Big Bang Nucleosynthesis and Relic Neutrinos

Abstract

The concerns of cosmology include the birth of the Universe, its evolution, its structure, and its ultimate fate. At present the most convincing theoretical paradigm in cosmology is the so-called Big Bang model, which attributes the origin of the Universe to an extremely hot and dense plasma that was highly isotropic and homogeneous. In this intriguing picture the evolution of the early Universe underwent a number of crucial events, such as the inflation, quark-baryon transition, neutrino decoupling, nucleosynthesis and photon decoupling. The Big Bang nucleosynthesis (BBN) has so far provided us with the most reliable probe of the early Universe based on the well-established knowledge of particle physics and nuclear physics. In this chapter we shall make use of the standard Big Bang model to describe the irreplaceable role of neutrinos in the history of the Universe. In particular, their contributions to the energy density of the Universe, their effects on the BBN and their properties as the cosmic neutrino background (CνB) will be discussed in some detail. Possible ways and experimental challenges to directly detect these Big Bang relic neutrinos will also be introduced.