This, the epoch of “let there be light,” has a significant effect on the structure of matter. Electrons, relieved from constant harassment by the photons, are now free to settle into orbit around nuclei, forming hydrogen and helium atoms. With atoms on hand, chemistry can proceed, to lead, eons hence, to the formation of alcohol and formaldehyde in interstellar clouds and the building of biotic molecules in the oceans of the early earth.
The ambient temperature of the universe rises rapidly as we continue up the stairway. It was less than 3 degrees above absolute zero on the bottom step, reached room temperature by the third step, and by the sixth step has risen to 10,000 degrees Kelvin—hotter than the surface of the sun. By the eleventh step, at which point the universe is a little under one month old, the temperature everywhere surpasses that of the center of the sun, and at the fifteenth step (five minutes ABT) it is fully a billion degrees Kelvin.
Energetic as this may be, the universe at the age of five minutes has already become cool enough for nucleons to stick together to make permanent atomic nuclei. We watch as protons and neutrons adhere to make nuclei of deuterium (a form of hydrogen) and deuterium nuclei pair off to form the nuclei of helium (two protons and two neutrons). In this fashion, one quarter of all the matter in the universe is rapidly combined into helium nuclei—along with traces of deuterium, helium-3 (two protons, one neutron), and lithium. The whole process is over in three minutes twenty seconds.
Above this point—prior to about one minute forty seconds ABT—there are no stable atomic nuclei. The ambient energy level exceeds the nuclear binding energy. Consequently, any nuclei that form are quickly torn apart again.
Between the seventeenth and eighteenth steps, at about one second ABT, we encounter the epoch of neutrino decoupling. Though the universe at this time is denser than rock (and as hot as the explosion of a hydrogen bomb) it has already begun to look vacuous to the neutrinos. Since neutrinos react only to the weak force, which is extremely short in range, they now find that they can escape its clutches and fly along indefinitely without experiencing any significant further interaction. Thus emancipated, they are free hereafter to roam the universe in their aloof way, flying through most matter as if it weren’t there. (Ten million trillion neutrinos will speed harmlessly through your brain and body in the time it takes to read this sentence. By the time you have read this sentence, they will be farther away than the moon.) The flood of neutrinos released at one second ABT therefore persists ever after, forming a cosmic neutrino background radiation comparable to the microwave background radiation produced by the decoupling of the photons. If these “cosmic” neutrinos (as they are called, to differentiate them from neutrinos released later on by supernovae) could be observed by a neutrino telescope of some sort, they would provide a direct view of the universe when it was only one second old.
As we climb on, the universe continues to become hotter and denser, and the level of structure that can exist becomes ever more rudimentary. There are of course no molecules or atoms or atomic nuclei at this early time, and by about the twenty-second step, some 10-6 (0.000001) second ABT, there are no protons or neutrons, either. The universe is an ocean of free quarks and other elementary particles.
If we take the trouble to count we will find that for every billion antiquarks there are a billion and one quarks. This asymmetry is important: The few excess quarks destined to survive the general quark-antiquark annihilation will form all the atoms of matter in the latter-day universe. The origin of the inequity is unknown; presumably it involved the breaking of a matter-antimatter symmetry at some earlier stage.
We are approaching a time when the basic structures of natural law, and not only those of the particles and fields whose behavior they dictate, were altered as the universe evolved. The first such transition comes at the twenty-seventh step, 10−11 second ABT, when the functions of the weak and electromagnetic forces are found to be handled by a single force, the electroweak. There is now enough ambient energy available to support the creation and maintenance of large numbers of W and Z bosons. These particles—the same kind the conjuring up of which in the CERN accelerator verified the electroweak theory—mediate electromagnetic and weak force interactions interchangeably, making the two forces indistinguishable. Prior to the twenty-seventh step the universe is ruled by only three forces—gravity, and the strong nuclear and electroweak interactions.
The next two dozen or so steps of our ascent are clouded in mystery. Some say that they traverse a “desert,” a bleak stretch of time in which little of importance occurred. But it remains to be seen, given further accelerator experiments and the development of more sophisticated theories, whether the desert will prove to have bloomed.
According to the “inflationary universe” theory (about which more in the next chapter) there may here have been a brief period, upward of the fortieth step, during which the universe expanded much more rapidly than it did thereafter. During this inflationary epoch the universe would have been empty, all its latent matter and energy swallowed up by the rapidly expanding vacuum. There would be nothing to write home about (no material structure at all!) other than the vacuum itself, its unfolding fields pregnant with potential but devoid of tangible objects.
Prior to the start of the inflationary epoch—at about the fifty-first step, only 10−35 second ABT—we enter a realm in which cosmic conditions are even less well understood. If the grand unified theories are correct, there here occurred a symmetry-breaking event in which the unified electronuclear force split into the electroweak and strong forces. If supersymmetry theory is correct, the transition may have come earlier, and would have involved gravitation. Writing a fully unified theory amounts to trying to understand what went on at this early time, when the symmetry thought originally to have characterized the universe shattered into the broken symmetries we find around us today.
