In Chapter 2, we discussed the very reliable observations that show that the universe is expanding at the present era. The current expansion of the universe represents the first argument in favour of the big bang model, although it is not proof by itself, as explained in Figure 4.3. Two other important observations support the model. The first is the relative abundances of hydrogen and helium in the universe. The second is the existence of cosmic radiation at the present era.

The amount of helium that was produced during the nucleosynthesis era of the big bang was determined by the relative numbers of neutrons and protons that were present at that time. These particles were formed shortly after the quark era, when the quarks became confined by the strong force. The universe was about 10−4 seconds old when this occurred. It took another three minutes or so for the universe to cool sufficiently for the synthesis of helium to be completed.

Thus, the neutrons and protons had to wait before they could begin to produce atomic nuclei. Because the mass of the neutron is slightly higher than that of the proton, the neutron has slightly more energy, and a free neutron may decay into a proton. What happens is that a down quark changes into an up quark, and this transformation is made possible by the weak interaction.