The term nucleosynthesis refers to the formation of heavy elements, atomic nuclei with many protons and neutrons, from the collision of light elements. The Big Bang theory predicts that the early universe was a very hot place. One second after the Big Bang, the temperature of the universe was roughly 10 billion degrees and was filled with a sea of neutrons, protons, electrons, anti-electrons (positrons), photons and neutrinos. As the universe cooled, the neutrons either decayed into protons and electrons or combined with protons to make deuterium.
During the first three minutes of the universe, most of the deuterium combined to make helium. Trace amounts of lithium were also produced at this time. The predicted abundances of deuterium, helium and lithium depend on the density of ordinary matter in the early universe: if the density of the ordinary matter is roughly 3% of the critical density, then the theory correctly predicts the abundances of these ``light'' elements.
The MAP satellite (microwave anisotropy probe) should be able to directly measure this matter density and compare the observed value to the predictions of Big Bang nucleosynthesis. This will be an important test of the model.
NUCLEOSYNTHESIS, the formation of atomic nuclei, started instants after the big bang, as the universe cooled, when the fundamental particles called free quarks (a) condensed into protons and neutrons (b). Protons (red) and neutrons (blue) paired off to form deuterons, but because the former outnumber the latter, most of the protons remained alone and became hydrogen nuclei (c). Almost all the deuterons in turn combined to form helium nuclei (d), leaving a tiny remnant to be detected today.
Image: Ian Worpole
Elements heavier than lithium are all synthesized in stars. During the late stages of stellar evolution, massive stars burn helium to carbon, oxygen, silicon, sulfur, and iron. Elements heavier than iron are produced in two ways: in the outer envelopes of supergiant stars and in the explosion of a supernovae.