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The Big Bang


The next major advance occurred soon after the Second World War when George Gamow realised that, in an expanding Universe, the early stages must have been very hot indeed – the temperature was so high that the dynamics of the expansion were dominated by the energy density of thermal radiation rather than by its matter content, in other words, the Universe was radiation-dominated. Following an earlier suggestion of Lemaître, he attempted to explain the origin of the chemical elements by primordial nucleosynthesis, that is, by nuclear fusion processes as the Universe cooled down from its very hot initial stages. The reasons for adopting this picture were twofold. Firstly, following the work of Cecilia Payne, the abundances of the chemical elements in stars seemed to be remarkably uniform and secondly it was thought that the central temperatures of the stars were not high enough for nucle-osynthesis to take place. Gamow’s programme was not successful because of the problem of synthesising elements heavier than helium – there are no stable isotopes
with atomic mass numbers 5 and 8. Therefore, in the short time-scales available
in the hot early phases of the expansion, there was not time to synthesis elements heavier that helium. Gamow’s coworkers Ralph Alpher and Robert Herman showed that only deuterium, helium-3 and helium-4 were created in significant quantities(Alpher and Herman, 195).

n the course of their calculations, Alpher and Herman worked out the thermal
history of the Universe in some detail and predicted that there should be present in the Universe today a diffuse background of black-body radiation with temperature about 5 K, the cooled remnant of its very hot early phases (Alpher and Herman, 1948). The detection of this background radiation was far beyond the capabilities of the technology of the 1940s and the lack of success of Gamow’s programme of primordial nucleosynthesis resulted in the neglect of this key prediction for many years. Furthermore, in the 1950s, Fred Hoyle discovered the triple-α resonance, which leads to the formation of carbon from three helium nuclei (Hoyle, 1954). Soon after, he and his colleagues, Margaret Burbidge, Geoffrey Burbidge and William Fowler, showed how the heavy elements could be accounted for by nucleosynthesis in stars(Fig. 1) (Burbidge et al., 1957).

Fig. 1.6. The evolution of the fraction (by number) of the light nuclei in a radiation-dominated Universe, according to calculations by Fermi and Turkevich and published by Alpher and Herman in 1950 (Alpher and Herman, 1950). The models began with 100% of the material in the form of neutrons. The tritium 3 H and neutrons shown surviving to 2000 seconds decay radioactively with half-lives of 12.46 years and 10.25 minutes respectively

Interest in what is now referred to as the Big Bang model of the Universe grew
steadily through the 1950s and early 1960s as evidence was found for cosmological evolutionary effects in the distribution of faint radio sources (Ryle, 1955, 1958). On the theoretical side, interest was rekindled in the question of the synthesis of elements in the early Universe, not now with a view to creating all the elements, but rather to account for the cosmic abundance of helium. By 1964, it was appreciated that, wherever helium could be observed in the Universe, it is present with a very high chemical abundance, about 24% by mass. This figure far exceeded what could be explained by stellar nucleosynthesis. I remember vividly attending a course of post-graduate lectures given by Fred Hoyle in Cambridge in 1964 entitled Problems of Extragalactic Astrophysics in which this problem was discussed. During the lecture course, Hoyle, Roger Tayler, and John Faulkner carried out detailed computations of the expected abundance of helium produced by primordial nucleosynthesis. Within a week of the topic being raised, they had shown that about 23 to 25% of helium by
mass is created by this process and that the percentage is remarkably independent of the precise initial conditions. The paper by Hoyle and Tayler was published in Nature in 1964 (Hoyle and Tayler, 1964). Subsequent more detailed calculations by Robert Wagoner, Fowler and Hoyle confirmed these conclusions and suggested that other elements which are difficult to account for by stellar nucleosynthesis, the light isotope of helium, 3 He, deuterium, D, and lithium, 7 Li, could also be accounted for in this way (Wagoner et al., 1967). Equally important, the success of these computations resulted in an upper limit to the mean baryon mass density of the Universe of about one tenth the critical density – if the density were any higher, less than the observed abundances of deuterium and helium-3 would be created primordially.

By the early 1960s, as the sensitivity of receivers for centimetre wavelengths
improved, it became feasible to search for the cool background radiation left over from the early stages of the Big Bang. The predicted remnant of the Big Bang was discovered, more or less by accident, by Arno Penzias and Robert Wilson in 1965 (Penzias and Wilson, 1965). The Cosmic Microwave Background Radiation was the second key discovery of twentieth century observational cosmology. Observations by the Cosmic Background Explorer (COBE), launched in 1989, showed that, away from the Galactic plane, the radiation is uniform over the sky to better than one part in 100,000 on angular scales greater than 7 ◦ and that its spectrum is of black-bod form with a quite remarkable precision (Smoot et al., 1992; Fixsen et al., 1996). These observations provided compelling evidence that our Universe went through a very hot, dense phase when the matter and radiation were in thermal equilibrium in its early stages.

The upshot of these discoveries was that there were four independent pieces of
evidence for the Big Bang picture of the origin and evolution of our Universe. Firstly, the expansion of the distribution of galaxies discovered by Hubble; secondly, the black-body spectrum and isotropy of the Cosmic Microwave Background Radiation; thirdly, the formation of the light elements by primordial nucleosynthesis; and fourthly, the fact that the ages of the oldest stars and nucleochronology ages were of the same order as the expansion age of the Universe. Thus, the Big Bang provided a natural framework within which to tackle the problems of galaxy and structure formation.

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