From Chapter 1, The Shores of Light The so-called big bang model arose from thinking about what an expanding universe would have been like in its infancy. The observable universe today is roughly 15 billion light-years in radius. When its radius was much smaller -- only one light-year, say -- all the matter in the universe must have been packed together in a lot less space. Any given quantity of matter, compressed to a higher density, gets hotter: That's why a penny, lifted off a railroad track moments after being flattened by a passing train, is hot to the touch, and why compressing air in a bicycle pump heats the air, making the pump warm. So it seems reasonable to imagine that the early universe may have been not only dense, but also hot. Very hot: When the universe was one second old, in this scenario, every spoonful of stuff was denser than stone and hotter than the center of the sun. The expansion and resultant cooling of the universe permitted the formation of atoms, molecules, galaxies, and living creatures. What we call matter is frozen energy. It froze because the universe, owing to its expansion, cooled.
The big bang theory implied that as the young universe expanded there should have come a time, nowadays reckoned at about five hundred thousand years after the beginning, when the primordial plasma thinned out sufficiently to become transparent to light. Physicists call this event photon decoupling, meaning that photons, the particles that constitute light and other forms of electromagnetic energy, were at this point set free. Thereafter they did not often interact with one another, or with matter, but went soaring unhampered through the constantly expanding reaches of cosmic space. Hence most of them should still be around today. Cosmic expansion would have stretched them out, increasing their wavelengths from those of light to the wavelengths we call microwave radio. In microwave frequencies it is convenient to express energy in terms of temperature -- as does, say, the instruction manual that accompanies a microwave oven -- so another way to reason through this argument is to say that the universe, having once been hot, should remain a bit warm even today. Physicists theorizing about the existence of this cosmic microwave background, or CMB, calculated that it should have a temperature of about three degrees above absolute zero. They also noted that it would display a "black body" spectrum, as is dictated by the relevant quantum physics equations and that it should be isotropic, meaning that any observer, anywhere in the universe, should measure the background as having the same temperature everywhere in the sky. One can think of the CMB as a haze of photons that has permeated space ever since the big bang. As we look far out in space -- and, therefore, backward in time, to when the CMB photons were more energetic -- we find the haze thickening. At the ultimate distance, where we are peering back into the first million years of time, the haze becomes opaque. Every observer using a microwave radio telescope thus sees the universe as a sphere that is almost transparent nearby but is opaque at its distant and fiery walls.
When this prediction was first made, in the 1940s, it was quickly forgotten. The big bang theory was not yet taken very seriously and there was no such thing as a microwave radio receiver. Then, in 1965, two physicists working with a radio receiver built for communications satellite experiments detected the CMB. Interest mounted as scientists came to appreciate that by studying the CMB they could make direct observation of the universe as it was only half a million years after the beginning of time. In 1989, the American space agency launched a satellite designed to study the CMB from orbit, where its detectors were free from the interference of Earth's atmosphere. Preliminary findings obtained by the COBE (Cosmic Background Explorer) satellite were announced the following year, and turned out to constitute a stunning confirmation of the big bang model. The CMB is indeed isotropic -- that is, it has equal intensity all over the sky, as anything genuinely universal must. And, as expected, its temperature is about three degrees above absolute zero -- 2.726 degrees, to be exact. And its spectrum conforms to a black body spectrum: The fit is so precise that the researchers making the announcement had to enlarge the size of the error bars on their diagrams: Otherwise the observational data points would have disappeared into the thin, inked line describing the theoretical prediction.
A final triumph for the COBE scientists came in 1992, when an all-sky map, carefully compiled by repeated observations that pushed the sensitivity of the COBE instruments to their limits, confirmed another important prediction of the big bang theory -- that matter, though generally distributed uniformly throughout the cosmos, began fairly early to clump into dense regions from which clusters of galaxies were to form. This was good news for theorists who argued that the vast clusters, superclusters, and bubbles of galaxies we see in the universe today formed by gravitational attraction from inhomogeneities in the early universe. The clumps of matter are thought to have originated as quantum fluctuations, microscopic departures from the generally homogeneous distribution of matter in the very early universe. Much remains to be studied about the spectrum and sizes of these inhomogeneities, and how, exactly, they resulted in the large-scale structures we see in the universe today. These findings led most cosmologists to agree that the universe emerged from a hot big bang state.
Several other sorts of evidence support the big bang theory.
One of these is the intriguing fact that the cosmic element abundance fits the predictions of the theory. Here the line of reasoning is that as the primordial fireball cooled, protons and neutrons would have joined up to form the nuclei of atoms. The calculations of the nuclear physicists -- who have had a lot of experience in this sort of thing, since similar processes occur in the explosions of thermonuclear bombs -- indicate that about a quarter of the atom-making stuff should have been converted into helium in the big bang, along with a bit of lithium, while the remainder survived as hydrogen (the simplest atom, whose nucleus in its rudimentary form consists of a single proton). And this is just what we do find: The universe at large is 25 percent helium and 73 percent hydrogen. The theory postulates that all the heavier elements were forged inside stars, notably in supernovae -- exploding stars, which seed space with clouds of debris, enriched with the heavier elements, from which condensed latter-day stars and planets, the earth and the sun among them. If this theory is correct we should find that older stars are poorer in heavy elements than younger stars are. And this, too, turns out to be the case.
In a big bang universe it ought to be possible to see direct evidence of cosmic evolution by looking out to great distances since light reaching us from billions of light-years away is billions of years old and so reveals what things were like billions of years ago. Such evidence has indeed been found. A dramatic example is provided by quasars, which are the bright cores of galaxies going through a stage in which they emit enormous amounts of energy, enough to make them visible at substantial distances. The nearest quasars lie more than a billion light-years from Earth: the most distant are more than 10 billion light-years away. Surveys of the hundreds of quasars found between these extremes show that they become increasingly numerous as we look deeper into space and farther into the past. Astronomers conclude that the quasar phase of galactic evolution is something galaxies exhibit when they are young, and seldom thereafter.