But until we have such a theory, we cannot expect to understand what went on in the infant universe. We approach the limits of our present conjecture at the sixtieth step, when the age of the universe is but 10−43 second. Here we encounter a locked door. On the other side lies the Planck epoch, a time when the gravitational attraction exerted by each particle was comparable in strength to the strong nuclear force.* The theoretical key that could open the door would be a unified theory that includes gravitation. The person who arrives at that theory will gaze deepest into the dawn of time. What will he or she see?
One possibility, of course, is that there will be more doors. This prospect has been raised by several researchers, among them Michael Turner, an American cosmologist working on early-universe theory at Fermilab and the University of Chicago. “I suspect that we may always find ourselves in this position—that to go the next tiny fraction of a second we will need some further knowledge that we won’t yet have,” Turner suggested in a 1985 interview. “If so, it may be a very long time, if ever, before we can answer the question that everyone would like to know—the question of what caused creation.”4 Another possibility is that we will find the answer, behind the Planck door or the one after that. The conviction that such an outcome is possible was expressed this way by the American physicist John Archibald Wheeler:
To my mind there must be, at the bottom of it all, not an equation, but an utterly simple idea. And to me that idea, when we finally discover it, will be so compelling, so inevitable, that we will say to one another, “Oh, how beautiful. How could it have been otherwise?”5
Suppose that a unified theory is written—a year from now or a century from now—that succeeds in delivering up just such a transcendental vision of perfection. How could we be sure that we could trust it? As Kepler realized after wasting years on his spherical universe of Platonic solids, one needs not only elegance from a theory but the verdict of experimental or observational test as well. A fully unified theory would in all likelihood purport to describe the universe as it was at less than 10−43 second ABT, when the ambient energy level was more than 1019 GeV. To re-crea
te such conditions would require an accelerator far beyond the reach of any foreseeable technology. Experimental verification of such a theory might remain forever out of reach.
The big bang itself, however, can be regarded as a gigantic accelerator experiment, and the universe we live in as its result. Viewed in this way, our microwave radiotelescopes are like Carlo Rubbia’s detectors at CERN, inasmuch as the particles they intercept were hurled off by the first (and still the greatest) experimental run of all time. A proper unified theory ought to specify just how that run turned out, by predicting the existence of all the particles in the present-day universe. Some of these, presumably, would not yet have been detected: One could then test the theory by searching for such “relic” particles in the here and now. Supersymmetry theory, as we saw in the previous chapter, predicts the existence of enormous numbers of as yet undetected particles left over from the early universe.* If the theory ripened to the point that it could specify the masses of these particles, it might be possible to test it by looking for them.
A ghostly clue that there may be such undetected material in the universe today is proffered by what astronomers call the “dark matter” problem. The masses of galaxies and their clusters can be deduced by measuring the velocity at which stars orbit the centers of the galaxies to which they belong, and at which galaxies orbit the centers of clusters of galaxies.† In case after case, this turns out to add up to something like five or ten times the mass of all the visible stars and nebulae. The startling implication is that everything we see and photograph in the sky amounts to only a fraction of the gravitationally interacting matter in our quarter of the universe. The unseen matter might, of course, consist of relatively large objects, such as brown dwarf stars or small black holes. But it might also consist of subatomic particles, many of them left over from the high-energy days of the early universe, in which case the identity of the particles would provide an observational test of supersymmetry or of any comparable unified theory of the early universe.
While awaiting the wished-for apotheosis of supersymmetry theory, we may care to reflect on the role played by symmetry in cosmic history. In doing so we soon confront the realization that perfect symmetry, though beautiful in the abstract, is also sterile. If, for instance, the matter-antimatter symmetry thought to have existed at the outset of cosmic evolution had been preserved, the particles of matter and those of antimatter would have mutually annihilated in the big bang, and no matter of either kind would have survived from which to make stars and planets and people. Had the putative primal force not scattered into the four forces, the universe today would be very different, perhaps uninhabitably so. It just may be, then, that we owe our existence, and that of the stars in the sky, to imperfections born of broken symmetry. To investigate the riddle of creation would then involve envisioning a perfectly symmetrical but unlivable universe, then trying to determine how it devolved from that sterile, pristine state toward becoming the less perfect but more variegated and hospitable universe in which we find ourselves today.
*There are exceptions, of course, notably those mathematicians who come into physics with little or no grounding in experimental science. But generally speaking, the best theoretical physicists are willing, if only during their student days, to get their hands dirty in the laboratory. Recall that the young Einstein nearly lost a hand this way.
