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date: 21 October 2020

(p. v) Preface

(p. v) Preface

In 1967 and 1975 the two pioneering Russian astrophysicists Yakov Zeldovich and Igor Novikov published two monographs under the common title Relyativistskaya Astrofizica (Relativistic Astrophysics) which were subsequently translated into English. The second of the volumes, titled The Structure and Evolution of the Universe, was a comprehensive account of modern cosmology with an emphasis on the victorious big-bang model. In their introduction Zeldovich and Novikov stated that ‘Our philosophy is that the history of the Universe is infinitely more interesting than the history of the study of the Universe.’ This is a defensible view but not the philosophy underlying the present handbook on the history of modern cosmology. This volume aims at describing in detail how our present understanding of the universe has emerged through a long and complex series of investigations with roots back in the nineteenth century and even earlier.

There are indeed two ways in which one can speak of the history of the universe. We know today and have known since about 1930 that the universe has its own physical history, in the sense that it evolves over time and can be ascribed a definite age. The details of this evolution, ranging from the big bang to the present and even into the future, are the business of astronomers, astrophysicists and cosmologists. But evolution is not the same as history in the ordinary sense of the term. Darwin’s masterwork of 1859 carried the title ‘The Evolution of Species’ and not ‘The History of Species’. History of cosmology is not primarily concerned with stars, galaxies, curved space, and dark energy, but with the scientists who explored the universe and the collective results of whom have provided the picture of the universe accepted at any given time. The ‘history’ of the universe is without a doubt highly interesting but not necessarily more interesting than the history of the study of the universe. Moreover, the two kinds of history are evidently closely connected, their relationship being synergetic rather than contradictory. It is only through the historical approach that we recognize that our present picture of the universe is not inevitable but the result of a long, bumpy and contingent development which might conceivably have resulted in a very different picture.

It is worthwhile contemplating the terminology and changed meanings of some of the key concepts of cosmology. First, to the ancient Greeks the term cosmos (κoσμoς) carried connotations such as order, regular behaviour, and beauty—it is no coincidence that the term appears also in cosmetics and cosmetology. The Greeks boldly claimed that the universe as a whole is a cosmos rather than a chaos and for this reason it must be possible to understand it rationally. Cosmology must be possible. It was a wildly, almost reckless claim but more than two thousand years later it turned out to be more than just a beautiful dream. While cosmology was for a long time the playground of speculative natural philosophy, today it is no less scientific than other branches of the physical sciences. And yet it is and will always remain a different science, principally (p. vi) because the domain of cosmology is the universe at large, a unique and epistemically extraordinary concept.

The universe or cosmos is everything that has, has had, or will have physical existence, whether matter, energy, space, or time. There is nothing outside the universe and literally everything of a physical nature is inside it. Cosmology in the traditional sense refers to the study of the structure of the universe, what in the seventeenth century was often known as cosmography, a term which stresses the mapping of the universe and which could also refer to what we would consider as geography today. Although the term is rarely used today, much of modern cosmology is in the older cosmographical tradition except that the universe under study is no longer assumed to be static. Edwin Hubble, aptly described by his biographer Gale Christianson as a ‘mariner of the nebulae’, was as much a cosmographer as a cosmologist. The dynamic conception of the universe is largely a product of the twentieth century, and yet scientists of the past often dealt with cosmogony, which literally means the study of how the universe came to be what it is. The earliest cosmological views we know of, those of the Mesopotamian and Egyptian cultures, were cosmogonies rather than cosmographies. They were mythical tales of how the world and the gods came into existence, not science.

While the name cosmology does not in itself include a temporal dimension, cosmogony does. However, from a modern perspective older terms such as cosmography and cosmogony can be misleading as they often referred to the description and formation of the planetary system and not to the universe as a whole. Henri Poincaré’s important work of 1913 entitled Hypothèses Cosmogoniques (Cosmogonic Hypotheses) was primarily concerned with theories of the formation of the planetary system and had almost nothing to say about cosmology in the modern meaning of the term.

These and other aspects of a semantic nature are not unimportant when it comes to a historical understanding of how the science of the universe has progressed. They illustrate the simple point known from all branches of culture that words come and go. Moreover, although the same word may be used during a very long period of time, in many cases the meaning of the word changes and sometimes drastically so. Just as the ‘electron’ of 1900 was very different from what physicists called an electron a few decades later, so the name ‘galaxy’ changed its meaning during roughly the same period. For practical reasons we retain the name universe, but we should not believe that when astronomers wrote about the universe in the early twentieth century they referred to the same concept that the name encompasses today.

