, China
(around third century)
Before heaven and earth had taken form all was vague and amorphous … That which was clear and light drifted up to become heaven, while that which was heavy and turbid solidified to become earth. It was very easy for the pure, fine material to come together, but extremely difficult for the heavy, turbid material to solidify. Therefore heaven was completed first and earth assumed shape after. When heaven and earth were joined in emptiness and all was unwrought simplicity, then without having been created things came into being. This was the Great Oneness. All things issued from this Oneness but all became different …
—Huai-nan Tzu, China
(around first century B.C.)
These myths are tributes to human audacity. The chief difference between them and our modern scientific myth of the Big Bang is that science is self-questioning, and that we can perform experiments and observations to test our ideas. But those other creation stories are worthy of our deep respect.
Every human culture rejoices in the fact that there are cycles in nature. But how, it was thought, could such cycles come about unless the gods willed them? And if there are cycles in the years of humans, might there not be cycles in the aeons of the gods? The Hindu religion is the only one of the world’s great faiths dedicated to the idea that the Cosmos itself undergoes an immense, indeed an infinite, number of deaths and rebirths. It is the only religion in which the time scales correspond, no doubt by accident, to those of modern scientific cosmology. Its cycles run from our ordinary day and night to a day and night of Brahma, 8.64 billion years long, longer than the age of the Earth or the Sun and about half the time since the Big Bang. And there are much longer time scales still.
There is the deep and appealing notion that the universe is but the dream of the god who, after a hundred Brahma years, dissolves himself into a dreamless sleep. The universe dissolves with him—until, after another Brahma century, he stirs, recomposes himself and begins again to dream the great cosmic dream. Meanwhile, elsewhere, there are an infinite number of other universes, each with its own god dreaming the cosmic dream. These great ideas are tempered by another, perhaps still greater. It is said that men may not be the dreams of the gods, but rather that the gods are the dreams of men.
In India there are many gods, and each god has many manifestations. The Chola bronzes, cast in the eleventh century, include several different incarnations of the god Shiva. The most elegant and sublime of these is a representation of the creation of the universe at the beginning of each cosmic cycle, a motif known as the cosmic dance of Shiva. The god, called in this manifestation Nataraja, the Dance King, has four hands. In the upper right hand is a drum whose sound is the sound of creation. In the upper left hand is a tongue of flame, a reminder that the universe, now newly created, will billions of years from now be utterly destroyed.
These profound and lovely images are, I like to imagine, a kind of premonition of modern astronomical ideas.* Very likely, the universe has been expanding since the Big Bang, but it is by no means clear that it will continue to expand forever. The expansion may gradually slow, stop and reverse itself. If there is less than a certain critical amount of matter in the universe, the gravitation of the receding galaxies will be insufficient to stop the expansion, and the universe will run away forever. But if there is more matter than we can see—hidden away in black holes, say, or in hot but invisible gas between the galaxies—then the universe will hold together gravitationally and partake of a very Indian succession of cycles, expansion followed by contraction, universe upon universe, Cosmos without end. If we live in such an oscillating universe, then the Big Bang is not the creation of the Cosmos but merely the end of the previous cycle, the destruction of the last incarnation of the Cosmos.
Neither of these modern cosmologies may be altogether to our liking. In one, the universe is created, somehow, ten or twenty billion years ago and expands forever, the galaxies mutually receding until the last one disappears over our cosmic horizon. Then the galactic astronomers are out of business, the stars cool and die, matter itself decays and the universe becomes a thin cold haze of elementary particles. In the other, the oscillating universe, the Cosmos has no beginning and no end, and we are in the midst of an infinite cycle of cosmic deaths and rebirths with no information trickling through the cusps of the oscillation. Nothing of the galaxies, stars, planets, life forms or civilizations evolved in the previous incarnation of the universe oozes into the cusp, flutters past the Big Bang, to be known in our present universe. The fate of the universe in either cosmology may seem a little depressing, but we may take solace in the time scales involved. These events will occupy tens of billions of years, or more. Human beings and our descendants, whoever they might be, can accomplish a great deal in tens of billions of years, before the Cosmos dies.
If the universe truly oscillates, still stranger questions arise. Some scientists think that when expansion is followed by contraction, when the spectra of distant galaxies are all blue-shifted, causality will be inverted and effects will precede causes. First the ripples spread from a point on the water’s surface, then I throw a stone into the pond. First the torch bursts into flame and then I light it. We cannot pretend to understand what such causality inversion means. Will people at such a time be born in the grave and die in the womb? Will time flow backwards? Do these questions have any meaning?
Scientists wonder about what happens in an oscillating universe at the cusps, at the transition from contraction to expansion. Some think that the laws of nature are then randomly reshuffled, that the kind of physics and chemistry that orders this universe represent only one of an infinite range of possible natural laws. It is easy to see that only a very restricted range of laws of nature are consistent with galaxies and stars, planets, life and intelligence. If the laws of nature are unpredictably reassorted at the cusps, then it is only by the most extraordinary coincidence that the cosmic slot machine has this time come up with a universe consistent with us.*
Do we live in a universe that expands forever or in one in which there is an infinite set of cycles? There are ways to find out: by making an accurate census of the total amount of matter in the universe, or by seeing to the edge of the Cosmos.
Radio telescopes can detect very faint, very distant objects. As we look deep into space we also look far back into time. The nearest quasar is perhaps half a billion light-years away. The farthest may be ten or twelve or more billions. But if we see an object twelve billion light-years away, we are seeing it as it was twelve billion years ago in time. By looking far out into space we are also looking far back into time, back toward the horizon of the universe, back toward the epoch of the Big Bang.
The Very Large Array (VLA) is a collection of twenty-seven separate radio telescopes in a remote region of New Mexico. It is a phased array, the individual telescopes electronically connected, as if it were a single telescope of the same size as its remotest elements, as if it were a radio telescope tens of kilometers across. The VLA is able to resolve or discriminate fine detail in the radio regions of the spectrum comparable to what the largest ground-based telescopes can do in the optical region of the spectrum.
Sometimes such radio telescopes are connected with telescopes on the other side of the Earth, forming a baseline comparable to the Earth’s diameter—in a certain sense, a telescope as large as the planet. In the future we may have telescopes in the Earth’s orbit, around toward the other side of the Sun, in effect a radio telescope as large as the inner solar system. Such telescopes may reveal the internal structure and nature of quasars. Perhaps a quasar standard candle will be found, and the distances to the quasars determined independent of their red shifts. By understanding the structure and the red shift of the most distant quasars it may be possible to see whether the expansion of the universe was faster billions of years ago, whether the expansion is slowing down, whether the universe will one day collapse.
Modern radio telescopes are exquisitely sensitive; a distant quasar is so faint that its detected radiation amounts perhaps to a quadrillionth of a watt. The total amount of energy from outside the solar system ever received by all the radio telescopes on the planet Earth is less than the energy of a single snowflake striking the ground. In detecting the cosmic background radiation, in counting quasars, in searching for intelligent signals from space, radio astronomers are dealing with amounts of energy that are barely there at all.
Some matter, particularly the matter in the stars, glows in visible light and is easy to see. Other matter, gas and dust in the outskirts of galaxies, for example, is not so readily detected. It does not give off visible light, although it seems to give off radio waves. This is one reason that the unlocking of the cosmological mysteries requires us to use exotic instruments and frequencies different from the visible light to which our eyes are sensitive. Observatories in Earth orbit have found an intense X-ray glow between the galaxies. It was first thought to be hot intergalactic hydrogen, an immense amount of it never before seen, perhaps enough to close the Cosmos and to guarantee that we are trapped in an oscillating universe. But more recent observations by Ricardo Giacconi may have resolved the X-ray glow into individual points, perhaps an immense horde of distant quasars. They contribute previously unknown mass to the universe as well. When the cosmic inventory is completed, and the mass of all the galaxies, quasars, black holes, intergalactic hydrogen, gravitational waves and still more exotic denizens of space is summed up, we will know what kind of universe we inhabit.
In discussing the large-scale structure of the Cosmos, astronomers are fond of saying that space is curved, or that there is no center to the Cosmos, or that the universe is finite but unbounded. Whatever are they talking about? Let us imagine we inhabit a strange country where everyone is perfectly flat. Following Edwin Abbott, a Shakespearean scholar who lived in Victorian England, we call it Flatland. Some of us are squares; some are triangles; some have more complex shapes. We scurry about, in and out of our flat buildings, occupied with our flat businesses and dalliances. Everyone in Flatland has width and length, but no height whatever. We know about left-right and forward-back, but have no hint, not a trace of comprehension, about up-down—except for flat mathematicians. They say, “Listen, it’s really very easy. Imagine left-right. Imagine forward-back. Okay, so far? Now imagine another dimension, at right angles to the other two.” And we say, “What are you talking about? ‘At right angles to the other two’! There are only two dimensions. Point to that third dimension. Where is it?” So the mathematicians, disheartened, amble off. Nobody listens to mathematicians.
Every square creature in Flatland sees another square as merely a short line segment, the side of the square nearest to him. He can see the other side of the square only by taking a short walk. But the inside of a square is forever mysterious, unless some terrible accident or autopsy breaches the sides and exposes the interior parts.
