Farewell to Reality

by Jim Baggott

  I am convinced that Mr Friedmann’s results are both correct and clarifying. They show that in addition to the static solutions to the field equations there are time varying solutions with a spatially symmetric structure.5

  But the retraction referred only to the mathematical rigour of Friedmann’s analysis. Einstein was convinced that the idea of an expanding universe had nothing to do with reality.

  Tragically, Friedmann died in 1925. His expanding-universe solutions were independently rediscovered two years later by Belgian theorist (and ordained priest) Georges Lemaître. But Lemaître published his results in French in a rather obscure Belgian journal, and they attracted little attention.

  Hubble’s law

  By 1931, everything had changed. Einstein was forced to accept that an expanding universe was not only possible, but appeared to describe the universe we inhabit. He publicly acknowledged that Friedmann and Lemaître had been right, and he had been wrong.

  What had convinced him were the results of a series of observations reported by American astronomer Edwin Hubble and his assistant Milton Humason. The results pointed to a relatively unambiguous conclusion: most of the galaxies we observe in the universe are moving away from us.

  Hubble had already radically transformed our understanding of the universe in the early 1920s. He had shown that what had appeared to be wispy patches of interstellar gas and dust — called nebulae — were in fact vast, distant galaxies of stars much like our own Milky Way. This revolutionary revision in our understanding that the Milky Way is but one of a huge number of galaxies not only greatly increased the size of the known universe; it also begged questions concerning a growing mystery surrounding their speeds.

  Starting in 1912, American astronomer Vesto Slipher at Lowell Observatory in Flagstaff, Arizona, had used the Doppler effect to investigate the speeds of what were then still judged to be nebulae. The technique works like this. When we receive a wave signal (light or sound) from a moving object, we find that as the object approaches, the waves become bunched and their pitch (frequency) is detected to be higher than the frequency that is actually emitted. As the object moves away, the waves become spread or stretched out, shifting the pitch to lower frequencies. The effect is familiar to anyone who has listened to the siren of an ambulance or police car as it speeds past.

  If we know the frequency that is emitted by the source, and we measure the frequency that is detected, then we can use the difference to calculate the speed at which the source is moving, towards or away from us, the receiver.

  Stars are composed mostly of hydrogen and helium atoms. Hydrogen atoms consist of a single proton orbited by a single electron. Depending on its energy, the electron wave particle may be present in any one of a number of ‘orbitals’ inside the atom, forming discrete shapes or clouds of probability relating to where the electron actually is. Each orbital has a characteristic, sharply defined energy. Consequently, when an excited electron releases energy in the form of a photon, the frequency of the photon lies in a narrow range, determined by the difference in the energies of the two orbitals involved.

  The result is an atomic spectrum, a sequence of ‘lines’ with each line representing the narrow range of radiation frequencies absorbed or emitted by the various electron orbital states inside the hydrogen atom. These frequencies are fixed by the energies of the orbitals and the physics of the absorption or emission processes. They can be measured on earth with great precision.

  But if the light emitted by a hydrogen atom in a star that sits in a distant galaxy is moving relative to our viewpoint on earth, then the spectral frequencies will be shifted by an amount that depends on the speed with which the galaxy is moving.

  In his observations, Slipher actually used two light frequencies characteristic of calcium atoms. He discovered that light from the Andromeda nebula (soon to be relabelled the Andromeda galaxy) is blueshifted (higher frequencies), suggesting that the galaxy is moving at high speed towards the Milky Way.* However, as he gathered more data on other galaxies, he found that most are redshifted (lower frequencies), suggesting that they are all moving away.