There is another point with regard to the name cosmology that needs to be commented upon. While in the present context the name refers to the science of the universe, in other and equally legitimate contexts it refers to a world view or ideology common to a particular society or culture. Historians and anthropologists speak of the romantic ideology or the communist ideology; and they investigate the cosmology of Australian aboriginals or that of the Hopi Indians. In a broad historical perspective the two widely different meanings of cosmology were originally connected, but this is no longer the case. We generally have no problem distinguishing scientific cosmology from what may be loosely called philosophical cosmology. This volume is about the former and not about the latter kind of cosmology. Nonetheless, even today one cannot completely separate the philosophical-religious aspects of cosmology from the scientific aspects. As illustrated in (p. vii) some of the chapters, changes in scientific cosmology have often resulted in discussions of a much wider nature related to philosophical, political and religious issues.

When did the universe become modern? The question is largely equivalent to asking when the study of the universe as a whole became a science based on mathematical models combined with astronomical observations and deductions from physical theory. The seeds for a new and immensely fruitful chapter in the history of cosmology were planted in the second half of the nineteenth century when the galaxies moved to the forefront of astronomical research and the art of spectroscopy made it possible to study the heavenly bodies from the perspective of physics and chemistry. During the same period the second law of thermodynamics suggested that a cosmic arrow of time might be ascribed to the universe. And yet all this was only prolegomena to the later development. It was Einstein’s cosmological field equations of 1917 which provided a solid theoretical framework for understanding the universe at large and which today are often hailed as the true beginning of modern cosmology at least as far as theory is concerned. Based on the theory of general relativity and Hubble’s measurements of extragalactic nebulae the traditional static picture of the universe was eventually replaced by the then mind-boggling idea of a universe in expansion.

The further development from the Friedman–Lemaître expanding universe to the present standard model was far from smooth as it followed a route with many false trails and ideas later recognized to be wrong. These false trails and wrong ideas are as much part of the history of modern cosmology as are the developments which today enter textbooks as the milestones that led to our present picture of the universe. From a proper historical perspective the important developments were those which were considered important at that time and not those which current cosmologists recognize as the preferred cosmological model. Consequently a substantial part of the chapters in this volume deals with theories and observational claims which are not only outdated but wrong.

Until recently the historiography of modern cosmology and in particular the development in the post-World War II period was an underdeveloped field of history of science. The low priority was evident not only if compared to the history of other branches of the modern physical sciences but even more so in comparison to the hundreds or thousands of scholarly works devoted to ancient cosmology or the Copernican revolution. Still by 1980 and possibly even today the situation was that our knowledge of how the heliocentric system of the world came into existence was more complete and fine-grained than our knowledge of the emergence of big-bang theories in modern cosmology. And yet it would be difficult to argue that the twentieth-century picture of the evolution of the universe is less revolutionary than the pictures constructed by Ptolemy, Copernicus, and Kepler.

There were a few works which covered in a comprehensive of scholarly manner the post-war development, such as John North’s The Measure of the Universe and Jacques Merleau-Ponty’s Cosmologie du XXeme Siècle. But unfortunately both of these works were completed just before the magic year of 1965 when the cosmic microwave background (CMB) radiation was discovered. During the last two or three decades the situation has improved markedly with the publication of many books and articles describing the developments from about 1965 to the present. Although today there is no shortage of (p. viii) literature on the history of modern cosmology, by far most of it is either of a specialized nature or belonging to the genre of popular history of science.

The present volume aims at presenting a comprehensive overview of the development of cosmology from the late nineteenth century to the early twenty-first century and to include not only the necessary scientific details but also the broader contexts. Although comprehensive, of course it does not give a complete account of the history. One of the issues which are missing is a sociologically oriented analysis of how and when cosmology became professionalized and accepted as a proper scientific discipline with its own journals, associations, and reward system. After all, modern cosmology is more than just a subfield of astronomy and physics. Unfortunately the social aspects of modern cosmology are a subject which has received almost no serious attention from historians and sociologists of science.

