cphil_bigbang
THE BIG BANG THEORY
by Ray Shelton
INTRODUCTION
Einstein has been consider the “father of modern cosmology” because of his theory of gravity in his General Theory of Relativity, not only because he broke with the cosmological thinking of the past since Copernicus and Newton, but because he also introduced a new understanding of the universe. It would be probably more correct call Einstein the “grandfather” of modern cosmology, because it was not Einstein’s intention to develop a new cosmology. With his General Theory of Relativity, he unintentionally pointed in the direction of a new cosmology, but others pick up the ball and ran with it and developed mathematical models of the universe that included the expansion of the universe, even before Hubble and Humason observationally established the expansion of the universe and formulated the law of that expansion. But the key theoretical developments came from four people, exactly during the ten years or so during which Slipher, Hubble and Humason were gathering the evidence that the universe was expanding. These “fathers of cosmology” were Einstein, de Sitter, Friedmann, and Lemaitre — with Eddington, perhaps as the benevolent god-father who helped the infant along the first faltering steps on the road to maturity. By the early 1930s, theory and observation came together in a remarkable fashion to point firmly in the direction of the dynamic evolving model which was later called the Big Bang model. It was another decade until George Gamow, Friedmann’s former student, developed a fully worked out version of the new cosmology. Meanwhile, various alternative models had been developed more fully, among which is the steady-state model. But the discovery of the cosmic background radiation in 1965 that was predicted by Gamow and his colleagues in the 1950s and forgotten, put an end to the steady-state model and was the second piece of powerful evidence that the universe did emerge from a hot fireball, the Big Bang. Fred Hoyle was the person who introduced the term “Big Bang” into astronomy in a radio broadcast on the British Broadcasting Corporation (BBC) in 1950, as a derisory name for the rival to his steady-state theory.
One of the first to develop a model of the universe based on Einstein’s static model of the universe was Willem de Sitter, the Dutchman who passed on the news of General Theory of Relativity to Eddington in London. In 1915, when news of Einstein’s new theory arrived in Leiden, de Sitter was already an experienced and senior astronomer. He had born in 1872 and studied at the University of Groningen and at the Royal Observatory in Cape Town. He was awarded Ph.D. in 1901, and by 1908 he was professor of theoretical astronomy at the University of Leiden. He died of pneumonia in 1934.
Professor de Sitter was one of the first to consider the implications of the Special Theory of Relativity for his field of astronomy; the Special Theory was only of interest to mathematicians because no practical relevance of it could be found in the first decade after its publication in 1905. De Sitter was possibly the first person after Einstein to make application of General Relativity to astronomy, because he was one of the first people to hear of Einstein’s new theory. When Einstein sought a description of the universe in terms of the General Theory, Einstein sought a single solution to his field equations that was a description of universe. His static model with its cosmological constant seemed to be the solution that he sought. But in one of the papers sent by de Sitter to the Royal Astronomical Society in London in 1917, where it was read with great interest by the then Secretary of the society, Arthur Eddington, it showed that there was another solution to the field equations, a solution that represented a different model of the universe. Obviously, both solutions could not represent the real world. We now know that neither of them represent the real world. and that is not now seen as a problem. But at the time, this was something of a blow to Einstein’s theory, since it could be argued that if the theory offered a choice of universes, all consistent with the basic equations, it could not be telling us much, if anything, about the real universe. That argument can scarcely hold up once the expansion of the universe was understood, and astronomers realized that Einstein’s equations had indeed predicted the expansion ten years before Hubble’s announcement in 1929 of the red-shift-distance relation.
De Sitter’s model of the universe, like Einstein’s, was in a mathematical sense static (and like Einstein’s, his equation included a cosmological constant). But unlike Einstein’s model, it contained no matter at all; it was a mathematical description of a completely empty universe. It is difficult to determine what “static” means, when there is nothing in an empty universe that can be used as an indicator of motion. And when the theorists tried to sprinkle mathematically a few specks of matter into de Sitter’s universe, they found a curious thing — these specks of matter, as test particles, promptly rushed away from each other. In addition, when they calculated how light from one of the test particles would look from another one of the test particles, the mathematicians found a red-shift proportional to the distance between the two particles. The de Sitter universe seemed to be static, only because it is empty. Much later, Eddington summed up the difference between the two relativistic cosmological models: Einstein’s universe contains matter but no motion; and de Sitter’s universe contains motion, but no matter.
Eddington was one of the few people who took seriously the expansion of a de Sitter’s universe containing a trace of matter. From the limited red-shift data coming in during the early 1920s, especially from Vesto Slipher, Eddington concluded that de Sitter’s variation on the relativistic theme was telling the astronomers something about the real world. Later Eddington developed his own variation, a model in which the universe sat for a long time (perhaps an infinitely long time) in a static state, like the Einstein universe, and then began to expand, like the de Sitter universe, as galaxies formed. But this model turned out to have little bearing on the nature of the real universe.
Einstein and de Sitter themselves had important second thoughts about the cosmological models once the red-shift-relation began to look like an important feature of the real universe. In 1932 they put their heads together and came up with yet another model universe, the Einstein-de Sitter (not to be confused with either of their separate models), in which they went back to their roots. The cosmological constant had been originally introduced by Einstein to hold the model static, but the real universe was seen to be expanding — so they threw away the cosmological constant. But the earlier models involved curved space (and in de Sitter’s model, curved time also), but there was no direct evidence that space was curved — so they threw away the curved space (but not the curved space-time). The Einstein-de Sitter model is the simplest model of the universe that can be constructed using the basic equations of General Relativity. It expands, as the equations required, and the space that is expanding is flat, the space of Special Relativity. And since nothing had been added to the model to stop the inevitable from happening when looking back in time, the model required that there was a definite creation event, long ago, when the universe was born out of a mathematical point, a state of infinite density, called a singularity.