Distant galaxies are bluer than nearby galaxies. Since bright young stars are blue, this indicates that galaxies billions of years ago formed new stars more profligately than they do today. Observations of remote clusters of galaxies made using the Hubble Space Telescope show that "rich" clusters (those with lots of galaxies relatively close to one another) used to have a higher proportion of spiral galaxies than they do today. So something must have reduced the number of spiral galaxies in such clusters. Nobody is sure what that was. Perhaps the spirals merged to form elliptical galaxies, or perhaps they were torn to pieces to make small irregular galaxies. Much of the bright promise of deep-space astronomy comes from the prospect of directly observing cosmic evolution by using more powerful telescopes as time machines, to look at the universe as it was in the distant past.
The ages of stars fit the age of the universe deduced from its expansion rate -- according to some of the data, at least. As we will see, several persuasive sets of observations suggest the universe has been expanding for approximately 15 billion years. This accords with the ages of the oldest known stars, estimated by astrophysicists at about 14 billion years. But there are other observations, which some researchers regard as persuasive, that yield an age for the universe of only about 10 billion years. If these prove to be correct then something is wrong either with our understanding of the ages of the oldest stars or with some aspect of the big bang theory.
Which leads us to what might be called the theoretical proofs of the big bang scenario. It may seem perverse to speak of using one theory to prove another, since theories normally stand or fall on the verdict of observation and experiment. But facts in themselves are as disorderly as cornflakes without a bowl. In practice, Science does a lot of pouring cornflakes from box to bowl -- checking not just whether facts fit a given theory, but whether the theories work well together. If we ask in what regard the big bang accords with other well-established theories, we find several answers.
General relativity has survived a great many experimental tests and seems to be perfectly accurate insofar as one is concerned with making predictions about the behavior of gravity under conditions that currently prevail throughout most of the universe. And general relativity implies, as we have noted, that the universe must be either expanding or contracting. So the very fact that we find evidence of cosmic expansion in the sky means that a well-established theory, relativity, supports another more hypothetical one, the big bang.
Quantum physics, too, finds a gratifying place within the big bang scheme. Using quantum mechanics, physicists are able to predict the existence and spectrum of the cosmic microwave background, calculate how much of the primordial material was turned into helium in the big bang, and estimate the ages of the oldest stars. Quantum physics makes accurate predictions about events involving three of the four fundamental forces of nature -- the weak and strong nuclear forces at work in atoms, and electromagnetism, the force responsible for light and radio energy. But there is not as yet a fully accomplished quantum theory of the fourth force, gravity. This would not matter much were the province of physics limited to the contemporary universe: Gravitation is so weak that it can be disregarded when calculating the interactions of subatomic particles, which have such small mass that their gravitational pull on one another is negligible. But in the high-density early universe, subatomic particles weighed so much that their mutual gravitational influence was comparable to their interactions via the other three forces. To reconstruct events thought to have transpired during the very first fractions of a second of cosmic time will require a quantum account of gravity. Such a theory presumably would lay bare a single principle underlying both quantum mechanics and general relativity, which at our present level of understanding are based on contradictory ways of looking at the world. Candidates include supersymmetry, grand unified theory, and superstring theory, in which all the particles of matter and energy found in the universe today are said to be scraps of space, created from shards of a primeval, ten-dimensional geometry that shattered when the expansion of the universe began. Such speculations would be mere abstractions in a static universe, but they take on the many-colored cloak of history in a big bang universe, where they may speak of conditions that actually once prevailed.
The inflationary hypothesis has generated considerable interest in cosmology. It proposes that during a dawning moment of cosmic history the expansion of the universe proceeded much faster than had been thought -- indeed, at a rate far greater than the velocity of light. For reasons we shall strive to make clear, the inflationary hypothesis not only solves several problems that afflicted earlier versions of the big bang theory but indicates that the universe is extremely large, and flings open a door onto the startling speculation that our universe originated as a microscopic bubble arising from the space of an earlier universe, which may in turn be one among many universes strewn like stars across inaccessible infinities of random spaces and times and sets of natural laws.
To sum up, as the twentieth century draws to a close, the big bang theory looks to be in pretty good shape. It is supported by several solid and more or less independent lines of evidence, and has at present no serious rivals. If one were asked to make a list of the greatest scientific accomplishments of the century, somewhere on that list -- along with relativity and quantum theory, the elucidation of the DNA molecule, the eradication of smallpox and the suppression of polio, the discovery of digital computation, and many other worthy attainments -- there would be a place for big bang cosmology.
Yet the big bang has its woes. One problem, already mentioned, is that while some observations indicate an age for the universe consistent with other measurements, some do not; if the latter observations hold up, something must be wrong. Another difficult area involves the perplexing question of how, in a generally homogeneous universe, primordial fluctuations produced the vast structures represented by superclusters of galaxies. Quite possibly related to this issue is the riddle of what constitutes the dark matter, nonluminous material that evidently holds the clusters together. Until these puzzles are resolved we will not be sure that cosmologists are on the right track in working within a big bang context. And almost certainly there will be other vexations to come. The greatest of these surely is the question of how the universe came into being -- which is itself a form of the philosophical riddle posed by Leibniz when he asked why there is something rather than nothing.
In this book we will encounter many such questions. For consistency we will address them within the context of the big bang theory, but without pretending that the theory is perfect or assuredly factual, much less "true." We have mountains to climb, and must use the tools available if we are to get somewhere and not just stand and gawk and wait for the high clouds to clear.
Copyright © 1997 by Timothy Ferris