Introduction

Neutrinos play a very important role in astrophysics. Due to their weaklyinteracting nature, they give information about the interior of stars,supernova explosions, the distant galaxies, the possible origin of thecosmic rays, etc. In Chapter 17, we have observed that astrophysical sitesare the major contributors to low energy neutrinos. In this chapter, wediscuss the importance of neutrinos in the creation of the chemical elements(see Figure 19.1) in the universe. We know that the universe started around13.8 billion years ago with a singularity, that is, Big Bang and since then,it has been expanding and becoming cooler. The very early universe consistedonly of radiation, which during the expansion and cooling phase gave rise toquark–antiquark and lepton–antilepton pairs. With further fallin 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% heliumnuclei, and traces of lithium nuclei. It should be noted that hydrogen isthe only element that was solely created during the Big Bangnucleosynthesis; all the other elements including helium and lithium aresynthesized by several processes as shown in Figure 19.1. Therefore, all thehydrogen in water molecules were produced during the first few minutes ofthe Big Bang.

The process of creation of new nuclei from pre-existing nucleons is known asnucleosynthesis. The nucleosynthesis of the lighter elements does notrequire the emission or absorption of neutrinos while all the elementsheavier than lithium require neutrinos directly or indirectly in theirsynthesis. The nucleosynthesis of intermediate and heavy elements require avery high temperature and pressure environment. Elements up to iron were/aresynthesized in the core of stars through the nuclear fusion reaction and itis believed that the heavier elements were synthesized outside the newlyformed neutron star in a core collapse supernova. Without neutrinos, wecannot think of energy from the stars. Moreover, neutrino properties figureprominently in many astrophysical environment. Neutrinos are involved indifferent types of nucleosynthesis processes like theν-process, νp-process,etc., in the creation of proton-rich nuclei as well as in the synthesis ofneutron-rich nuclei through the r-process and s-process.