As the simplest solution of the field equations, the Einstein-de Sitter model is a very useful case study. It has been used in many courses of cosmology, on the reasonable grounds that students ought to start with the simplest examples, and work their ways up to more complex and more interesting things. With hindsight, the key feature of the Einstein-de Sitter model, and the reason it was so widely taught, is that it includes the moment of creation, what was later called the Big Bang.
The Russian mathematician and meteorologist Alexander Friedmann (1888-1925) was the first person to appreciate that expansion was an integral part of the relativistic description of the universe and that it should be incorporated into the cosmological model from the outset. Friedmann was born in Russia in the city then known as St. Petersburg (later Leningrad), He studied mathematics at the city’s university from 1906 to 1910. He became a member of the mathematics faculty of the university, served in the Russian air force during the First World War, lived through the revolution of 1917 to become a full professor at Perm University and then returned to St. Petersburg in 1920 to carry out research at the Academy of Sciences. By the time he died in 1925, the city was renamed Leningrad. Friedmann died in obscurity and his interest in meteorology may have caused his death. The official biographies says he died of typhoid, but according to the cosmologist George Gamow, the cause of his death was pneumonia contracted following a chill Friedmann caught while flying in a meteorological balloon. Gamow, who became a key figure among the next generation of cosmologist, ought have known, since he was one of Friedmann’s students in Russia, and only moved to the United States in the middle 1930s.
Friedman’s research interest originally centered on the Earth Sciences, geomagnetism, hydromechanics, and meteorology. But as an able mathematician he was keenly interested in Einstein’s work and in 1922 published his solution to the field equations of General Theory of Relativity. Two key features of this work are important to modern cosmology. First, Friedmann realized that he was dealing with a family of solutions to the equations. He realize that there is no unique solution, as Einstein had hoped, but instead there was a set of different variations, each describing a different kind of universe. Second, Friedmann incorporated expansion into his models from the outset. In a way, this echoed the work of Clifford in the 1880s, the idea that space might be uniformly curved, like the spherical surface of a soap bubble, but that this curvature might be changing with time — decreasing, perhaps — as the “bubble” expands. Friedmann’s models offered several variations on the theme. In some versions, the bubble expanded forever; in others, it expanded up to a certain limiting size and then collapsed back upon itself, as the force of gravity overcomes the expansion. There were other versions with a cosmological constant, and alternatives in which the cosmological constant is set to zero — the preferred alternative today. But in all the models there was at least a period — an interval time — during which the whole universe expanded in such a way that it would produce a recession velocity proportional to distance. Friedmann also appreciated that the red-shift of light in the expanding universe is not caused by the galaxies moving apart from each other through space. It is caused by space itself stretching, like a stretching rubber sheet, between the galaxies. Space — or better, space-time — expands, and carries the galaxies along with it for the ride.
But Friedmann’s work, which was published in a well-known and widely read journal, was largely ignored. It was called to Einstein’s attention by one of Friedmann’s colleagues on a visit to Berlin, and Einstein later acknowledged its accuracy in a brief note to Friedmann, but even Einstein failed to realize what it was telling us about the real universe. At first Einstein did not believe Friedmann’s calculations and thought that he had made a mistake. Like most physicists and astronomers of the time, Einstein thought the universe was static and existed from eternity past to eternity future. A dynamic, expanding universe seemed to be contrary to experience and a gratuitous novelty. Because he wanted a static universe, Einstein even went so far as to alter his general relativity equations, adding a “gravitational term” to the equations so that they would allow for a static solution. Einstein later called this mutilation of his equations “the biggest blunder of his life.” So it was Friedmann, not Einstein, who had discovered that general relativity required a expanding, changing universe. His dramatic prediction was made seven years before the great cosmological discovery of the American astronomer Edwin Hubble in 1929. From a detailed study of the distant galaxies, Hubble concluded that the universe was indeed expanding and formulated the law governing that expansion. Two years later, in 1931, Einstein finally dropped this “fudge” factor, the cosmological constant, from the equations of his General Relativity Theory and returned to the theory of expanding universe, which he had abandoned fourteen years earlier.
Mathematicians in the 1920s had little contact with astronomers, and astronomers seldom followed the developments in mathematics; and the scientific worlds of Europe and America were more separated in 1920s than they are today. So new mathematical ideas in Europe were not being matched up with new astronomical discoveries in the United States. And there was a bias against a Russian mathematician who was better known for his work as a meteorologist. Whatever the reason, Friedman never lived to see his work recognized as one of the key contributors to twentieth-century cosmology.
The next person to solve Einstein’s equations in the same way Friedmann did (but quite independently, and with no knowledge of Friedmann’s work) was the Belgian cosmologist, Georges Lemaitre, who made a breakthrough in seeing these solutions accepted as worthwhile tool for cosmologist to probe the nature of universe. His original publication, which was in an obscure Belgian journal in 1927, attracted little notice. But following the announcement of the red-shift-distance relation, which became known as Hubble’s law, Eddington learned of Lemaitre’s paper and arranged for a English translation, which appeared in the Monthly Notices of the Royal Astronomical Society in 1931. If anybody deserves the title “Father of the Big Bang”, it was Lemaitre — which led to some awful puns over the years, since Lemaitre was also a Roman Catholic priest, as well as cosmologist and mathematician. He was born in 1894 and was trained as a civil engineer; during the First World War he served as an artillery officer with the Belgian army. After the war, he studied at the University of Louvain, graduating in 1920 before entering a seminary and being ordained a Roman Catholic priest in 1923. He spent a year at Cambridge, where he worked with Eddington, and a year in the United States, dividing his time between Harvard and MIT, before returning to Louvain, where he became professor of astronomy in 1927 and stayed there for rest of his career; he died in 1966.
Throughout his long career, Lemaitre continued to develop his cosmological ideas, and he lived to see many of them incorporated into mainstream cosmology. The most important was the idea of the Big Bang itself, although he did not give it that name. Lemaitre’s solutions of the general relativity equations were essentially the same as those found by Friedmann, although Lemaitre preferred to retain throughout his life the cosmological constant that even Einstein abandoned in the 1930s. But Lemaitre, unlike Friedmann, or anyone else before him, tackled the problem of what these equations was telling us about the origin of universe.
