‘What exactly does the Big Bang theory explain? A theory should be able to explain phenomenon, otherwise it is a conjecture only.’
‘The most important contribution of Big Bang theory is interpreting the abundance of different elements in the universe.’
One of the most important weak interactions is the merger of a proton and an electron to yield a neutron and an antineutrino. Neutrons formed prolifically by this reaction and attained an abundance comparable to that of protons. However, a large number of electrons are required for neutron formation to proceed, and this favorable situation rather abruptly changed when the universe was about one second old. At this stage, the temperature dropped below 1 million electron volts, or 10 billion degrees Kelvin. It requires an energy of about 1 million electron volts to create a new electron-positron pair. Creation ceased, and the electron-positron pairs annihilated. The neutron-creating reactions stopped, but a considerable number of neutrons remained. There was in fact just one neutron for every six protons. This ratio of one neutron to six protons depends largely on the mass difference between the neutron and the proton, and it relies on the competition between two rates: the weak nuclear interactions and the expansion. Applying Einstein’s famous E=mc2 relation, we can think of this mass difference as an energy. The neutron is more massive than the proton by about 1.3 million electron volts. When the temperature of the universe, of the energy of any particle, drops below this typical energy, reactions involving the heavier neutrons are suppressed and the neutron-proton ratio is frozen out: neutrons are neither produced by weak interactions nor destroyed, except by decay of free neutrons.
The neutrons were the vital component in the next sequence of events. Free neutrons are unstable particles. A free neutron will spontaneously disintegrate in about eleven minutes. The role of neutrons is also essential in thermonuclear fission and fusion. The combining of neutrons with nuclei results in more massive and stable nuclei. Because of the strong nuclear bonding forces between neutrons and protons, there is a very small reduction in mass of the new nucleus relative to that of its constituent parts. The energy that is bound up in holding the nucleus together reveals itself as a mass deficiency and will be released when the nucleus is synthesized. In this way, an enormous amount of energy can be released. (Similarly, in a fusion bomb, hydrogen is converted to helium, and the tiny difference in mass between one helium nucleus and four protons provides the source of energy.) We describe this process, whereby heavier elements are synthesized from lighter elements, often accompanied by release of energy, as nucleosynthesis.