Neutrinos play a very important role in astrophysics. Due to their weakly interacting nature, they give information about the interior of stars, supernova explosions, the distant galaxies, the possible origin of the cosmic rays, etc. In Chapter 17, we have observed that astrophysical sites are the major contributors to low energy neutrinos. In this chapter, we discuss the importance of neutrinos in the creation of the chemical elements (see Figure 19.1) in the universe. We know that the universe started around 13.8 billion years ago with a singularity, that is, Big Bang and since then, it has been expanding and becoming cooler. The very early universe consisted only of radiation, which during the expansion and cooling phase gave rise to quark–antiquark and lepton–antilepton pairs. With further fall in temperature, the quarks combined to form
nucleons, which in turn fused to form the lighter elements like hydrogen, helium, and lithium, the first ever nuclei created in the universe. Thus, the early universe consisted of about 75% hydrogen nuclei, 25% helium nuclei, and traces of lithium nuclei. It should be noted that hydrogen is the only element that was solely created during the Big Bang nucleosynthesis; all the other elements including helium and lithium are synthesized by several processes as shown in Figure 19.1. Therefore, all the hydrogen in water molecules were produced during the first few minutes of the Big Bang.
The process of creation of new nuclei from pre-existing nucleons is known as nucleosynthesis. The nucleosynthesis of the lighter elements does not require the emission or absorption of neutrinos while all the elements heavier than lithium require neutrinos directly or indirectly in their synthesis. The nucleosynthesis of intermediate and heavy elements require a very high temperature and pressure environment. Elements up to iron were/are synthesized in the core of stars through the nuclear fusion reaction and it is believed that the heavier elements were synthesized outside the newly formed neutron star in a core collapse supernova. Without neutrinos, we cannot think of energy from the stars. Moreover, neutrino properties figure prominently in many astrophysical environment. Neutrinos are involved in different types of nucleosynthesis processes like the ν-process, νp-process, etc., in the creation of proton-rich nuclei as well as in the synthesis of neutron-rich nuclei through the r-process and s-process.