Soon afterward, the Dutch astronomer, Willem de Sitter, found a solution to Einstein's equations that described an expanding universe, and the Russian mathematician Alexander Friedmann found another. Einstein's static picture, it turned out, was one of three special cases among an infinity of possible solutions, some expanding, some contracting. Yet despite the excitement and publicity that the General Theory had aroused—publication of Einstein's special relativity theory in 1905 had made comparatively little impact; his Nobel Prize of that year was awarded for a paper on the photoelectric effect—the subject remained confined to the circle of probably not more than a dozen or so specialists who had mastered its intricacies until well into the 1920s. Then the possible significance began being recognized of observational data that had been accumulating since 1913, when the astronomer V. M. Slipher (who, as is often the case in instances like this, was looking for something else) inferred from redshifts of the spectra of about a dozen galaxies in the vicinity of our own that the galaxies were moving away at speeds ranging up to a million miles per hour.
An Aside on Spectra and Redshifts
A spectrum is the range of wavelengths over which the energy carried by a wave motion such as light, radio, sound, disturbances on a water surface, is distributed. Most people are familiar with the visible part of the Sun's spectrum, ranging from red at the low-frequency end to violet at the high-frequency end, obtained by separating white sunlight into its component wavelengths by means of a prism. This is an example of a continuous, or "broadband" spectrum, containing energy at all wavelengths in the range. Alternatively, the energy may be concentrated in just a few narrow bands within the range.
Changes in the energy states of atoms are accompanied by the emission or absorption of radiation. In either case, the energy transfers occur at precise wavelength values that show as "lines," whose strength and spacings form patterns—"line spectra"—characteristic of different atomic types. Emission spectra consist of bright lines at the wavelengths of the emitted energy. Absorption spectra show as dark lines marking the wavelengths at which energy is absorbed from a background source—for example, of atoms in the gas surrounding a star, which absorb certain wavelengths of the light passing through. From the line spectra found for different elements in laboratories on Earth, the elements present in the spectra from stars and other astronomical objects can be identified.
A "redshifted" spectrum means that the whole pattern is displaced from its normal position toward the red—longer wavelength—end. In other words, all the lines of the various atomic spectra are observed to lie at longer wavelength values than the "normal" values measured on Earth. A situation that would bring this about would be one where the number of waves generated in a given time were stretched across more intervening space than they "normally" would be. This occurs when the source of the waves is receding. The opposite state of affairs applies when the source is approaching and the wavelengths get compressed, in which case spectra are "blue-shifted." Such alteration of wavelength due to relative motion between the source and receiver is the famous Doppler shift. 43 Textbooks invariably cite train whistles as an example at this point, so I won't.
A Universe in the Red and Lemaître's Primeval Atom
By 1924 the reports of redshifts from various observers had grown sufficiently for Carl Wirtz, a German astronomer, to note a correlation between the amounts of galactic redshift and their optical faintness, which was tentatively taken as a measure of distance. The American astronomer Edwin Hubble had recently developed a new method for measuring galactic distances using the known brightnesses of certain peculiar variable stars, and along with his assistant, Milton Humason, conducted a systematic review of the data using the 60-inch telescope at the Mount Wilson Observatory in California, and later the 100-inch—the world's largest at that time. In 1929 they announced what is now known as Hubble's Law: that the redshift of galaxies increases steadily with distance. Although Hubble himself always seemed to have reservations, the shift was rapidly accepted as a Doppler effect by the scientific world at large, along with the startling implication that not only is the universe expanding, but that the parts of it that lie farthest away are receding the fastest.
A Belgian priest, Georges Lemaître, who was conversant with Einstein's theory and had studied under Sir Arthur Eddington in England, and at Harvard where he attended a lecture by Hubble, concluded that the universe was expanding according to one of the solutions of GRT in which the repulsive force dominated. This still left a wide range of options, including models that were infinite in extent, some where the expansion arose from a state that had existed indefinitely, and others where the universe cycled endlessly through alternating periods of expansion and contraction. However, the second law of thermodynamics dictated that on balance net order degenerates invariably, one way or another, to disorder, and the process is irreversible. The organized energy of a rolling rock will eventually dissipate as heat in the ground as the rock is brought to a halt by friction, but the random heat motions of molecules in the ground never spontaneously combine to set a rock rolling. This carries the corollary that eventually everything will arrive at the same equilibrium temperature everywhere, at which point all further change must cease. This is obviously so far from being the case with the universe as seen today that it seemed the universe could only have existed for a limited time, and it must have arrived at its present state from one of minimum disorder, or "entropy." Applying these premises, Lemaître developed his concept of the "primeval atom," in which the universe exploded somewhere between 10 billion and 20 billion years ago out of an initial point particle identified with the initial infinitely large singularity exhibited by some solutions to the relativistic equations. According to this "fireworks model," which Lemaître presented in 1931, the primeval particle expanded and split up into progressively smaller units the size of galaxies, then stars, and so forth in a process analogous to radioactive decay.
This first version of a Big Bang cosmology was not generally accepted. The only actual evidence offered was the existence of cosmic rays arriving at high energies from all directions in space, which Lemaître argued could not come from any source visible today and must be a leftover product of the primordial breakdown. But this was disputed on the grounds that other processes were known which were capable of providing the required energy, and this proved correct. Cosmic-ray particles were later shown to be accelerated by electromagnetic forces in interstellar space. The theory was also criticized on the grounds of its model of stellar evolution based on a hypothetical process of direct matter-to-energy annihilation, since nuclear fusion had become the preferred candidate for explaining the energy output of stars, and Willem de Sitter showed that it was not necessary to assume GRT solutions involving a singularity. Further, the gloomy inevitability of a heat death was rejected as not being necessarily so, since whatever might seem true of the second law locally, nothing was known of its applicability to the universe as a whole. Maybe the world was deciding that the period that had brought about such events as the Somme, Verdun, and the end of Tsarist Russia had been an aberration, and was recovering from its pessimism. Possibly it's significant, then, that the resurrection of the Big Bang idea came immediately following World War II.
After the Bomb: The Birth of the Bang
Gamow's Nuclear Pressure-Cooker
In 1946, Russian-born George Gamow, who had worked on the theory of nuclear synthesis in the 1930s and been involved in the Manhattan Project, conjectured that if an atomic bomb could, in a fraction of a millionth of a second, create elements detectable at the test site in the desert years later, then perhaps an explosion on a colossal scale could have produced the elements making up the universe as we know it. Given high enough temperatures, the range of atomic nuclei found in nature could be built up through a succession starting with hydrogen, the lightest, which consists of one proton. Analysis of astronomical spectra showed the universe to consist of around 75 percent hydrogen, 24 percent helium, and the rest a mix cont
inuing on through lithium, beryllium, boron and so on of the various heavier elements. Although all of the latter put together formed just a trace in comparison to the amount of hydrogen and helium, earlier attempts at constructing a theoretical model had predicted far less than was observed—the discrepancy being in the order of ten orders of magnitude in the case of intermediate mass elements such as carbon, nitrogen, and oxygen, and getting rapidly worse (in fact, exponentially) beyond those.
Using pointlike initial conditions of the GRT equations, Gamow, working with Ralph Alpher and Robert Herman, modeled the explosion of a titanic superbomb in which, as the fireball expanded, the rapidly falling temperature would pass a point where the heavier nuclei formed from nuclear fusions in the first few minutes would cease being broken down again. The mix of elements that existed at that moment would thus be "locked in," providing the raw material for the subsequently evolving universe. By adjusting the parameters that determined density, Gamow and his colleagues developed a model that within the first thirty minutes of the Bang yielded a composition close to that which was observed.
Unlike Lemaître's earlier proposal, the Gamow theory was well received by the scientific community, particularly the new generation of physicists versed in nuclear technicalities, and became widely popularized. Einstein had envisaged a universe that was finite in space but curved and hence unbounded, as the surface of a sphere is in three dimensions. The prevailing model now became one that was also finite in time. Although cloaked in the language of particle physics and quantum mechanics, the return to what was essentially a medieval worldview was complete, raising again all the metaphysical questions about what had come before the Bang. If space and time themselves had come into existence along with all the matter and energy of the universe as some theorists maintained, where had it all come from? If the explosion had suddenly come about from a state that had endured for some indefinite period previously, what had triggered it? It seemed to be a one-time event. By the early 1950s, estimates of the total amount of mass in the universe appeared to rule out the solutions in which it oscillated between expansion and contraction. There wasn't enough to provide sufficient gravity to halt the expansion, which therefore seemed destined to continue forever. What the source of the energy might have been to drive such an expansion—exceeding all the gravitational energy contained in the universe—was also an unsolved problem.
Hoyle and Supernovas as "Little Bang" Element Factories
Difficulties for the theory mounted when the British astronomer Fred Hoyle showed that the unique conditions of a Big Bang were not necessary to account for the abundance of heavy elements; processes that are observable today could do the job. It was accepted by then that stars burned by converting hydrogen to helium, which can take place at temperatures as low as 10 million degrees—attainable in a star's core. Reactions beyond helium require higher temperatures, which Gamow had believed stars couldn't achieve. However, the immense outward pressure of fusion radiation balanced the star's tendency to fall inward under its own gravity. When the hydrogen fuel was used up, its conversion to helium would cease, upsetting the balance and allowing the star to collapse. The gravitational energy released in the collapse would heat the core further, eventually reaching the billion degrees necessary to initiate the fusion of helium nuclei into carbon, with other elements appearing through neutron capture along the lines Gamow had proposed. A new phase of radiation production would ensue, arresting the collapse and bringing the star into a new equilibrium until the helium was exhausted. At that point another cycle would repeat in which oxygen could be manufactured, and so on through to iron, in the middle of the range of elements, which is as far as the fusion process can go. Elements heavier than iron would come about in the huge supernova explosions that would occur following the further collapse of highly massive stars at the end of their nuclear burning phase—"little bangs" capable of supplying all the material required for the universe without need of any primordial event to stock it up from the beginning.
This model also accounted for the observational evidence that stars varied in their makeup of elements, which was difficult to explain if they all came from the same Big Bang plasma. (It also followed that any star or planet containing elements heavier than iron—our Sun, the Earth, indeed the whole Solar System, for example—must have formed from the debris of an exploded star from an earlier generation of stars.) Well, the images of starving postwar Europe, shattered German cities, Stalingrad, and Hiroshima were fading. The fifties were staid and prosperous, and confidence in the future was returning. Maybe it was time to rethink cosmology again.
The Steady-State Theory
Sure enough, Fred Hoyle, having dethroned the Big Bang as the only mechanism capable of producing heavy elements, went on, with Thomas Gold and Herman Bondi, to propose an alternative that would replace it completely. The Hubble redshift was still accepted by most as showing that the universe we see is expanding away in all directions to the limits of observation. But suppose, Hoyle and his colleagues argued, that instead of this being the result of a one-time event, destined to die away into darkness and emptiness as the galaxies recede away from each other, new matter is all the time coming into existence at a sufficient rate to keep the overall density of the universe the same. Thus, as old galaxies disappear beyond the remote visibility "horizon" and are lost, new matter being created diffusely through all of space would be coming together to form new galaxies, resulting in a universe populated by a whole range of ages—analogous to a forest consisting of all forms of trees, from young saplings to aging giants.
The rate of creation of new matter necessary to sustain this situation worked out at one hydrogen atom per year in a cube of volume measuring a hundred meters along a side, which would be utterly undetectable. Hence, the theory was not based on any hard observational data. Its sole justification was philosophical. The long-accepted "cosmological principle" asserted that, taken at a large-enough scale, the universe looked the same anywhere and in any direction. The Hoyle-Bondi-Gold approach introduced a "perfect cosmological principle" extending to time also, making the universe unchanging. It became known, therefore, as the steady-state theory.
The steady-state model had its problems too. One in particular was that surveys of the more distant galaxies, and hence ones seen from an earlier epoch because of the delay in their light reaching Earth, showed progressively more radio sources; hence the universe hadn't looked the same at all times, and so the principle of its maintaining a steady, unvarying state was violated. But it attracted a lot of scientists away from the Big Bang fold. The two major theories continued to rival each other, each with its adherents and opponents. And so things remained through into the sixties.
Then, in 1965, two scientists at Bell Telephone Laboratories, Arno Penzias and Robert Wilson, after several months of measurement and double-checking, confirmed a faint glow of radiation emanating evenly from every direction in the heavens with a frequency spectrum corresponding to a temperature of 2.7ºK. 44 This was widely acclaimed and publicized as settling the issue in favor of the Big Bang theory.
The Cosmic Background Radiation: News but Nothing New
Big Bang had been wrestling with the problem of where the energy came from to drive the expansion of the "open" universe that earlier observations had seemed to indicate—a universe that would continue expanding indefinitely due to there being too little gravitating mass to check it. Well, suppose the estimates were light, and the universe was in fact just "closed"—meaning that the amount of mass was just enough to eventually halt the expansion, at which point everything would all start falling in on itself again, recovering the energy that had been expended in driving the expansion. This would simplify things considerably, making it possible to consider an oscillating model again, in which the current Bang figures as simply the latest of an indeterminate number of cycles. Also, it did away with all the metaphysics of asking who put the match to whatever blew up, and what had been going on before.
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br /> A group at Princeton looked into the question of whether such a universe could produce the observed amount of helium, which was still one of Big Bang's strong points. (Steady state had gotten the abundance of heavier elements about right but was still having trouble accounting for all the helium.) They found that it could. With the conditions adjusted to match the observed figure for helium, expansion would have cooled the radiation of the original fireball to a diffuse background pervading all of space that should still be detectable—at a temperature of 30ºK. 45 Gamow's collaborators, Ralph Alpher and Robert Herman, in their original version had calculated 5ºK for the temperature resulting from the expansion alone, which they stated would be increased by the energy production of stars, and a later publication of Gamow's put the figure at 50ºK. 46
The story is generally repeated that the discovery of the 2.7ºK microwave background radiation confirmed precisely a prediction of the Big Bang theory. In fact, the figures predicted were an order of magnitude higher. We're told that those models were based on an idealized density somewhat higher than that actually reported by observation, and (mumble-mumble, shuffle-shuffle) it's not really too far off when you allow for the uncertainties. In any case, the Big Bang proponents maintained, the diffuseness of this radiation across space, emanating from no discernible source, meant that it could only be a relic of the original explosion.
It's difficult to follow the insistence on why this had to be so. A basic principle of physics is that a structure that emits wave energy at a given frequency (or wavelength) will also absorb energy at the same frequency—a tuning fork, for example, is set ringing by the same tone that it sounds when struck. An object in thermal equilibrium with—i.e., that has reached the same temperature as—its surroundings will emit the same spectrum of radiation that it absorbs. Every temperature has a characteristic spectrum, and an ideal, perfectly black body absorbing and reradiating totally is said to be a "blackbody" radiator at that temperature. The formula relating the total radiant energy emitted by a blackbody to its temperature was found experimentally by Joseph Stefan in 1879 and derived theoretically by Ludwig Boltzmann in 1889. Thus, given the energy density of a volume, it was possible to calculate its temperature.
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