The volume describes and explains the historical background to what we know about the universe today but does so in a non-linear, more historically authentic way that pays attention also to the many developments that turned out to be unfruitful or just wrong. The book is organized in thirteen roughly chronologically ordered chapters, with some focusing on theory and others more on observations, experiments and technological advances. A few of the chapters are of a more general nature, relating to larger contexts such as politics, economy, philosophy and world views. While each chapter can be read separately it also connects to the other chapters, and the book thus aims at describing the history coherently and as a whole. The content and style of the chapters differ, of course, but they all form part of the same grand story.

The authors come from diverse backgrounds, some professional astrophysicists and cosmologists who have lived through the exciting and dramatic developments of the last 50 years. Others come from a background in the history and philosophy of science where the perspective is different from and complementary to that of the practitioners at the coalface. As editors, we have made no to effort to smooth out the difference in approach and perception. We find the synergies and tensions between the approaches stimulating, revealing, and thought-provoking.

The contents

The first chapter in this ‘Handbook of the History of Modern Cosmology’, written by Helge Kragh, surveys cosmological ideas in the pre-Einsteinian era from approximately 1860 to 1910. During this period cosmology was not recognized as a distinct field of scientific study and yet there was a lively discussion of subjects which were de facto cosmological. Some of these discussions were stimulated by Olbers’ paradox and others by the controversial application of the law of entropy increase to the universe at large. Importantly, astrophysics emerged as a new and powerful transdisciplinary science which substantially changed classical astronomy and eventually also cosmology. During the same period the idea of non-Euclidean geometry aroused great mathematical interest and was even embraced by a few astronomers and physicists. But throughout the half-century cosmological theory remained characteristically speculative and qualitative, largely without a stable foundation in the form of relevant observations. Most (p. ix) professional astronomers either adopted an agnostic attitude or simply ignored the universe at large as a possible domain of science.

Chapter 2 by Robert W. Smith covers the same period as Chapter 1 but from the perspective of observational astronomy and cosmology and extending the period up to about 1940. With the establishment of state-of-the-art observatories in the US in the early twentieth century astronomers began to address the nebulae, one of the major results being V.M. Slipher’s discovery of redshifts in the spectra of spiral nebulae. The so-called Great Debate in 1920 was primarily concerned with the relationship of spiral nebulae to the Milky Way Galaxy, but the result of the debate was inconclusive. It was only with E. Hubble’s discovery of Cepheids in the Andromeda Nebula that a distance indicator was established and the question resolved. With the new distance scale and more data for nebular redshifts astronomers looked for a redshift–distance relation that would eventually become the main evidence for the expanding universe. To a large extent through the work of Hubble, extragalactic astronomy became a reality in the 1930s and immediately applied to test cosmological models.

As detailed in Chapter 3 by Matteo Realdi, during the 1920s cosmological models based on general relativity theory meant essentially the matter-filled Einstein model or the empty universe of Willem de Sitter. Only in 1930 was it recognized that none of the static models were adequate and that a better choice was the expanding universe which had earlier been proposed by Alexander Friedman and Georges Lemaître. With the new revelation physicists and astronomers such as Otto Heckmann, Howard P. Robertson, and Richard C. Tolman systematized the various models satisfying the requirements of general relativity. Some of the models led to a finite age of the universe smaller than the age of the Earth and much smaller than the age of stars and galaxies. The notorious timescale difficulty continued to haunt relativistic cosmology until the mid-1950s. Nonetheless, by the late 1930s the expanding universe was generally accepted and disseminated to a broader public in the form of popular books and articles. On the other hand, Lemaître’s suggestion of a kind of big-bang model was unfavourably received.

It is tempting but unhistorical to identify cosmological models with solutions to Einstein’s field equations. As discussed in Chapter 4 by Helge Kragh, since the 1920s a series of alternative cosmological theories have been proposed, some of them radically alternative and others mere modifications of relativistic models. Apart from discussing the general issue of what ‘alternative’ means, the chapter focuses on a small number of more or less heterodox theories. Some of them, such as E.A. Milne’s kinematic cosmology, once played an important role but now belongs to history. The same is the case with the steady-state theory which, because of its historical importance, is considered separately (Chapter 5). Other ideas, such as oscillating models and the introduction of time-varying constants of nature, are still discussed if mainly outside mainstream cosmology. The chapter also describes plasma models and tired-light hypotheses, and it briefly relates to more recent sceptical views regarding cosmology’s status as a proper science. Such views, found in the recent literature, have a long historical heritage but are increasingly marginalized by the remarkable developments related in this Handbook.

