In 1942 George Gamow began talking about the big-bang origin of the elements. In the 1948 paper that marks the beginning of big-bang nucleosynthesis (BBN), Gamow, his student Ralph Alpher, and Hans Bethe proposed that the periodic table was built up by neutron capture minutes after the big bang. Critical physics corrections made by Chushiro Hayashi, Enrico Fermi and Anthony Turkevich led to the seminal 1953 paper of Alpher, Robert Herman and James Follin that described correctly the big-bang synthesis of large amounts of 4He and little else. As Fermi and Turkevich had pointed out, Coulomb barriers and the lack of stable nuclei with mass 5 and 8 preclude significant nucleosynthesis beyond 4He. BBN required a hot beginning, and in 1949 Alpher and Herman predicted a 5 K temperature for the relic radiation now known as the Cosmic Microwave Background (CMB).

As the above diagram shows, within all theoretical and observational errors, the Big Bang Nucleosynthesis are impressively accurate. Except in the leftmost part, the pale blue strip indicating the observed value for helium-4 can hardly be distinguished from the theoretical curve. The green deuterium curve meets the pale green strip indicating the observed value almost exactly at the value indicated by the WMAP observations (vertical golden strip), and similarly the WMAP observations, the magenta helium-3 curve and the observed upper limit for helium-3 coincide very well. Only for lithium-7 is there an appreciable gap between prediction and observation though, given the uncertainties of determining the initial abundance of this element from observations, this discrepancy is likely to teach us more about stellar physics than about Big Bang Nucleosynthesis. All in all, this match between theory and observation constitutes one of the big successes of the standard models of cosmology.

The reason why something like inflation was needed in cosmology was highlighted by discussions of two key problems in the 1970s. The first of these is the horizon problem -- the puzzle that the Universe looks the same on opposite sides of the sky (opposite horizons) even though there has not been time since the Big Bang for light (or anything else) to travel across the Universe and back. So how do the opposite horizons "know" how to keep in step with each other? The second puzzle is called the flatness problem This is the puzzle that the spacetime of the Universe is very nearly flat, which means that the Universe sits just on the dividing line between eternal expansion and eventual recollapse.

The flatness problem can be understood in terms of the density of the Universe. The density parameter is a measure of the amount of gravitating material in the Universe, usually denoted by the Greek letter omega (O), and also known as the flatness parameter. It is defined in such a way that if spacetime is exactly flat then O = 1. Before the development of the idea of inflation, one of the great puzzles in cosmology was the fact that the actual density of the Universe today is very close to this critical value -- certainly within a factor of 10. This is curious because as the Universe expands away from the Big Bang the expansion will push the density parameter away from the critical value.

Inflation was proposed in January 1980 by Alan Guth as a mechanism for resolving these problems. At the same time, Alexei Starobinsky argued that quantum corrections to gravity would replace the initial singularity of the universe with an exponentially expanding deSitter phase. In October 1980 Demosthenes Kazanas suggested that exponential expansion could eliminate the particle horizon and perhaps solve the horizon problem, and Sato suggesting that an exponential expansion could eliminate domain walls (another kind of exotic relic.) In 1981 Einhorn and Sato published a model similar to Guth's and showed that it would resolve the puzzle of the magnetic monopole abundance in Grand Unified Theories. Like Guth, they concluded that such a model not only required fine tuning of the cosmological constant, but also would very likely lead to a much too granular universe, i.e., to large density variations resulting from bubble wall collisions.

Caption: The composite image on the left is of the galaxy cluster Abell 85, located about 740 million light years from Earth. The illustration on the right shows snapshots from a simulation, representing the growth of cosmic structure when the Universe was 0.9 billion, 3.2 billion and 13.7 billion years old (now).