  Hubble and Humason now used the more powerful 100-inch telescope at Mount Wilson near Pasadena, California, to gather data on more galaxies. What they found was that the majority of galaxies are indeed receding from us. Hubble discovered, in what appears to be an almost absurdly simple relationship, that the speed at which each galaxy is receding is proportional to the galaxy’s distance. This is Hubble’s law.6

  The fact that most of the galaxies are receding from us does not place us in an especially privileged position at the centre of the universe. In an expanding universe it is spacetime that is doing the expanding, with every point in spacetime moving further away from every other point. The standard analogy is to think of the three-dimensional universe in terms of the two dimensions of the skin of a balloon. If we cover the deflated balloon with evenly spaced dots, then as the balloon is inflated the dots all move away from each other. And the further away they are, the faster they appear to be moving.

  Lemaître had actually predicted this kind of behaviour in 1927, and had even derived a version of Hubble’s law. But in an English translation of his 1927 paper that was published in 1931, all references to his derivation were inexplicably (and very carefully) excised.

  If the universe is expanding, then simple logic suggests that we can ‘wind the clock back’ and conclude that it must have had an origin at some point in time. In 1927, Lemaître had not speculated on such a moment of ‘creation’, but he now went on to envisage this to involve what he called a ‘primeval atom’: essentially all the material substance of the universe compressed into a single atomic structure. He then likened the creation of the expanding universe to the process of radioactive disintegration.

  There were plenty of problems still. Hubble’s original work suggested that the universe is younger than the estimated age of the earth. But these problems were eventually resolved in the 1950s, most notably by Allan Sandage, another Mount Wilson astronomer, who would eventually come to be seen as Hubble’s successor. Today, the value of the Hubble constant — the constant of proportionality between galactic speed and distance — is determined to be about 70 kilometres per second per megaparsec,* and the age of the universe is 13.7 billion years.

  The big bang

  Lemaître’s concept of a ‘primeval atom’ was obviously speculative. Indeed, it might be thought that any suggestion concerning a moment of creation extrapolated backwards from the large-scale structure of today’s universe would remain for ever beyond science and therefore intangible.

  But George Gamow thought differently. He realized that when aspects of atomic and nuclear physics are applied to the problem, the large-scale structure of today’s universe inherently limits the creation possibilities. It was possible to say an awful lot more about the early universe.

  The disintegration of Lemaître’s primeval atom could not explain the relative abundances of hydrogen, helium and the sprinkling of other atoms in the universe. An atom whose nucleus contained all the protons and neutrons of the universe would indeed be unstable and would decay rapidly, but most nuclear fission reactions involve the splitting of nuclei rather than their utter fragmentation into individual protons and neutrons. It seemed rather more logical to start with an early universe consisting of primordial protons, neutrons and electrons and apply the principles of nuclear physics to work out what would most likely happen next.

  With support from postgraduate student Ralph Alpher, Gamow did precisely this. Estimating the conditions likely to have prevailed in the first few minutes after the moment of creation, Alpher and Gamow successfully predicted the relative abundance of hydrogen and helium in today’s universe.7

  Predicting the abundance of hydrogen and helium in the universe may not sound like much of an achievement, but at this time the amount of helium was not precisely known, and this was big news. Through the work of Hubble and Humason, the evidence for
an expanding universe was becoming overwhelming. But, although an expanding universe would seem to imply an inevitable beginning, the simple fact that it is expanding cannot be taken as evidence that it expanded from some origin, or that this origin represents the creation of all material substance and radiation.

  Indeed, maverick English astronomer Fred Hoyle had developed an alternative explanation. Together with Austrian astronomers Thomas Gold and Hermann Bondi, Hoyle developed a ‘steady-state’ model of the universe in which the expansion of spacetime is eternal and new matter is constantly emerging from a hypothetical ‘C-field’, or creation-field, which pervades the universe.

  Now this might seem rather far-fetched, but creation of new matter at a rate of one atom per year in a volume of space equal to St Paul’s Cathedral in London is all that is required to maintain a universal steady state. Over time, the new matter would be drawn together by gravity, forming first gas clouds, then stars and galaxies.

