Steven Weinberg

The FirstThreeMinutesA modem viewof the origin ofthe universe

FLAMINGOPublished by Fontana Paperbacks

Contents

Preface 91 Introduction: the Giant and the Cow 132 The Expansion of the Universe 203 The Cosmic Microwave Radiation Background 524 Recipe for a Hot Universe 815 The First Three Minutes 1026 A Historical Diversion 1207 The First One-hundredth Second 1308 Epilogue: the Prospect Ahead 145

Afterword 151TABLES : 1. Properties of Some Elementary Particles 163

2. Properties of Some Kinds of Radiation 164Glossary 165

Preface

This book grew out of a talk I gave at the dedication of theUndergraduate Science Center at Harvard in November1973. Erwin Glikes, president and publisher of Basic Books,heard of this talk from a mutual friend, Daniel Bell, andurged me to turn it into a book.

At first I was not enthusiastic about the idea. Although Ihave done small bits of research in cosmology from time totime, my work has been much more concerned with thephysics of the very small, the theory of elementary particles.Also, elementary particle physics has been extraordinarilylively in the last few years, and I had been spending toomuch time away from it, writing non-technical articles forvarious magazines. I wanted very much to return full timeto my natural habitat, the Physical Review.

However, I found that I could not stop thinking about theidea of a book on the early universe. What could be moreinteresting than the problem of Genesis? Also, it is in theearly universe, especially the first hundredth of a second, thatthe problems of the theory of elementary particles cometogether with the problems of cosmology. Above all, this isa good time to write about the early universe. In just the lastdecade a detailed theory of the course of events in the earlyuniverse has become widely accepted as a 'standard model'.

It is a remarkable thing to be able to say just what theuniverse was like at the end of the first second or the firstminute or the first year. To a physicist, the exhilaratingthing is to be able to work things out numerically, to be ableto say that at such and such a time the temperature anddensity and chemical composition of the universe had such

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and such values. True, we are not absolutely certain aboutall this, but it is exciting that we are now able to speak ofsuch things with any confidence at all. It was this excitementthat I wanted to convey to the reader.

I had better say for what reader this book is intended.I have written for one who is willing to puzzle through somedetailed arguments, but who is not at home in either mathe-matics or physics. Although I must introduce some fairlycomplicated scientific ideas, no mathematics is used in thebody of the book beyond arithmetic, and little or no knowl-edge of physics or astronomy is assumed in advance. I havetried to be careful to define scientific terms when they arefirst used, and in addition I have supplied a glossary ofphysical and astronomical terms (p. 165). Wherever possible,I have also written numbers like 'a hundred thousand million'in English, rather than use the more convenient scientificnotation: 1011.

However, this does not mean that I have tried to write aneasy book. When a lawyer writes for the general public, heassumes that they do not know Law French or the RuleAgainst Perpetuities, but he does not think the worse of themfor it, and he does not condescend to them. I want to returnthe compliment: I picture the reader as a smart old attorneywho does not speak my language, but who expects nonethe-less to hear some convincing arguments before he makes uphis mind.

For the reader who does want to see some of the calcula-tions that underlie the arguments of this book, I have pre-pared 'A Mathematical Supplement', which follows the bodyof the book (p. 175). The level of mathematics used herewould make these notes accessible to anyone with an under-graduate concentration in any physical science or mathe-matics. Fortunately, the most important calculations incosmology are rather simple; it is only here and there thatthe finer points of general relativity or nuclear physics comeinto play. Readers who want to pursue this subject on amore technical level will find several advanced treatises

Preface 11

(including my own) listed under 'Suggestions for FurtherReading' (p. 189).

I should also make clear what subject I intended this bookto cover. It is definitely not a book about all aspects ofcosmology. There is a 'classic' part of the subject, which hasto do mostly with the large-scale structure of the presentuniverse: the debate over the extragalactic nature of thespiral nebulae; the discovery of the red shifts of distantgalaxies and their dependence on distance; the general relativ-istic cosmological models of Einstein, de Sitter, Lemaitre, andFriedmann; and so on. This part of cosmology has beendescribed very well in a number of distinguished books, andI did not intend to give another full account of it here. Thepresent book is concerned with the early universe, and inparticular with the new understanding of the early universethat has grown out of the discovery of the cosmic microwaveradiation background in 1965.

