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Preface to the First Edition

The progress of modern cosmology has been guided by both observational and theoretical advances. The subject really took off in 1917 with a paper by Albert Einstein that attempted the ambitious task of describing the universe by means of a simplified mathematical model. Five years later Alexander Friedmann constructed models of the expanding universe that had their origin in a big bang. These theoretical investigations were followed in 1929 by the pioneering work on nebular redshifts by Edwin Hubble and Milton Humason, which provided the observational foundations of present-day cosmology. In 1948 the steady state theory of Hermann Bondi, Thomas Gold, and Fred Hoye added a spice of controversy that led to many observational tests, essential for the healthy growth of the subject as a branch of science. Then in 1965 Arno Penzias and Robert Wilson discovered the microwave background, which not only revived George Gamow's concept of the hot big bang proposed nearly two decades before, but also prompted even more daring speculations about the early history of the universe.

The landmarks mentioned above have led to many popular and technical books on cosmology. In particular, the rapid growth of interest in the areas of general relativity and cosmology during the 1970s was reflected in a number of classic textbooks that came out in the early 1970s. The purpose of the present textbook is to introduce the reader to the state of the subject in the early 1980s. However, the approach adopted here is different from that found in most other texts on the subject, and it is perhaps desirable to state what the differences are and why they have been introduced.

For example, it is usual to find cosmology appearing at the end of a text on general relativity, introduced more as an appendage than as a subject in its own right. Perhaps this is one reason why cosmology still stands apart from the rest of astronomy, to which it really belongs. The astronomer tends to regard cosmology as a playground for general relativists rather than as a logical extension of extragalactic astronomy. To correct this tendency, the relative importance of cosmology and general relativity has been inverted in this text. Chapter 2 introduces general relativity more as a tool for studying cosmology than as a subject in its own right. Thus the relativist may find many topics dealt with at a superficial level or not at all. This chapter covers only those topics that are really necessary for understanding the large-scale geometrical properties of the universe. I have taken this approach in the hope that the relatively elementary treatment of general relativity will not put a newcomer off, as a more exhaustive treatment might well do. The expert relativist may skip this chapter and refer to it only for fixing the notation.

Chapters 3 and 4 introduce the standard models of cosmology as solutions of Einstein's equations. The tools developed in Chapter 2 will be found applicable here, and the reader will find the pace more relaxed than in Chapter 2.

Chapters 5, 6, and 7 concentrate on the physical aspects of standard cosmology. Gamow's idea of primordial nucleosynthesis, the current state of ignorance on galaxy formation, the properties of the microwave background, and the various recent contributions of particle physics to our understanding of the early universe are discussed here.

Perhaps this would have been the appropriate stage to move on to observational cosmology. However, I felt that the reader should also be taken on a short excursion into nonstandard cosmology. Contrary to the view propagated (unfortunately) by many experts in cosmology today, the subject is not a closed book, nor is standard cosmology the only answer to the problem of the origin and the evolution of the universe. Part III of this book introduces some alternatives to the standard models.

Although some readers may prefer to see an observational test discussed immediately after the theoretical prediction, I have left observations to the last part of the book. This approach has made an overall assessment of the various models possible. A survey of cosmological observations shows how better techniques and a better appreciation of errors and uncertainties have led to frequent reassessments (a classic example being the value of Hubble's constant, which is still uncertain!). I have therefore not gone into very many observational details, but have emphasized how the observations are made and the likely sources of errors. In any case it would be unwise to go into too many details in an introductory text.

In spite of many remarkable advances, cosmology is still very much an open subject. On the observational side, the launching of the space telescope in the mid-1980s is likely to revolutionize our view of the universe. On the theoretical side, the Grand Unified Theories (GUTs) are still grappling with the problem of the early universe, while quantum cosmology is in a rudimentary state. Cosmologists have yet to appreciate the problems posed by life in the universe. How did life come into existence? Is it confined to the Earth or is it widespread in the universe? A text of the future may well devote a large part of its discussion on cosmology to contributions from biology.

It is assumed that the reader is familiar with standard mathematical methods like differential equations, vector analysis, Fourier series and transforms, the calculus of variations, and so on. A knowledge of basic physics including mechanics, elementary thermodynamics, electromagnetic theory, atomic structure, and fluid dynamics is also assumed. Similarly, basic knowledge of elementary astronomy will be useful. The text is intended for advanced undergraduates, graduate students, and teachers of astronomy and cosmology.

This book contains over 400 exercises, of which over 80 percent are of a computational nature. Many of them are designed to illustrate or amplify the material described in the text. It is hoped that they serve their intended purpose.

I thank Art Bartlett for encouraging me to write the book. Comments received from Bob Gould, Bob Wagoner, Dimitri Mihalas, Richard Bowers, and Geoff Burbidge were of great help during the preparation of the manuscript. Last, but not least, it was Fred Hoyle who introduced me to the fascinating field of cosmology as a graduate student, and I am indebted to him for agreeing to write the Foreword.

I began writing this book while visiting the Department of Applied Mathematics and Astronomy at the University College, Cardiff, Wales. I am grateful to the head of the department, Chandra Wickramasinghe, for the facilities extended to me at Cardiff. For the prompt typing of the manuscript I am indebted to Ms Suzanne Ball and Mr P. Joseph. It is also a pleasure to acknowledge the help I received from the Drawing Office and Xerox Facility of the Tata Institute of Fundamental Research.

Bombay, India Jayant Narlikar

Preface to the Second Edition

I am happy that the revised second edition of Introduction to Cosmology is seeing the light of the day. The motivation and format of this edition continue to be the same as for the earlier edition and hence this preface only supplements the more detailed preface of the first edition given above.

The changes incorporated in this edition broadly reflect the new developments in cosmology that came in the 1980s, e.g. inputs from particle physics including the inflationary universe, new attempts at structure formation, recent observations of the large-scale structure and the improved (more sensitive) limits on the intensity fluctuations of the microwave background. The observational sections have been updated although no text book can really keep pace with the rapid advances in cosmological observations.

A comparison of the two editions will reveal a slight rearrangement of the chapters including a streamlining of the part devoted to alternative cosmologies. The final chapter is perhaps more critical of standard cosmology than before. This is necessary, in my opinion, in order to correct the prevailing impression that the standard hot big bang model describes the universe so well that no significant new or alternative inputs are required.

I thank Simon Mitton for encouraging me to proceed with the job of revising the book for Cambridge University Press. Thanks to speedy typing by Santosh Khadilkar and help with artwork by Arvind Paranjpye, the job could be completed within the time frame set by Simon. I also thank the numerous reviewers of the first edition whose constructive comments helped in preparing the revised manuscript.

Inter-University Centre for
Astronomy and Astrophysics
Pune, India
Jayant V. Narlikar

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