  Hoyle rejected the theory that there could ever have been a ‘moment of creation’. In a radio programme broadcast by the BBC in 1949, he introduced a new term — ‘big bang’ — intended to disparage the idea:

  On scientific grounds this big bang hypothesis is much the less palatable of the two [i.e. less palatable compared to the steady-state model]. For it is an irrational process that cannot be described in scientific terms … On philosophical grounds too I cannot see any good reasons for preferring the big bang idea. Indeed it seems to me … a distinctly unsatisfactory notion, since it puts the basic assumption out of sight where it can never be challenged by a direct appeal to observation.8

  The importance of Alpher and Gamow’s work derives from the fact that this was the first attempt to describe the immediate aftermath of the big bang in scientific terms. The conclusion was that if the universe had begun in a big bang, then the operation of the principles of nuclear physics could explain the relative abundance of hydrogen and helium, which together constitute 99.99 per cent of all visible matter.

  The cosmic microwave background radiation

  Predicting the relative abundance of hydrogen and helium was an encouraging result, but not one that could be considered to constitute a proof of the big bang model. Hoyle, for one, was unimpressed.

  Alpher had become rather frustrated that the model he had developed with Gamow didn’t predict the synthesis of elements heavier than helium.* In the meantime, Gamow had forged ahead on other aspects of early post-big-bang physics. In the summer of 1948 he sent Alpher a manuscript of a paper he had recently submitted to the British journal Nature. The paper was concerned with the densities of matter and radiation up to the point of recombination.

  At the temperatures and pressures prevailing inside the primordial fireball, all material substance would be present in the form of hot plasma. Protons (hydrogen nuclei), clusters of two protons and two neutrons (helium nuclei) and electrons would move freely within such a plasma, exchanging electromagnetic radiation (photons) between them. As the universe expanded and cooled, the temperature would eventually drop to around 3,000°C, at which point recombination would occur. Electrons would be captured by the hydrogen and helium nuclei to form neutral atoms. After recombination, the hot radiation that had been exchanged between the constituents of the plasma would be released. The previously opaque universe would become transparent.

  Let there be light. Literally.

  Alpher and fellow physicist Robert Herman realized that Gamow’s estimates of the densities of matter and radiation were seriously wrong, and sent a telegram to Gamow at the US atomic weapons research laboratory at Los Alamos, where he was working through the summer months. Gamow judged that it was too late to retract his paper, and urged Alpher and Herman to submit a note correcting his error for publication in the same journal.

  Alpher and Herman went a little further. They took the opportunity in this short note to explain that the radiation released after recombination would have persisted to the present day. They estimated that this cosmic background radiation would have an average temperature just five degrees above absolute zero (5 kelvin). Although they didn’t say so, it was implicit in their proposal that the radiation would be present in the form of microwaves.

  Now this was an out-and-out prediction. Unlike the relative abundances of hydrogen and helium, the existence of cosmic microwave background radiation had not been anticipated. And this was a prediction that only a model based on a hot big bang could make.

  The prediction was largely ignored. In his BBC radio broadcast Hoyle had demanded a ‘direct appeal to observation’, of a kind of which he believed the big bang model to be incapable. Yet here was a simple test. Did the CMB radiation exist, or not?

  Alpher and Herman later explored possible reasons why their work did not attract much attention:

  There was the just mentioned age problem; by the 1950s new values of Hubble’s parameter had eliminated this. Also, some scientists had a predilection toward a steady-state universe. Finally, there was the above-mentioned view of Gamow’s work and, perhaps by association, our work [Gamow’s reputation for playfulness meant that his work wasn’t always taken seriously].9

  It is also a fact that whilst Alpher and Herman had worked in an academic institution, cosmology hadn’t yet established itself as an important scientific discipline, and neither physicist belonged to the recognized communities of astrophysicists and astronomers. By the time cosmology had become more fashionable, both Alpher and Herman had left academia to pursue careers in industrial science.