Unlike Friedmann, Lamaitre clearly knew something about galaxy red-shift being detected at the time. In a 1927 paper, Lamaitre recognized that galaxies might provide the “test particles” by which the universal expansion might be measured, and he gave, without any reference, a value to the constant of proportionality in the red-shift-distance relation (what became known later as Hubble’s constant) so close to the value that Hubble published a little later, that some cosmologists believe that there must have been communication between Lamaitre and Hubble, which there was none. Lamaitre saw what both observations and the Theory of General Relativity were saying. If galaxies are far apart today and are getting farther apart, that must mean that they were closer together in past. If we look far enough back in time, there can be no empty space between the galaxies. And earlier than that there would be no space between the stars, and even earlier time there would be no space between the atoms, or between the nuclei of the atoms. That was as far back as even the imagination of Lamaitre could go. He envisaged a time when the entire content of the universe was packed into a sphere only about thirty times bigger than the sun, what he called the “primeval atom”. This atom then at the beginning of time exploded outwards, breaking up into fragments that became atoms, stars, and the galaxies that we now know, with the galaxies still moving away from each other because of the expansion of the universe. The process was likened to the way an unstable, radioactive atomic nucleus may spontaneously split into pieces that go their own way — nuclear fission, the power source of atomic or nuclear bomb. This simple idea has been developed and modified considerably since the 1930s. But modern cosmology keeps at its heart this idea, first propounded by Lemaitre, that our universe was born out of a superdense state that gave birth to everything we see in the expanding universe. With the observations of Hubble and Humason that showed that our universe to be expanding, and with Lemaitre’s idea of the primeval atom, being published in English in early 1930s, modern cosmology was off and running. By the mid-1960s the Big Bang cosmology, in one or another of its many versions, has been accepted as the framework of cosmological theory. Today it is virtually without competitors.
Edwin Hubble was born in 1889 in Marshfield, Missouri, the fifth of seven sons of a local lawyer, and attended both high school and college in Chicago, at the University of Chicago. Hubble was a superb athelete and was offered a chance to turn professional as a boxer to fight the great Jack Jackson. Instead, he took up an offer of a Rhodes Scholarship to travel to Oxford University in England, where he studied law, represented the university as an athelete, and fought the French boxer and chamption Georges Carpentier as an amateur in an exhibition bout. On his return to the United States in 1913, Hubble joined the Kentucky bar, but practiced law for only a few months before deciding that this was not the career he wanted. Reverting to an interest in astronomy that had been partly stimulated by his University of Chicago astronomy professor, George Ellery Hale, who was probably the greatest telescope builder of the twentieth century, Hubble returned to that university, studying astronomy at Yerkes Observatory. He finished his research in 1917 and was awarded a Ph.D.; Hale offered him a post at Mount Wilson Observatory in California, but first Hubble enlisted in the infantry to fight in France where he was wounded by shell fragments in his right arm. So in 1919 he arrived at Mount Wilson just as the new hundred-inch telescope was coming into full use and just as Howard Shapley, who had established that the Cepheid variables were not binaries but pulsating stars, had departed for Harvard to become director of Harvard College Observatory. Hubble’s timing could not have been better.
Hubble picked up where Shapley had left off; Hubble built on Shapley’s technique of estimating distances using the Cepheids and globular clusters. It had only been recently as 1917 that a nova had been idenified for the first time on a photographic plate by George Ritchey at Mount Wilson, stimulating Heber Curtis, an astronomer at Lick Observatory, to search back through the photographic records at Lick and find the evidence that gave him the first direct measure of distance to extragalatic nebulae. For hundreds of years, the nature of those nebulae had been open to debate: Were the spiral nebulae galaxies outside the Milky Way or not? Although Hubble believed that the spiral nebulae were galaxies far beyond the Milky Way, he wasn’t going to be rushed into a hasty attempt to prove it. First, he tackled the problem of the other nebulae, the ones that did not show the characteristic spiral structure and that were almost certainly part of the Milky Way system. By 1922, Hubble, using the 60-inch and 100-inch telescopes at Mount Wilson, completed a major study that showed that these gaseous nebulae were indeed part of the Milky Way. But what about the spiral nebulae? Hubble now turned his attention to this problem. But in the middle of his observation program, another extragalactic Cepheid was identified in the Andromedia Nebula. The discovery was made in the autumn of 1923, during a survey aimed at finding a novae in Andromedia Nebula, also known as M31 (number 31 in Messier’s catalog of stars), a novae that might be used to test Curtis’ ideas about the nature of nebulae. It was established that what was first thought to be a nova was a variable star and it was a typical Cepheid with a period of about a month and the required distance was on the order of 900,000 light-years. With no new assumptions at all (unlike Curtis, who could only guess that the novae in Andromedia were the same intrinsically as novae in the Milky Way) but using the same yardstick that had been used by Shapley to map the Milky Way, Hubble could now measure the distances to the nearer external galaxies. In 1923, with improved photographic emulsions, Hubble succeeded in resolving the outward part of Andromedia Nebula into dense swarms of stars, and over the next few months and years, identified more Cepheids in both M31 and in another spiral nebula about the same distance away, M33. The Andromedia Nebula was a galaxy outside the Milky Way and the Milky Way Galaxy was not the only galaxy in the universe. So by the end of 1924, the debate was over; there was sufficient evidence to settle the problem of the nature of spiral nebulae, and Hubble presented it to a meeting of the American Astronomical Society. With the combination of the hundred-inch telescope and careful analysis, Edwin Hubble had given a new picture of the universe, but more startling discoveries still to come.
