Big-Bang Nucleosynthesis: Thermonuclear History in the Early Universe

The origin of elements has been one of the most interesting issues in science, because it is closely connected with that of ourselves and the solar system. At present, this subject is considered to be related to the evolution of the universe. The Big Bang model, based on the theory of General Relativity, has proven to be the most reliable theory to describe the evolution of the universe. This is supported by the following observational facts: the expansion of the universe, the cosmic microwave background (CMB) radiation with a tiny fluctuation in temperature, and the primordial abundance of light elements, 4He, D, and 7Li. Among many kinds of books concerning the Big Bang theory, we present a book which includes a detailed description of Big Bang nucleosynthesis (BBN) whose theme has not been explained well so far. BBN is the interdisciplinary field which incorporates nuclear physics, elementary particle physics, and astronomy. In particular, it is indispensable to obtain important information on the earth, such as the results of experiments by particle accelerators and observational findings from stars and galaxies. Originally, nucleosynthesis in the early universe had been studied fully by nuclear (particle) physicists, which were based on experimental data of nuclear fusion and β-decays. The basic idea that the light elements could be produced during the BBN epoch – the first 3 min – still holds now. We are now in an amazing stage of development in modern cosmology. To derive matter density of the universe, we need to measure the mass of a typical galaxy and intergalactic distance. To this end, we climb up the cosmic distance ladder from the mean distance between the Earth and the Sun, 1.5 × 1013 cm, to the size of the large-scale structure of the universe, 1026 cm, which is an interesting and instructive subject on observational cosmology in its own right. Nevertheless, using calculated abundance of BBN, the matter density of baryons except dark matter is determined with great accuracy from the observed primordial abundance of D. In this book, we start to make introductory explanations of General Relativity and physical processes which are necessary to construct a model of the Big Bang universe. Moreover, to explain the consistency between the calculated and observed abundances of light elements, we must explore many physical possibilities. In particular, we emphasize the importance of the idea beyond the standard BBN, because fundamental questions of both dark matter and dark energy are not answered at all. This book is meant to become a survey of the state of physical cosmology, including observational and theoretical parts that establish the subject as a mature physical science and the more notable attempts to improve and extend the picture. The goal of this book is to present theoretical bases and applications of BBN. Readers can appreciate the current status of BBN and acquire techniques to calculate nucleosynthesis. As a result, they obtain a new idea for BBN in the framework of modern cosmology. The contents include a basic curriculum of physics, that is, mechanics, thermodynamics, quantum mechanics, and statistical physics. If a reader is unfamiliar with General Relativity, we recommend to skip complicated expressions, because he or she can comprehend the fundamental concept of BBN without the intricate formulae. Thus, most of this book is accessible to people who have studied physics to an advanced level at university; we hope that not only graduated students but also undergraduates would become familiar with BBN and receive inspiration from the early universe. March 2018"