  Consequently, when in the summer of 1964 Princeton physicist Robert Dicke suggested: ‘Wouldn’t it be fun if someone looked for this radiation?’ he was unaware of Alpher and Herman’s earlier prediction.* Dicke had independently rediscovered the possible existence of the CMB and assigned the task of working out how to detect the radiation to two young radio astronomers, Peter Roll and David Wilkinson. Turning to Jim Peebles, a theorist from Manitoba, he said: ‘Why don’t you go and think about the theoretical implications?’10

  Peebles went home and thought about it. He reinvented the big bang model that Gamow, Alpher and Herman had developed and used it to predict a CMB radiation with a temperature about ten degrees above absolute zero. But when he submitted this work for publication, it was rejected, on the basis that Alpher, Herman and Gamow had already covered this ground some years before.

  When, in early 1965, Dicke, Peebles, Wilkinson and Roll assembled for a lunchtime meeting to discuss the design of an apparatus to detect the CMB radiation, Dicke was interrupted by a phone call from radio astronomer Arno Penzias at Bell Laboratories in Holmdel, New Jersey. Penzias had caught sight of a preliminary manuscript on the CMB radiation by Dicke and Peebles and realized that this was a solution to a problem that he and fellow Bell Labs radio astronomer Robert Wilson had been puzzling over for some months. Dicke put the phone down. ‘Well, boys,’ he said. ‘We’ve been scooped!’11

  Dicke, Roll and Wilkinson piled into a car and drove the thirty-odd miles to Holmdel. They found that Penzias and Wilson, accomplished radio astronomers, had constructed a highly sensitive radiowave detector with a twenty-foot horn antenna. However, when they had first switched it on, they had picked up a persistent and annoying hiss of microwave radiation that appeared to come uniformly from all directions in the sky. It was only when Penzias had seen Dicke and Peeble’s manuscript at the beginning of 1965 that the penny dropped.

  The radiation that Penzias and Wilson had stumbled on quite by accident fitted closely with the Princeton physicists’ predictions, indicating a radiation temperature of about 3 kelvin. Penzias and Wilson and the Princeton group published companion papers announcing the discovery in May 1965.

  The big bang was no longer idle theoretical speculation. It was observational fact.

  The flatness and horizon problems

  The discovery of the CMB radiation was a triumph for big bang cosmology, and the alternative steady-state model disappeared from discussion almost overnight.
We now knew that the universe had ‘begun’ in a tiny cosmic fireball which had expanded to form the universe we can observe today. It was, admittedly, a complex creation story, involving aspects of quantum physics, nuclear physics, the dynamics of gas clouds, star formation, stellar nucleosynthesis, galaxy and galactic cluster formation, supernovae and the formation of heavier elements, more gas clouds, more stars (this time with planetary systems), earth, life and us. It was obvious that there were still some gaps to fill.

  But the big bang model also had some problems. Einstein had based his initial calculations on the perfectly reasonable assumptions that the universe is more or less the same in all directions (it is ‘isotropic’), and that the stars and galaxies are not vastly different from one another in composition (the universe is homogeneous). What’s more, the universe that we observe today is ‘flat’, meaning that there are three spatial dimensions, in which Euclidean geometry applies — the angles of a triangle add up to 1800, the square of the hypotenuse of a right-angled triangle is equal to the sum of the squares of the other two sides (Pythagoras’ theorem), the ratio of the area of a circle and the square of its radius is equal to π and so on.

  This might seem a statement of the blindingly obvious, but the ‘flatness’ of space depends critically on the density of mass-energy in the universe.

  The problem was that, in order to create a perfectly flat universe, with a density parameter Ω of precisely 1, the conditions that prevailed during the big bang would have had to have been rather special, and extremely fine-tuned. Dicke had first raised his hand to declare that this was a problem in the 1960s, and in 1979 he and Peebles published an important paper on the subject. Physicists grow increasingly nervous when confronted with inexplicable coincidences, as these are often open to misinterpretation as evidence of design.*