Still in 1960 it was far from evident that we live in a big-bang universe. Chapter 5, also by Helge Kragh, deals with the extensive and on occasion dramatic cosmological controversy that raged throughout the period 1948–1965 between two radically different (p. x) conceptions of the universe. In the late 1940s George Gamow and collaborators revived the idea of a finite-age, exploding universe which they developed into a quantitative model of the early hot universe. At the same time Fred Hoyle, Thomas Gold, and Hermann Bondi proposed the steady-state model resting on assumptions quite different from those of relativistic cosmology. The chapter follows the controversy up to the discovery of the CMB radiation which essentially sounded the death knell of the steady-state theory. And yet, according to a minority of cosmologists the death sentence was premature. They continued to fight the big-bang orthodoxy and suggest modified steady-state models in accordance with observations. But their fight was unsuccessful and, some may say, misguided. The case of the cosmological controversy is not only of historical importance, it also provides a nice illustration of the classical problem of theory choice in science.

Chapter 6 by Malcolm Longair is devoted to observational and astrophysical cosmology in the period from 1940 to 1980 and thus continues the earlier story covered in Chapter 2. The timescale problem which had plagued most relativistic models was solved with Walter Baade’s revision of Hubble’s constant in about 1953, but later in the century a new dispute emerged regarding the value of the constant. Other disputes of mainly an observational nature concerned the value of the deceleration parameter and the possibility of a non-zero cosmological constant. Perhaps the leading observational cosmologist of the period, Allan Sandage, believed that the correct cosmological model could be nailed down with the 200-inch telescope on Mount Palomar. Other issues dealt with in the chapter, apart from the determination of cosmological parameters, are the formation of large-scale structures in the universe and the role played by radio astronomy in the resolution of the cosmological controversy. The chapter also discusses the work that led to a reliable thermal history of the universe based on observations and begins the search for the density fluctuations in the CMB radiation which are further dealt with in other chapters.

The subject of Chapter 7, also by Malcolm Longair, is relativistic astrophysics and the role it has played as a tool in the cosmological arena. Although astrophysics in the relativistic domain began in the 1930s with work on supernovae and neutron stars, it was only three decades later that the new research area attracted massive attention and became of crucial importance in cosmology and in the testing of theories of gravity. The discovery of extragalactic radio sources and quasars entered the controversy over the steady-state theory and it was followed in 1967 by the discovery of pulsars. The new and strange objects provided valuable input for cosmological and gravitational theories. The same was the case with the surprising discovery in 1997 that the high-redshift gamma bursts are at cosmological distances and thus must have enormous energies. The chapter also recounts the story of celestial X-rays and gamma rays which dates back to rocket flights in the early 1960s, and it discusses the attempts to detect gravitational waves which culminated in the recent successful LIGO experiments rewarded with the 2017 Nobel Prize in physics. Through the whole of this period, general relativity changed from a specialised mathematical discipline to an everyday tool of the astrophysicist.

The discovery and further study of the CMB has been of unrivalled importance to our understanding of the early universe and to cosmology generally. Chapter 8 by Bruce (p. xi) Partridge is devoted to the epic story of the CMB with a focus on the years from the mid-1960s to the present day. Although predicted by Ralph Alpher and Robert Herman as early as 1948, the CMB became a reality only with its serendipitous discovery by Arno Penzias and Robert Wilson in 1964. Initially it was questioned whether or not the CMB was cosmic, but measurements at other wavelengths soon silenced the scepticism. The chapter deals with the advances in instrument technologies that turned the CMB to a phenomenon of quantitative studies and ultimately discovered the tiny anisotropies predicted by theory. The detailed study of the radiation’s power spectrum opened up a Pandora’s box full of new insights and new problems. The CMB became an foundational tool for calculations of nucleosynthesis and led to the necessity of dark matter and dark energy as major constituents of our universe, not to mention hypotheses of an early inflationary phase. This study reveals the enormous effort needed from many physicists, engineers and cosmologists, as well as funding agencies, to determine unambiguously the central role which the CMB occupies in modern cosmology.

The era of big science space programmes coincided with the Cold War. As suggested by Silvia De Bianchi in Chapter 9, even cosmology became involved in the political, military, and technological competition between the USA and the Soviet Union. Rather than focusing on the ideological scene, she tells the story, based on archival sources, of how the two superpowers developed research programmes of both scientific and military significance to test theories of cosmology and gravitation. Radio astronomy and radar imaging techniques were in part developed for the purpose of planetary astronomy and in part for military purposes. Advanced laser technology, of obvious military importance, provided a tool for testing general relativity and determining whether or not the constant of gravitation varied slowly in time. Although the general picture during the 1960s and 1970s was characterized by competition, there were also collaborative space science programmes. Generally the chapter argues that the historiography of cosmology and gravitation in the period needs to take into account Cold War space science and its associated technologies.

