Final Things

Abstract

In the course of last 19 chapters, we have attempted to tie together many diverse observations and facts about the Universe into a self-consistent picture, within the context of the standard Big Bang scenario. It is, however, incomplete in a number of important ways. A useful comparison may be drawn with the history of our understanding of the origin of the chemical elements, a pleasant analogy drawn to my attention by Dr. Martin Harwit. In the 1930s, the origin of the chemical elements was a mystery. The tools were not available to understand the physical processes by which the synthesis of the chemical elements could have taken place. They might well have been laid down primordially by processes which took place in the inaccessible early Universe. The picture changed dramatically as the role of nuclear processes and the physics of the early Universe became better understood. One of the key events was the discovery of the triple-α resonance by Hoyle in 1953, which showed how the barriers to the synthesis of carbon from three helium nuclei in the central regions of stars could be overcome. Over the succeeding years, Burbidge, Burbidge, Fowler and Hoyle and Cameron elucidated the processes by which the chemical elements could be synthesised in stars. There remained the problem of the synthesis of light elements, such as helium and deuterium, but this was resolved with the realisation that they could be synthesised by non-equilibrium processes in first few minutes of the Big Bang, a topic dealt with in some detail in Chap. 10. It is now univerally accepted that there is no need for the chemical abundances of the elements to be laid down primordially - they can be accounted for by physical processes which take place naturally in the course of primordial and stellar evolution. The analogy with the problems of contemporary astrophysical cosmology is that it remains to be seen what parts of the most promising scenarios for the origin of galaxies and the large-scale structure of the Universe will be explainable by physical processes which have not yet been established by laboratory experiments, which can be established convincingly by astronomical observations, and which may remain inaccessible to us. To put it another way, what is the minimum set of axioms needed in order to account for the Universe as we know it today? As we will discuss, it might turn out that some parts of the story are simply beyond what can be treated by the physical tools we are likely to possess and then some reasonable set of initial conditions would have to be adopted. On the other hand, optimists, such as the present author, believe that precise astronomical observations and their judicious interpretation are likely to be very fruitful routes to understanding physical processes which cannot be reproduced in the laboratory. Indeed, this is a belief which is held fervently by many of us.

In this final chapter, some of these uncharted areas are surveyed. We begin with topics which are well within current and future observational capababil-ities, and then deal with deeper and more difficult issues. Specifically, these topics are:

• Can the vast range of observations of galaxies throughout the redshift interval 0 < z < 5 be reconciled on an empirical basis with some overall scenario for the formation of galaxies and larger-scale structures?.

• What is the origin of the rotation of galaxies and their magnetic fields?.

• What are the essential features of theories of the very early Universe required by our analysis of the problems of astrophysical cosmology and the formation of galaxies?

These are somewhat diverse topics, but they have been lurking in the background of the astrophysical discussions of much of this book.