When reasonable reliable distance to nebulae became known, a startling discovery was made. Vesto M. Slipher (1875-1969) at Lowell Observatory in the early 1900s had obtained the spectra of a number of nebulae; he noticed that in their spectra that invariably the absorption lines were displaced to the red end of the spectra, and the fainter galaxies usually show the greater shifts. By 1929, Slipher had only forty-six red-shift spectra of galaxies beyond the Milky Way that were known then. Hubble followed Slipher’s work with a exhaustive study, assisted by Humason, using the 100-inch telescope at Mount Wilson. Except for some galaxies in the local group, Hubble confirmed that galaxies always show spectral lines shifted to the red.
By the late 1920s, enough distances to the galaxies had been measured so that Hubble was able to compare the observed red shifts with the distances to the galaxies. To his surprise, the red shifts in the spectra of the galaxies increased proportionally to the distances to the galaxies. The farther away the galaxies the greater the red shifts. This means that since the red shifts are Doppler shifts, then galaxies are all moving away from us. (According to the Doppler effect, light or sound from an object coming toward us have an increase to higher frequencies and the light or sound from an object moving away from us have a decrease to lower frequencies. These changes of frequency of the light or sound of moving object are called the Doppler effects. The decrease of frequency of light is called red-shifts since the decrease is toward the red end or lower frequencies of the light spectra. Correspondingly, the increase of frequency of light is called the blue-shifts since the increase is toward the blue end or higher frequencies of the light spectra. The red-shifts of the spectra of galaxies means they are moving away from us.) And the further away the galaxies, the faster they are receding. Hubble summarized his results in a formula, which is now called Hubble’s Law of Red Shifts:
v = Hd,
where v is the velocity of recession of the galaxy,
H is a constant of proportionality known as Hubble’s Constant,
d is the distance to the galaxy.
When v is given in kilometers per second and d is in megaparsec (1 million par secs = 3.26 million light-years), Hubble’s Constant H was found to be between 40 and 100 kilometers per second per megaparsecs. The presently accepted value is 55 kilometers per second per megaparsecs. Hubble’s law predicts that the plot of the galactic recessional velocities against the distances galaxies is a straight line. The uncertainties in the value of H arise mainly from the difficulties in measuring the distances to the galaxies used to calibrate Hubble’s law. It is not now uncommon, since the law is rather well established, to use it to measure the distance to a galaxy once its velocity of recession has been measured.
Milton Humason was born at Dodge Center, Minnesota, in 1891. When he was fourteen, he was sent to a summer camp on Mount Wilson, and he enjoyed himself so much that within days of going back to high school, he had persuaded his parents to let him take a year off from school to go back to the mountain. He never returned to formal education, but by circuitous route became one of the foremost observational astronomers of his generation. As an academic dropout he became for a time a mule driver, guiding packtrains up the trails to the mountain top while the Mount Wilson Observatory was being constructed. He was facinated with both the mountain and observatory work, but also found time to fall in love and marry the daughter of the observatory’s engineer in 1911. With new responsibility as a married man, Humason gave up mule skinning and tried to settle down on a ranch owned by a relative in La Verne. But in 1917, when a janitor’s job at the Mount Wilson Observatory fell vacant, Humason’s father-in-law urged him to apply and hinted that a bright young man who loved mountains and observatories might find this a stepping-stone to greater heights. The twenty-six-old janitor joined the staff of the observatory in 1917. From janitor Humason was soon promoted to night assistant with the job of looking after the telescope and helping the observational astronomers go about their tasks; he soon wangled some observing time of his own, and Humason showed so much skill with the telescope that in 1919 he was appointed Assistant Astronomer and became a junior member of the observatory’s academic staff. Edmund Hale had to fight off a lot of opposition to the appointment — after all Humason was a mule skinner and janitor with no formal education since the age of fourteen, and the fact he was married to the engineer’s daughter also counted against him in the eyes of those who suspected foul play concerning his promotion. But Hale knew his man and he stuck to his guns. Humason stayed Assistant Astronomer until 1954, when he became a full Astronomer at Mount Wilson and Palomar observatories; since 1947 he had been Secretary of the Observatories, responsible for public relations and various administrative duties. He received honorary degrees, but never one of the more common kind, and he lived within a few weeks of his eighty-first birthday in 1972. His meticulous handling of delicate instruments and his skill with the large telescopes enabled him to provide the base-line data with which the cosmologists were able to build their first detailed imaginative models of the universe and to fit their ideas into the theory of the Big Bang itself. And this all began in 1928, when Hubble first steered Humason toward the task of measuring the red-shift of faint and distant galaxies. These observations required some new instruments, new photographic techniques, and the almost unique combination of patience and skill that enabled Humason to spend hours at the telescope, spread over several nights, guiding it precisely so that he could obtain the spectrum of a far distant galaxy on a tiny photographic plate just half an inch wide. By 1935, he added 150 red-shifts to Slipher’s list and was clocking up red-shifts corresponding to recession velocities in excess of 40,000 kilometer per second — more than one eigth of the speed of light. From the end of the 1920s onward, it became increasingly clear that the universe is a very large place indeed, that galaxies, or clusters of galaxies, are its largest building blocks, and the red-shifts rule the roost. The universe is expanding like crazy.
While the big bang cosmology was making its way to public consciousness, there were other physicists that did not agree with it. They put together a theory that has come to be known as the Steady State Theory. The steady state theory of the universe was the twentieth century cosmology that holds that the large scale features of the universe do not change with time, and that the universe does not have a beginning in time. It was devised by the British astronomer Sir Fred Hoyle (1915-), and the British cosmologist William H. McCrea, and the two Austrian-born astronomers Thomas Gold (1920-) and Hermann Bondi (1919-). In 1948, the journal Monthly Notices of the Royal Astronomical Society published two papers in which three young Cambridge physicists proposed a new cosmological theory. One paper was written by Thomas Gold and Hermann Bondi, and the other by Fred Hoyle. All three had spent part of World War Two in radar research, in which capacity they got to know each other well. They planned to return to Cambridge University after the war. However, since the Gold and Bondi were at the time Austrian citizens (both were born in Vienna), they had first to spend one year in internment. So after the war, when they returned to Cambridge, they continued to meet and discuss the problems of physics. One of these problems was relativistic cosmology, a field with which none of them had previously been occupied but which they agreed was unsatisfactorily developed. Gold suggested that perhaps the universe was unchanging but dynamic. This thought, and the discussion that followed, may have been the beginning of steady state theory. At any rate, Gold’s idea required a creation of matter out of nothing, and in early 1948 Gold and Bondi, and independently, Hoyle worked out two versions of the steady-state theory.