Chapter 10 by Malcolm Longair continues the story from Chapter 6, now bringing the development in observational and astrophysical cosmology up to the present. Since about 1980 our empirical knowledge of the universe has progressed tremendously and what was once a dream, precision cosmology, has become a reality. The remarkable development has largely been technology-driven, the result of increased computer power, new generations of telescopes for all wavebands, semiconductor-based detectors (CCDs), and much more. The discipline has also benefitted from the influx of experimental and theoretical physicists into the cosmological arena. The chapter describes how the accuracy and reliability of the cosmological parameters have increased drastically and established the ΛCDM consensus model. The age of the universe has been determined by different techniques resulting in the same age of about 13.8 billion years. A major advance in this period involved the supernovae projects which in the late 1990s led to the conclusion that the universe is in a state of acceleration. Most of its gravitating mass is non-baryonic and most of its total energy density is due to vacuum energy. The chapter also surveys several other crucial areas of modern cosmology, including light element nucleosynthesis and the formation of galaxies and large-scale structures.

(p. xii) Chapter 11, written by Malcolm Longair and Chris Smeenk, deals with concepts that are key components of modern cosmology but the nature of which are not well understood. While dark non-baryonic matter is generally accepted on observational grounds, there is no sure knowledge of what this kind of matter consists of. The history of dark energy is part of the history of Einstein’s cosmological constant, or perhaps it is the other way around. It may well be that the dark energy is the cosmological constant, but several alternatives are still discussed. With the proposal of the inflationary scenario for the very early universe in the early 1980s, a major revision of the very early phases of the big bang model saw the light of day. Studies of the CMB later resulted in a more sophisticated picture of inflation and its role in structure formation. Although many cosmologists assert that inflation has become part of the present consensus model, critics argue that it is too early to claim with confidence that the universe started in an explosive inflationary phase.

Some of the themes in Chapter 11 are further discussed in Chapter 12 by Milan M. Ćirković if from a different, theoretical and to some extent speculative perspective. The history of the universe supposedly started with the big bang at t = 0 but theories of quantum gravity make it possible to extend the history arbitrarily into the past, so-called pre-big-bang theories. And as to the far future, physicists have produced scenarios of the evolution of the universe and its constituent objects zillions of years from now, what is sometimes known as physical eschatology. The notorious anthropic principle entered cosmology in 1974 as a selection effect for observations and an argument for other possible universes. The controversial idea of a multiverse has several roots, including string theory, the many-worlds interpretation of quantum mechanics, and self-reproducing inflationary models. Thus, cosmology in the twenty-first century is not only characterized by precise measurements and an observationally confirmed standard model but also by an increased engagement with other fields of fundamental physics, resulting in fields such as string cosmology and its bedfellows. Once again the development has stimulated discussions of a philosophical nature.

Although cosmology has long ago become a genuine physical science, it is not and cannot be totally divorced from philosophical concerns. The final Chapter 13 by Chris Smeenk takes up some of the pertinent philosophical issues relating to modern cosmology. Theology has a place too, as in the classical and still living discussion of the ultimate origin of the universe. During the controversy over the steady-state theory in the 1950s (Chapter 5) the very nature and aims of cosmology were hotly debated and not all of the questions raised then have received a final answer. Cosmology rests on extrapolations. It is empirically underdetermined in a strong sense as observational knowledge is restricted to just a small part of the universe. More recently the question of the initial state has been reconsidered from the perspective of theories of quantum gravity. Anthropic reasoning and the epistemic status of multiverse hypotheses have attracted as much philosophical as scientific interest. In short, for philosophers, aspects of modern cosmology question traditional modes of scientific inquiry related to fundamental issues such as explanation, prediction, and falsification. And there is more.

(p. xiii) A number of the authors of the papers which include images reproduced in this handbook are deceased or uncontactable. We are most grateful to the publishers who have been helpful in giving permission for the use of the figures from journals, books, and other media for which they hold the copyright. Every effort has been made to track down the copyright owners for all the pictures, but a very few of them have proved to be beyond what we consider to be reasonable effort.

Helge Kragh and Malcolm S. Longair