*At this point gravitons, the carriers of gravitational force, would have decoupled from the other particles, producing a gravitational background radiation much like those generated later by the decoupling of neutrinos and photons. The present-day temperature of the cosmic gravitational background radiation, however, is only 1 degree Kelvin, placing it far below the sensitivity of any conceivable gravitational detector. Still, it is there, and if we could find a way to observe it we could see all the way back to the Planck epoch.
*String theory postulates that there exists and has existed only a single variety of particle, but that this particle has an infinite number of manifestations—as in the innumerable tunes that may be composed on a single string of Pythagoras’s lyre. Thus a single supersymmetric variety of particle shows up in various harmonics as gravitons and gravitini, quarks and squarks, photons and photinos, and so forth. Since, as Gell-Mann noted, “these infinitely many particles all obey a single very beautiful master equation,” the theory suggests how maximum complexity could have arisen from maximum simplicity.
†Recall that, as Newton found, the gravitational force of any object may be regarded as emanating from a point at its center. Each star in a galaxy responds to the total gravitation of the mass of the galaxy that lies within its orbit, as if the gravity were coming from a point source at the galactic center. The orbital velocity of a star lying near the edge of a galaxy therefore constitutes an index of the total mass of the galaxy.
18
THE ORIGIN OF THE UNIVERSE
Where wast thou when I laid the foundations of the earth? Declare if thou hast understanding!
—Yahweh, to Job
Who really knows?
—Rig-Veda
Speculation about the origin of the universe is an old and notorious human activity. Old, I suppose, because there is no birth certificate for the human species: We are obliged to investigate our origins on our own, and in doing so have found it necessary to ponder as well the derivation of the wider world of which we are a part. Notorious, because the cosmogonic speculations that resulted told us more about ourselves than about the universe they claimed to describe: All, to some extent, were psychological projections, patterns cast outward from the mind onto the sky, like dancing shadows from a jack-o’-lantern.
Prescientific creation myths depended for their survival less on their accordance with the data of observation (of which there was in any event very little) than on the extent to which they were satisfying or reassuring or poetically resonant. Cherished insofar as they were our own, these tales emphasized what mattered most to the societies that preserved them. The Sumerians, living at a confluence of rivers, envisioned creation as having resulted from what amounted to a mud-wrestling match among the gods. (From a clod thrown off, the earth congealed.) The Mayans, obsessed with ball playing, conjectured that their creator was transformed into a solar kickball each time the planet Venus disappeared behind the sun. Tahitian fisherman told of an angler god who tugged their islands from the ocean floor; the Japanese sword-wielders formed their islands from drops of blood dripping from a cosmic blade. To the logic-loving Greeks, creation was elemental: For Thales of Miletus, the universe originally was water; for Anaximenes (also of Miletus), air; for Heraclitus, fire. In the fecund Hawaiian Islands, genesis was managed by a team of spirits skilled in embryology and child development. African bushmen huddled around a fire watched the sparks fly upward into the night sky and recited these words:
The girl arose; she put her hands into the wood ashes; she threw up the wood ashes into the sky. She said, “The wood ashes must become the Milky Way. They must lie white along in the sky, that the stars may stand outside of the Milky Way, and the Milky Way be the Milky Way, while it used to be wood ashes.”1
The advent of science and technology has brought about an improvement in the sophistication of cosmogonic theorizing—relative at least to what preceded it, if not to the bald reality (if there be such) of the great yawning cosmos (if it be a cosmos). But science has by no means freed the creation question from its old entanglement in human presuppositions and desires. The question of how the universe began is at best elusive, and when we hunt after it, our quivers bristling with quarks and leptons and curved space tensors and quantum probabilities, we have an only marginally better justification for our audacity than was enjoyed by Tahitian visionaries who imagined that God might cast his fishing line and catch not a fish but an emerald isle. Many scientists understood this very well, and many, consequently, would have nothing to do with cosmogony, the study of the origin of the universe. Some left the matter alone simply because they could see
no practical way of approaching it. Others, adhering to the doctrine of causation, banished the issue of a first cause to exile in realms beyond science. As the astronomer Allan Sandage said:
If there was a creation event, it had to have had a cause. This was Aquinas’s whole question, one of the five ways he established the existence of God. If you can find the first effect, you have at least come close to the first cause, and if you find the first cause, that to him was God. What do astronomers say? As astronomers you can’t say anything except that here is a miracle, what seems almost supernatural, an event which has come across the horizon into science, through the big bang. Can you go the other way, back outside the barrier and finally find the answer to the question of why is there something rather than nothing? No, you cannot, not within science. But it still remains an incredible mystery: Why is there something instead of nothing?2
Such reservations notwithstanding, a few scientists did attempt to investigate the question of how the universe might have originated, while admitting that their efforts were probably “premature,” as Weinberg mildly put it. At its best, if viewed with an encouraging squint, their work appeared to shine a lamp into the anterooms of genesis. What they illuminated there was very strange, but this was, if anything, encouraging: We should hardly expect to find the familiar at the wellsprings of creation.
Coming of Age in the Milky Way Page 36