One day a three-dimensional creature—shaped like an apple, say—comes upon Flatland, hovering above it. Observing a particularly attractive and congenial-looking square entering its flat house, the apple decides, in a gesture of interdimensional amity, to say hello. “How are you?” asks the visitor from the third dimension. “I am a visitor from the third dimension.” The wretched square looks about his closed house and sees no one. What is worse, to him it appears that the greeting, entering from above, is emanating from his own flat body, a voice from within. A little insanity, he perhaps reminds himself gamely, runs in the family.
Exasperated at being judged a psychological aberration, the apple descends into Flatland. Now a three-dimensional creature can exist, in Flatland, only partially; only a cross section can be seen, only the points of contact with the plane surface of Flatland. An apple slithering through Flatland would appear first as a point and then as progressively larger, roughly circular slices. The square sees a point appearing in a closed room in his two-dimensional world and slowly growing into a near circle. A creature of strange and changing shape has appeared from nowhere.
Rebuffed, unhappy at the obtuseness of the very flat, the apple bumps the square and sends him aloft, fluttering and spinning into that mysterious third dimension. At first the square can make no sense of what is happening; it is utterly outside his experience. But eventually he realizes that he is viewing Flatland from a peculiar vantage point: “above.” He can see into closed rooms. He can see into his flat fellows. He is viewing his universe from a unique and devastating perspective. Traveling through another dimension provides, as an incidental benefit, a kind of X-ray vision. Eventually, like a falling leaf, our square slowly descends to the surface. From the point of view of his fellow Flatlanders, he has unaccountably disappeared from a closed room and then distressingly materialized from nowhere. “For heaven’s sake,” they say, “what’s happened to you?” “I think,” he finds himself replying, “I was ‘up.’ ” They pat him on his sides and comfort him. Delusions always ran in his family.
In such interdimensional contemplations, we need not be restricted to two dimensions. We can, following Abbott, imagine a world of one dimension, where everyone is a line segment, or even the magical world of zero-dimensional beasts, the points. But perhaps more interesting is the question of higher dimensions. Could there be a fourth physical dimension?*
We can imagine generating a cube in the following way: Take a line segment of a certain length and move it an equal length at right angles to itself. That makes a square. Move the square an equal length at right angles to itself, and we have a cube. We understand this cube to cast a shadow, which we usually draw as two squares with their vertices connected. If we examine the shadow of a cube in two dimensions, we notice that not all the lines appear equal, and not all the angles are right angles. The three-dimensional object has not been perfectly represented in its transfiguration into two dimensions. This is the cost of losing a dimension in the geometrical projection. Now let us take our three-dimensional cube and carry it, at right angles to itself, through a fourth physical dimension: not left-right, not forward-back, not up-down, but simultaneously at right angles to all those directions. I cannot show you what direction that is, but I can imagine it to exist. In such a case, we would have generated a four-dimensional hypercube, also called a tesseract. I cannot show you a tesseract, because we are trapped in three dimensions. But what I can show you is the shadow in three dimensions of a tesseract. It resembles two nested cubes, all the vertices connected by lines. But for a real tesseract, in four dimensions, all the lines would be of equal length and all the angles would be right angles.
Imagine a universe just like Flatland, except that unbeknownst to the inhabitants, their two-dimensional universe is curved through a third physical dimension. When the Flatlanders take short excursions, their universe looks flat enough. But if one of them takes a long enough walk along what seems to be a perfectly straight line, he uncovers a great mystery: although he has not reached a barrier and has never turned around, he has somehow come back to the place from which he started. His two-dimensional universe must have been warped, bent or curved through a mysterious third dimension. He cannot imagine that third dimension, but he can deduce it. Increase all dimensions in this story by one, and you have a situation that may apply to us.
Where is the center of the Cosmos? Is there an edge to the universe? What lies beyond that? In a two-dimensional universe, curved through a third dimension, there is no center—at least not on the surface of the sphere. The center of such a universe is not in that universe; it lies, inaccessible, in the third dimension, inside the sphere. While there is only so much area on the surface of the sphere, there is no edge to this universe—it is finite but unbounded. And the question of what lies beyond is meaningless. Flat creatures cannot, on their own, escape their two dimensions.
Increase all dimensions by one, and you have the situation that may apply to us: the universe as a four-dimensional hypersphere with no center and no edge, and nothing beyond. Why do all the galaxies seem to be running away from us? The hypersphere is expanding from a point, like a four-dimensional balloon being inflated, creating in every instant more space in the universe. Sometime after the expansion begins, galaxies condense and are carried outward on the surface of the hypersphere. There are astronomers in each galaxy, and the light they see is also trapped on the curved surface of the hypersphere. As the sphere expands, an astronomer in any galaxy will think all the other galaxies are running away from him. There are no privileged reference frames.* The farther away the galaxy, the faster its recession. The galaxies are embedded in, attached to space, and the fabric of space is expanding. And to the question, Where in the present universe did the Big Bang occur? the answer is clearly, everywhere.