The fundamental idea of their theory was that, because the observed expansion of the universe, it is necessary to compensate for the separating effect by the continuous formation of new galaxies and clusters of galaxies. The rate of formation just compensates for the effect of expansion and gives a stable situation. Bondi and Gold adopted the “perfect cosmological principle” that the large-scale features of the universe are the same not only for every location in space but for every instant in time. This leads immediately to a steady-state universe and it is immaterial whether observers compare their pictures “at the same time” or not. On the other hand, McCrea and Hoyle started with a mathematical definition of continuous creation within the framework of general relativity and then derived the steady-state solution as a consequence of the field equations. On this model, the large-scale properties of distant parts of the universe should be the same as those of nearer parts.
Fred Hoyle seems to be the person who introduced the term “Big Bang” into astronomy in a radio broadcast on the BBC in 1950, as a derisory name for the rival to his steady-state theory. Many astronomers don’t like the term, because it is in some ways misleading. It gives the impression of that universe began as a single huge explosion occurring in the middle of empty space, like a big firecracker or a large-scale nuclear bomb. But space is what expands and matter is carried along with the expanding space. At the beginning of the universe, there is no change of pressure forcing it to expand, and there were no sound waves to make the “bang” audible. The bomb analogy breaks down as a description of the beginning of the expanding universe. But though the purist may bemoan its misleading impression, we are stuck with the name Big Bang.
The steady state theory is no longer believed to hold. For all practical purposes, the steady state theory died in 1965, with the discovery of the cosmic microwave background radiation, which is the afterglow or “echo” of the titanic explosion of the Big Bang. Two researchers at Bell Labs in Crawford Hill, New Jersey, Arno Penzias and Robert Wilson, verified the existence of the background radiation predicted by the Big Bang theory. The Russian physicist, George Gamow, years earlier in the 1940s predicted that there might be a way to verify experimentally once and for all that the Big Bang actually took place. Gamow maintained that the original radiation left over from the Big Bang should still be circulating around the universe, although its temperature would be quite low after 10 to 20 billion years. He predicted that the “echo” from the Big Bang would be evenly distributed around the universe, so that it would appear the same no matter where we looked. His collaborators, Ralph Alpher and Robert Herman in 1948 published a paper in which they calculated the temperature to which the cosmic fireball would have had now cooled down: 5 degrees Kelvin; 5°K. [1]. In 1965 there was a spectacular verification of the Gamow-Alpher-Herman prediction of this “echo” or background radiation left by the original Big Bang. At Bell Telephone Laboratories in Holmdel, New Jersey, scientists had constructed a hugh radio antenna, the Holmdel Horn Antenna, that would relay mesages between the earth and communication satellites. When Penzias and Wilson set up their experiment, to measured the microwave radiation, they found a bothersome background radiation in the microwave region; there was an excess noise being picked up by their equipment. There shouldn’t be that much radiation. After adjusting their instruments and cleaning pigeon droppings from the 20 foot horn antenna used in some experiments with communication satellites, they could not get rid of the excess noise. No matter where the antenna was pointed, this strange radiation was being received. They even put their instruments on high-altitude jet airplanes and balloons to get rid of the interference from the earth, but this strange noise became stronger.
In January of 1965, when someone suggested that Robert Dicke at Princeton University might be able to explain the puzzling background radiation, they got in touch with the Princeton group that was planning to built the equipment needed to detect the cosmic background radiation predicted by the Big Bang theory. Penzias phoned Dicke, and the four members of the Princeton group made the half-hour drive to Crawford Hill to find out what was going on there. The Princeton team was excited about what Penzias and Wilson had found; it was in line with the predictions made by the Big Bang theory. And Penzias and Wilson were relieved to find out what was the explanation of the radio noise that they could not get rid of. When the scientists plotted the relation between the intensity of the radiation and the frequency, it resembled the curve predicted so many years earlier by Gamow and others. The measured temperature of 3 degrees Kelvin [1] was remarkably close to the original prediction of the temperature of the radiation from the cosmic fireball. They discovered, much to their delight, that this radiation was exactly the background radiation that had been predicted. And this 3 degree radiation is still the most conslusive evidence that the universe began with a cataclysmic explosion. With the confirmation of the predicted radiation, scientists became convinced that the Big Bang was “the answer” to the problem of the origin of universe. For the discovery of the radio signals remaining from the Big Bang. Penzia and Wilson received the 1978 Nobel Prize in Physics. When Alan Guth, the young elementary particle physicist who first proposed the inflationary universe model in 1980, was asked about its relation to the steady-state theory, his answer was, “What is the steady-state theory?”
George Gamow was born in Russia in the Ukraine, at Odessa, in 1904. After he moved permanently to the United States in mid-1930s, he always signed his letters to his friends, “Geo.,” an abbreviation that he was unshakably convinced was pronounced “Joe”; so he was “Joe” to a very large number of friends, until his death in 1968. Having lived through the turmoil of the revolution and the civil war in Russia, in 1922 Gamow enrolled at Novorossysky University but soon transferred to the University of Leningrad, where he stayed until 1928, gaining a Ph.D. and learning about Friedmann’s models of the universe from Friedmann himself. Once qualified, Gamow traveled to the University of Gottingen, then to the Institute of Theoretical Physics in Copenhagen, then Cavendish Laboratory at Cambridge, then back to Copenhagen. The three scientific centers that he visited in the years from 1928 to 1931 were at the heart of the revolution in physics then taking place, the discovery of quantum physics and the beginning of the application of the new theory to an understanding of atoms. Gamow learned his quantum physics from the pioneers. And during his visit to Gottingen he made the first of his major contributions to science, applying quantum theory to explain how an alpha particle could escape from an atomic nucleus.
