The second plank of evidence for the Big Bang model lies in the area of nucleosynthesis. In English, this means that the abundance of the elements seen in the universe at large matches what one would expect in a universe that began in a state that was almost infinitely hot and infinitely dense.
The great Russian theoretician George Gamow was one of the first to take Lemaitre’s prediction of an early universe the size of an atom seriously (see post below). However, Gamow went even further. A specialist in nuclear physics, he calculated that the temperatures in the early universe would have been too high for atoms to form. Instead, matter would have existed as elementary particles, only gradually forming atoms as the universe expanded and cooled in the first few minutes.
Applying simple mathematics to the expanding universe led to a surprising result – that the matter of our universe should be composed almost entirely of the simplest two elements, Hydrogen and Helium (specifically, about 75% H and about 25% He). This prediction turned out to be correct – all the other elements of the Periodic Table account for about 0.1% of the matter of the universe!
The irrepressible George Gamow
This was an impressive early victory for the Big Bang model – however, it failed to explain where the other elements came from (Gamow’s own belief that the other elements are also made in the Big Bang was soon shown to be false, as the universe cools too quickly). It was later shown that all the other elements are ‘cooked in the stars’.
Essentially, what happens is this: a young star burns Hydrogen as fuel, fusing it into Helium, a process that balances the intense inward gravitational force on the star. As the star ages, it runs out of fuel to burn, becomes denser and the temperature increases. Helium then fuses into Lithium, Lithium into the next element, and so on. This process continues right down to Iron. At this point, some stars lose the battle with gravity, turning into neutron stars or even black holes.
Others have a different fate. By a process of accretion of a nearby star, they go supernova – a cataclsmic process at extreme temperatures during which all the heavy elements of the Periodic Table are fused in turn, and then all spat out in one great explosion…
And that’s where you and I, and all the little children and everything we see about us comes from….stardust!
Note: an interesting sidenote is that the formation of carbon in the fusion chain of stars was long a stumbling block. The puzzle was eventually solved by the British astrophysicist Fred Hoyle, by postulating the existence of a new, unkown isotope of carbon. He badgered an American experimental group to look for this isotope and they soon found it. The riddle of the formation of heavier elements was solved….but Hoyle was to become a controversial figure despite this great success. He was bitterly opposed to the Big Bang model on philosophical grounds – as the evidence mounted, he concocted ever more convoluted alternative models, refusing to concede defeat right up until his death. More on this later…
Note 2: the above is a simplified version of Big Bang nucleosynthesis. In fact, traces of Lithium and Berellium were also formed in the Bang, you can read the details here