Of course, the theory of the expansion of the universe isan essential ingredient in our present view of the early uni-verse, so I have been compelled in Chapter 2 to provide abrief introduction to the more 'classic' aspects of cosmology.I believe that this chapter should provide an adequate back-ground, even for the reader completely unfamiliar withcosmology, to understand the recent developments in thetheory of the early universe with which the rest of the bookis concerned. However, the reader who wants a thoroughintroduction to the older parts of cosmology is urged toconsult the books listed under 'Suggestions for FurtherReading'.

On the other hand, I have not been able to find anycoherent historical account of the recent developments incosmology. I have therefore been obliged to do a little diggingmyself, particularly with regard to the fascinating questionof why there was no search for the cosmic microwave radia-tion background long before 1965. (This is discussed inChapter 6.) This is not to say that I regard this book as adefinitive history of these developments - I have far too much

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respect for the effort and attention to detail needed in thehistory of science to have any illusions on that score. Rather,I would be happy if a real historian of science would usethis book as a starting point, and write an adequate historyof the last thirty years of cosmological research.

I am extremely grateful to Erwin Glikes and FarrellPhillips of Basic Books for their valuable suggestions inpreparing this manuscript for publication. I have also beenhelped more than I can say in writing this book by the kindadvice of my colleagues in physics and astronomy. For takingthe trouble to read and comment on portions of the book, Iwish especially to thank" Ralph Alpher, Bernard Burke,Robert Dicke, George Field, Gary Feinberg, William Fowler,Robert Herman, Fred Hoyle, Jim Peebles, Arno Penzias, BillPress, Ed Purcell and Robert Wagoner. My thanks are alsodue to Isaac Asimov, I. Bernard Cohen, Martha Liller andPhilip Morrison for information on various special-topics.I am particularly grateful to Nigel Calder for reading throughthe whole of the first draft, and for his perceptive comments.I cannot hope that this book is now entirely free of errorsand obscurities, but I am certain that it is a good deal clearerand more accurate than it could have been without all thegenerous assistance I have been fortunate enough to receive.

Cambridge, MassachusettsJuly 1976

S T E V E N W E I N B E R G

Introduction: the Giant and the Cow

The origin of the universe is explained in the Younger Edda,a collection of Norse myths compiled around 1220 by theIcelandic magnate Snorri Sturleson. In the beginning, saysthe Edda, there was nothing at all. 'Earth was not found, norHeaven above, a Yawning-gap there was, but grass nowhere.'To the north and south of nothing lay regions of frost andfire, Niflheim and Muspelheim. The heat from Muspelheimmelted some of the frost from Niflheim, and from the liquiddrops there grew a giant, Ymer. What did Ymer eat? It seemsthere was also a cow, Audhumla. And what did she eat? Well,there was also some salt. And so on.

I must not offend religious sensibilities, even Viking reli-gious sensibilities, but I think it is fair to say that this is nota very satisfying picture of the origin of the universe. Evenleaving aside all objections to hearsay evidence, the storyraises as many problems as it answers, and each answerrequires a new complication in the initial conditions.

We are not able merely to smile at the Edda, and forswearall cosmogonical speculation - the urge to trace the historyof the universe back to its beginning is irresistible. From thestart of modem science in the sixteenth and seventeenthcenturies, physicists and astronomers have returned again andagain to the problem of the origin of the universe.

However, an aura of the disreputable always surroundedsuch research. I remember that during the time that I was astudent and then began my own research (on other problems)in the 1950s, the study of the early universe was widelyregarded as not the sort of thing to which a respectablescientist would devote his time. Nor was this Judgement

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unreasonable. Throughout most of the history of modemphysics and astronomy, there simply has not existed anadequate observational and theoretical foundation on whichto build a history of the early universe.

Now, in just the past decade, all this has changed.