In 1931, Gamow was called back to the USSR, where he was appointed Master of Research at the Academy of Sciences in Leningrad and Professor of Physics at Leningrad University. But his ebulient nature and independence of mind hardly suited him to a happy life under Stalin’s regime in the 1930s, and when he was allowed to attend a scientific conference in Brussels in 1933, he seized the opportunity not to return to the USSR, moving to George Washington University in Washington, D.C., where he was professor of physics from 1934 to 1956, and then to the University of Colorado in Boulder, where he stayed until his death in 1968.
During the Second World War and the development of the atomic or nuclear bomb, the next version of the Big Bang came into existence. George Gamow, who was one of the Manhattan Project scientists, became one of the persons who pushed his view of the origin of the universe into the forefront of science. Upon seeing the explosion of the atomic bomb, he drew an analogy between that explosion and the beginning of the universe. If the A-bomb can, in a hundred millionth of a second, creating elements still detected years later, why couldn’t a huge explosion at the beginning of time produce all the elements we have today? If the universe did come from a single point, using the equations of General Relativity, Gamow theorized that the nuclear reactions created during the explosion would create all the light elements like hydrogen and helium. And eventually, as the universe continued to cool down, the heavier elements would be produced as well. By making some adjustments to the mathematics that explains the density of the matter in the universe, he was able to produce data that agreed pretty close to what was observed.
During the 1940s, Gamow was joined at George Washington University by Ralph Alpher, a graduate student to whom Gamow assigned the task of working out the details of how more complex nuclei might have been built up from hydrogen (a process known as nucleosynthesis) in the Big Bang. The model they developed depended upon collisions between the particles in the cosmic soap of dense material in the first few minutes of the life of the universe. The calculations showed that it would be relatively easy for a proton (hydrogen nucleus) and a neutron to collide strongly enough to overcome the electrostatic repulsion and stick together to form the nucleus of deuterion, also known as heavy hydrogen. Another collison with a neutron would produce a nucleus of tritium, heavy-heavy hydrogen, containing one proton and two neutrons. But tritium is unstable, so that one of its neutrons soon spits out an electron and becomes a proton. The nucleus has now changed into one that corresponds to the isotope of helium, containing two protons and one neutron, and is called, for obvious reasons, helium-3. All it now needs is for another neutron to stick to the growing nucleus to make an alpha particle, the nucleus of helium-4 atom. Now these helium nuclei once formed could easily pick up the electrons they needed from the swarm of particles in the primeval soap. But here the model ran into a snag. The helium-4 nucleus, the alpha particle, is particularly stable. It is very disinclined either to break up into smaller components, or accept additional components and grow into something more complex. Worse, there is no naturally occurring element that has a nucleus containing five particles, and when such a nucleus is made artifically in the lab by bombarding helium-4 with neutrons, it immediately breaks down into helium-4 again. To get around this difficulty Gamow and Alpher had to speculate that a single helium-4 nucleus might occasionally be struck by simulaneously by two particles and capture them both to form a nucleus containing six particles. And if the two particles are helium-4 nuclei, a nucleus would be formed, containing eight particles, boron-8. Even if this happens, the same problem arises for the nucleus containing eight particles, which very quickly breaks down into alpha particles. And with the universe rapidly thinning out as it expands away from the superdense state of the Big Bang, by the time it had made the helium, the chance of a double collision of this kind is small and rapidly getting smaller. In the 1940s, although the prospect getting over these gaps by the capture of two particles at once seemed unlikely, there was just enough ignorance about the conditions of the early universe, and about the rates at which nuclear reactions might occur, to allow Gamow and Alpher to get away with the idea as a working hypothesis. After all, as Gamow used to tell anyone who was interested, the theory explained where all of the hydrogen and all of the helium in the universe came from, and that accounted for more than 99 percent of the matter visisble in the stars and galaxies. Even if the theory did not explain the synthesis of the heavy elements (to an astronomer anything except hydrogen and helium is a “heavy” element), they represent less than 1 percent of the matter in the universe.
The detailed calculations of how the nuclei can capture neutrons or protons (the numbers that come out of the calculations are called capture cross sections) formed the basis of Alpher’s Ph.D. thesis, submitted in 1948. Clearly it deserved a wider audience, and Gamow and Alpher wrote a paper for submission to the Physical Review. At this point, Gamow’s sense of fun overcame him, and he perpetrated his most famous scientific joke. He wrote later,
“It seemed unfair to the Greek alphabet to have the article signed by Alpher and Gamow only, and so the name of Dr. Hans A. Bethe (in absentia) was inserted in preparing the manuscript for print. Dr. Bethe, who received a copy of the manuscript, did not object.”
So the classic paper, in which the modern version of the Big Bang model first saw the light of day, appeared, on April 1, 1948 (a coincidence that delighted Gamow), under the names of Alpher, Bethe, and Gamow. To this day, it is known as the “alpha, beta, gamma” paper, a suitable reflection of the fact that it deals with the beginning of things, and the importance of particle physics to cosmology (alpha particles which are another names for helium nuclei, beta particles which are is another name for electrons, and gamma rays are the intense pulse of electromagnetic radiation, an energetic photons).
This early version of the Big Bang appeared in the same year, 1948, that Fred Hoyle, Tommy Gold, and Hermann Bondi came up with their idea of an expanding Steady State universe. Right through the 1950s and into the early 1960s the two rival concepts of the universe stirred debate among the experts, with Hoyle leading the Steady Staters and Gamow as the leading Big Banger in friendly rivalry. Ironically, it was to be Hoyle who was to show how to resolve the greatest difficulty with Gamow’s model of the Big Bang, finding a way to make the heavy elements inside the stars, once the initial job of cooking up helium in the Big Bang had been carried out.
