Cosmos (39 page)

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.
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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: