Creation of Light Elements The Universe consists mostly of hydrogen, which makes up almost three-quarters of all conventional (baryonic) mass. It also contains about 24% by weight of Helium-4 (helium atoms with nuclei that consist of two protons and two neutrons), and smaller amounts of Deuterium (Hydrogen-2), Helium-3 and Lithium-6. These are thought to have been manufactured early in the Big Bang, when the Universe was between about one second and 100 seconds old. At that time, the temperature of the Universe would have approached a thousand million degrees, and the Universe would have been so dense that nuclear reactions would have been common. These would have fused almost all the neutrons available in the early Universe with protons to make nuclei of Helium-4, with just a few neutrons left over to make Deuterium and Helium-3. Some of the these lighter elements would have been combined to make most of the Lithium-7 seen in the Universe today. However, this mechanism would not have been able to make heavier elements, most of which are known to have been made in the stars that formed later. Agreement between theory and observations Calculations of light-element production agree very well with the astrophysical observations, as shown in the picture on the right. It can be seen that the observations agree with the calculations, if the total amount of conventional matter in the Universe is a few per cent of the critical density required to reverse the expansion of the Universe, as shown by the vertical pink stripe. This estimate agrees, to within a factor of about two, with an entirely independent estimate based on observations of the cosmic background radiation. The agreement between the two estimates is a powerful confirmation of the Big Bang model. In order to calculate accurately the amounts of the different light elements, it is necessary to know how quickly the Universe was expanding when it was very young, which depends on the number of different types of particles present at the time. In particular, the light-element calculations are sensitive to the total number of different types of light neutrinos. Experiments at the LEP particle accelerator showed that there are just three different types of neutrino, in which case the light-element calculations work particularly well. This success provides further confirmation of the Big Bang model.

The top panel shows the amount of Helium-4, the middle panel the amounts of Deuterium and Helium-3, and the bottom panel the amount of Lithium-7, all as functions of the ratio of matter to all particles in the Universe. The theoretical calulations are shown with yellow bands to indicate their uncertainties. The dashed boxes indicate the ranges observed by astrophysicists.