An obvious requirement for the inventors of the Steady State theory was the need to find a way to synthesize elements in the stars. For, if there was no Big Bang, then the Steady Staters had to make the elements within the stars. And the problem began to be a pressing one for astronomers, not because of any interest in the Big Bang or Steady State models of the universe, but because their improving observations of stellar spectra increasingly showed, in the 1940s and 1950s, that different stars contained different amounts of the different elements. It might be speculated that the material from which the stars formed came from a Big Bang, or it may be speculated that the material to make new stars is being created continuously in the space between the stars. But when there is found that there are systematic differences in the composition of the stars, with some stars richer in the heavy elements than others, it might be suspected that some of those materials are being manufactured out of primordial materials (whatever that may have been) in side the stars themselves.
Fred Hoyle’s paper presented, for the first time, a clear exposition of the basic ideas of nucleosynthesis within the accepted framework and using the best information about nuclear reaction rates, cross sections, etc. As Gamow’s team struggled to find a way to make elements heavier than helium in the Big Bang, Hoyle struggled to find ways to make them within the stars. The key problem was how get past the instablity of the Boron-8 nucleus. The only way to do this was to invoke a triple collision, with three alpha particles colliding almost simultaneously to form a nucleus of carbon-12. Gamow couldn’t make this work in the Big Bang, because even in the few minutes after the moment of creation, matter in the universe was spread too thin, and the temperature was too low, for such collisions to happen often enough to produce the amount of heavy elements we see in the universe today. Inside the stars, however, it is both hot and dense for many millions of years, giving a much better opportunity for even relatively triple collisions to occur often enough to produce a significant amount of carbon-12.
The idea looked good, but it ran into problems rather like the problems Eddington encountered when the physicists told him the sun was not hot enough for hydrogen fusion to occur. When Ed Salpeter, a physicist visiting the Kellogg Lab in 1951, made the appropriate calculations, he found that the cross sections still were not big enough. Some carbon-12 could be made within the stars, but nowhere near enough. Now Hoyle made a dramatic contribution. He came to CalTech in 1953 convinced that all the heavy elements are made in stars. He had tackled the calculation from the other end, using the observed measurements (from spectroscopy) of the abundances of heavy elements in stars to deduce how fast the triple-collision reaction must proceed, and he found that it had to go much faster than Salpeter had calculated. So he predicted that the carbon-12 nucleus must be capable of existing in what is called an excited state, a state that has more energy than the minimum value appropriate for that nucleus. If such an excited state existed, with precisely the right amount of energy, then and only then could the collision of the three alpha particles together be encouraged to form carbon-12 nuclei sufficiently often to make all the carbon observed in the spectra of stars. The reaction is encouraged by a process called resonance between the energy state of the three alpha particles and the energy state of the carbon-12 nucleus, the resonance occurs only if the carbon-12 energy level is just right, which is how Hoyle was able to predict what the excited energy state of the carbon-12 must be.
Hoyle badgered the physicists at CalTech until a group of them went away to look for an excited state of carbon-12, using a reaction involving deuterion particles colliding with nitrogen-14 nuclei to create carbon-12 plus an alpha particle. And they found it, almost exactly where Hoyle had predicted. But this was still one step short of proving that the excited state of carbon-12 could be produced by the interaction of three alpha particles. Now Willy Fowler, the leader of the CalTech team investigating how star work, working with two Lauritsens, Charles and his son Thomas, and Charles Cook, manufactured “excited” carbon-12 from the decay of boron-12. They found that although some of the excited carbon-12 fell back into its minimum energy state (the ground state) and stayed as carbon-12, some of it broke up into three alpha particles. Other things being equal (and in this case are so), these kinds of nuclear reactions are reversible. Since excited carbon-12 can decay into three alpha particles, there is no doubt that three alpha particles can combine to form excited carbon-12. Here was proof that as well as burning hydrogen to make helium, stars could burn helium to make carbon. The helium-burning process explained how large stars, called red giants, are kept hot, and it got the astrophysicists over the hurdle of nuclear synthesis at element number eight. By looking at the information from stellar spectroscopy, Hoyle had correctly predicted what physicists would find in experiments in the lab here on earth. That gave them the confidence to continue their measurements of interaction rates in the lab, and use the information to calculate the whole chain of reactions needed to build up all the naturally occurring elements, in all their varieties of isotopes, in the stars.
Now it is easy to understand how all the elements are made in the stars. And it removed the major obstacle to accepting the hot Big Bang theory of the origin of the universe. All investigation, from Gamow onward, had shown that no elements heavier than helium could be made in the Big Bang. They had to be made somewhere else. The equations of stellar nucleosynthesis showed that everything heavier than helium could indeed be made in stars but the observed amount of helium in the universe could not; it must have been made somewhere else than in the stars. But Big Bang theory needed stellar nucleosynthesis; and stellar nucleosynthesis needs the Big Bang. Together, the combination of a hot Big Bang and nucleosynthesis inside the stars provided a beautiful, complete picture of where everything came from.
With his theory in hand, Gamow went on to popularize and publicize his ideas to the postwar population. Science writers and the general public quickly embraced his theory because it was easy to understand the analogy with the atomic bomb explosion. And as the popularity of the theory increased, it came to be accepted as fact rather than a theory. This was helped along by the publication of Gamow’s books One, Two, Three, Infinity, in which he presented the theory in the last chapter, and The Creation of the Universe, in which he presented his theory in non-technical language. Those who disagreed with Gamow dubbed his theory as “the Big Bang” as did Fred Hoyle, the physicist who later proposed the steady-state theory, who coined the phrase. The name was originally intended to poke fun at the Gamow’s concept of the origin of the universe, but it became the popular name for the cosmology that is widely accepted today. It was not long before scientists and theologians began to discuss the similarities between the Big Bang and the creation of the universe in the Bible. In 1951, Pope Pius XII made one of the first offical statements of the Roman Catholic Church regarding the Big Bang theory. He said, “Scientists are beginning to find the fingers of God in the creation of the universe.”