If there is insufficient matter to prevent the universe from expanding forever, it must have an open shape, curved like a saddle with a surface extending to infinity in our three-dimensional analogy. If there is enough matter, then it has a closed shape, curved like a sphere in our three-dimensional analogy. If the universe is closed, light is trapped within it. In the 1920’s, in a direction opposite to M31, observers found a distant pair of spiral galaxies. Was it possible, they wondered, that they were seeing the Milky Way and M31 from the other direction—like seeing the back of your head with light that has circumnavigated the universe? We now know that the universe is much larger than they imagined in the 1920’s. It would take more than the age of the universe for light to circumnavigate it. And the galaxies are younger than the universe. But if the Cosmos is closed and light cannot escape from it, then it may be perfectly correct to describe the universe as a black hole. If you wish to know what it is like inside a black hole, look around you.
We have previously mentioned the possibility of wormholes to get from one place in the universe to another without covering the intervening distance—through a black hole. We can imagine these wormholes as tubes running through a fourth physical dimension. We do not know that such wormholes exist. But if they do, must they always hook up with another place in our universe? Or is it just possible that wormholes connect with other universes, places that would otherwise be forever inaccessible to us? For all we know, there may be many other universes. Perhaps they are, in some sense, nested within one another.
There is an idea—strange, haunting, evocative—one of the most exquisite conjectures in science or religion. It is entirely undemonstrated; it may never be proved. But it stirs the blood. There is, we are told, an infinite hierarchy of universes, so that an elementary particle, such as an electron, in our universe would, if penetrated, reveal itself to be an entire closed universe. Within it, organized into the local equivalent of galaxies and smaller structures, are an immense number of other, much tinier elementary particles, which are themselves universes at the next level and so on forever—an infinite downward regression, universes within universes, endlessly. And upward as well. Our familiar universe of galaxies and stars, planets and people, would be a single elementary particle in the next universe up, the first step of another infinite regress.
This is the only religious idea I know that surpasses the endless number of infinitely old cycling universes in Hindu cosmology. What would those other universes be like? Would they be built on different laws of physics? Would they have stars and galaxies and worlds, or something quite different? Might they be compatible with some unimaginably different form of life? To enter them, we would somehow have to penetrate a fourth physical dimension—not an easy undertaking, surely, but perhaps a black hole would provide a way. There may be small black holes in the solar neighborhood. Poised at the edge of forever, we would jump off …
*This is not quite true. The near side of a galaxy is tens of thousands of light-years closer to us than the far side; thus we see the front as it was tens of thousands of years before the back. But typical events in galactic dynamics occupy tens of millions of years, so the error in thinking of an image of a galaxy as frozen in one moment of time is small.
*The object itself might be any color, even blue. The red shift means only that each spectral line appears at longer wavelengths than when the object is at rest; the amount of the red shift is proportional both to the velocity and to the wavelength of the spectral line when the object is at rest.
*The dates on Mayan inscriptions also range deep into the past and occasionally far into the future. One inscription refers to a time more than a million years ago and another perhaps refers to events of 400 million years ago, although this is in some dispute among Mayan scholars. The events memorialized may be mythical, but the time scales are prodigious. A millennium before Europeans were willing to divest themselves of the Biblical idea that the world was a few thousand years old, the Mayans were thinking of millions, and the Indians of billions.
*The laws of nature cannot be randomly reshuffled at the cusps. If the universe has already gone through many oscillations, many possible laws of gravity would have been so weak that, for any given initial expansion, the universe would not have held together. Once the universe stumbles upon such a gravitational law, it flies apart and has no further opportunity to experience another oscillation and another cusp and another set of laws of nature. Thus we can deduce from the fact that the universe exists either a finite age, or a severe restriction on the kinds of laws of nature permitted in each oscillation. If the laws of physics are not randomly reshuffled at the cusps, there must be a regularity, a set of rules, that determines which laws are permissible and which are not. Such a set of rules would comprise a new physics standing over the existing physics. Our language is impoverished; there seems to be no suitable name for such a new physics. Both “paraphysics” and “metaphysics” have been preempted by other rather different and, quite possibly, wholly irrelevant activities. Perhaps “transphysics” would do.
*If a fourth-dimensional creature existed it could, in our three-dimensional universe, appear and dematerialize at will, change shape remarkably, pluck us out of locked rooms and make us appear from nowhere. It could also turn us inside out. There are several ways in which we can be turned inside out: the least pleasant would result in our viscera and internal organs being on the outside and the entire Cosmos—glowing intergalactic gas, galaxies, planets, everything—on the inside. I am not sure I like the idea.
*The view that the universe looks by and large the same no matter from where we happen to view it was first proposed, so far as we know, by Giordano Bruno.