A new version of the bigbang model of the universe called the inflationary universe model provides a compelling explanation of why the density of the universe should exactly equals Friedmann’s critical value. Invented by Alan Guth in 1980, the inflationary model answers most of the previously unanswered questions of the original big bang model.
In the standard big bang model, the universe expands smoothly and adiabatically (where the temperature will drop due to expansion without loss of heat from the system) from the beginning onward. But in the inflanatory model there is a brief departure from the adiabatic expansion. A much faster, exponential expansion occurs about 10-35 and about 10-33 seconds after the big bang. After this inflationary expansion, the universe displays a high degree of homogeneity and isotropy.
Thus there are just three observations that confirm the Big Bang cosmology.
1. The first of these is that of the expansion of the universe, which was first discovered in 1929 by Edwin Hubble. He observed that distant galaxies are moving away from us, and moreover, the further away a galaxy is, the faster it is receding. This discovery is embodied in the equation called Hubble’s Law:
v = Hd,
where v is the velocity of recession of the galaxy,
H is a constant of proportionality known as Hubble’s Constant,
d is the distance to the galaxy,
where v is given in kilometers per second and d is in megaparsec (Mpc = 1 million par secs = 3.26 million light-years), We now know the value of Hubble’s Constant H is not strictly constant but is decreasing slowly with time. Its present value is little uncertain, owing to the difficulties that was found in measuring the galactic distances. The accepted range of values is
H = 100h km sec-1/Mpc,
where h is between 0.55 and 1.0. That is, the present accepted range of values for H is between 55 and 100 kilometers per second per megaparsecs in distance.
(A megaparsec has the value 1 Mpc = 3 × 106 light-years = 3 × 1024 cm.)
2. The second principal observation is that of the cosmological abundance of the light elements. In the late 1940s, George Gamow and his collaborators explained these observed abundancies in terms of an early universe which was very hot and very dense. The light elements, they proposed, were synthesized when the universe was at an absolute temperature of 109 °K (on the Kelvin temperature scale where 0°K is -273°C). This temperature is equivalent to a thermal energy 0.1 MeV. (It is often convient to express energy in electron volts (eV) where 1eV = 1.2 × 10 4 K.) This process is called “nucleosynthesis” and accounts for the formation of the light elements — heavy elements are formed much later inside stars and distributed throughout the universe by supernova explosions.
3. The third principal observation is that of the cosmic microwave background radiation, discovered in 1965 by Arno Penzias and Robert Wilson. This radiation, in which we are constantly bathed from all directions, is a “residue” of the original hot universe. However, it now has a temperature of only 2.7 °K, owing to the cooling of the expansion of the universe.
It was Einstein who should have predicted an expanding universe; that is, it implied that the universe is neither eternal or static. But he ignored this fact when he was unable to find a static cosmological solution to his general relativity equations, and he modified his equations by introducting a new term, a “cosmological constant “, into his equations. The cosmological constant term acted as a repulsive antigravity force that is not connected with the presence of matter in the universe: this corresponds to the energy in empty space, and, Einstein argued, exactly balances the gravitational attraction of all the matter in the universe. The net result of the constant is a static universe.
In 1922, the Russian mathematican and meteorologist Alexandre Friedmann (and much later in 1935 by the American physicist Howard Robertson and the British mathematican Arthur Walker, independently), working with Einstein’s unmodified equations, considered expanding cosmologies based on two assumptions:
(1) isotropic (that is, the universe looks the same in all directions), and
(2) homogeneous (that is, it looks the same from every point in the universe).
Although on the smallish scales the universe appears very different in different directions, on the very large length scales (much greater than the distances between galaxies), it is indeed remarkably uniform and isotropic. Distant galaxies are distributed more or less uniformly. But the cosmic microwave background radiation is uniform to within a few parts of 105, indicating that the universe is even more isotropic in the past.
From these two very simple assumptions about the universe alone, Friedman showed that the universe should not be expected to be static. But, in fact, in 1922, several years before Edwin Hubble’s discovery, Friedman predicted exactly what Hubble found! These two assumptions give rise to the so called Friedmann models which seems to describe our universe. The Friedmann models are of two types:
(1) If the average density of matter in the universe is less than a certain critical value, then the expansion of the universe will go on forever.
(2) If the average density of matter exceeds this critical value, then the gravity exerted by the matter will eventually stop the expansion and cause the universe to implode back on itself.
These Friedmann models form the basis for the standard Big Bang cosmology. Which one describes our universe depends upon the actual rate of expansion (H) and the density (ρ) neither of which is known with accuracy. However, despite their vastly different predictions for the eventual fate of the universe, these models do nevertheless paint a very similar picture of the early universe.
The expanding universe raises the question of whether the expansion will continue forever or will eventually end. In the framework of the Friedmann models, the answer depends upon
(1) how fast the universe is expanding, and
(2) how much matter there is.
If the mass/energy density of the universe is greater than a certain critical value, then gravitational attraction will eventually overcome the expansion of the universe and the universe will collapse. If, on the other hand, the density is less than the critical value, expansion will ad infinitum. The critical density is
ρc = 3H2/8πG =
2 × 10-29 h2 gm/cc =
1 × 104 h2 eV/cc,
and is equivalent to ten hydrogen atoms per cubic meter throughout the universe. Current observations suggested that the density of the universe is between 0.1ρc and 2ρc. So the fate of universe hangs in the balance, depending upon more accurate measurements.
General Relativity is above all about the geometry of the universe. If the density is greater than the critical density, then space (not space-time) is positively curved like the surface of a sphere, and the universe is said to be “closed”, expanding for a certain time before contracting. And if the density is less than critical value, then the space is negatively curved like a saddle, and the universe is said to be “open”, expanding forever. Finally, if the density just so happens to be exactly equal to critical density, then space is not curved at all but “flat” (but space-time is curved).