The Atom of the Universe - ebook

Phillip James Edwin Peebles
Albert Einstein Professor of Science Emeritus
Department of Physics, Princeton University
Princeton, New Jersey, USA

Preface

The demonstration that our physical universe expanded from a very different state grew out of advances in the 1920s and 1930s that to a striking degree were the work of a single person, Georges Lemaître. Dominique Lambert’s biography, Un atome d’univers. La vie et l’oeuvre de Georges Lemaître, shows us how Lemaître’s remarkable work was informed by his life outside science, and the context within his other interests in natural science. This welcome English translation makes Lambert’s biography accessible to more people who take an interest in how we arrived at our present understanding of cosmic evolution from the dense early conditions Lemaître termed the “Primeval Atom” and Fred Hoyle renamed the “Big Bang.”

Science builds on what came before. Critical for Lemaître was Einstein’s idea that a philosophically acceptable universe is the same everywhere, apart from small-scale irregularities such as our home on a planet near a star in a galaxy of stars. I see no indication Lemaître entertained doubts about Einstein’s idea; he seems to have been willing to work with it as a useful working hypothesis, as it has proved to be. Ideas can lead us to aspects of reality, on occasion.

Einstein’s idea of large-scale uniformity was daring: there was no evidence, or motivation from a full theory. Surely it was more daring still to imagine that the very spacetime of our universe is evolving. My thinking on who put the idea together, which leads me to Lemaître, goes as follows. The Russian Alexander Alexandrovich Friedmann was the first to find the mathematical solution of Einstein’s general relativity theory for a homogeneous expanding universe of matter. It was a great accomplishment. The difficult conditions under which Friedmann worked, and his tragic early death, make it quite understandable that he did not recognize the astronomers’ observational evidence for his expanding solution. But science demands credible relations between theory and observation. The German mathematician Hermann Weyl did know about the evidence that starlight from distant galaxies of stars tends to be shifted to the red, as if the light were Doppler shifted by the motions of the galaxies away from us. Weyl showed that, in a special solution of Einstein’s theory that the Dutch astronomer Willem de Sitter had found, a homogeneous distribution of massless particles could move in the manner astronomers later observed: the rate of separation of any pair of galaxies would be on average proportional to their separation. But de Sitter’s solution contains no matter, quite contrary to all the stars we see around us, and in this solution spacetime is not evolving: massless particles move in a fixed spacetime geometry. Friedmann’s solution contains matter, and spacetime evolves, but evidently Weyl was not aware of Friedmann. Two years after Weyl, in 1925, the Belgian Georges Lemaître published his independent discovery of Weyl’s result, and in 1927 Lemaître presented his independent discovery of Friedmann’s solution for an expanding universe filled with matter. Lemaître showed that if this solution describes our universe then the rate of separation of the galaxies of stars is proportional to their separation, as Weyl had done for an empty universe. Two years later the American astronomer Edwin Hubble announced the first observational evidence of this relation between velocity and distance. Another year after that the community noticed Lemaître’s work, and he became famous.

I see in this story several lessons on research in natural science. An idea may be discovered more than once, as in the independent recognition by Friedmann, Weyl and Lemaître of key elements of the idea of an expanding universe. A significant idea requires credible contact between theory and observation, as in the discovery that theory and observation agree on the proportionality of recession velocity and separation. An idea becomes an advance when the community is ready to recognize this contact. The evidence that the galaxies are moving apart was known in the early 1920s, but by and large it was taken without consideration that our universe is unchanging. Lemaître had the good fortune to hit on the idea of expansion later in the 1920s, when the problem of understanding the astronomers’ redshifts had become pressing enough that people were willing to entertain Lemaître’s idea. And consider yet another lesson. Lemaître published his paper on a matter-filled expanding universe in Annales de la Société scientifique de Bruxelles. I am sure this is an excellent journal, but it was not read by leading astronomers and relativists, so it not surprising that the paper was overlooked until Lemaître wrote several letters pointing it out to Arthur Stanley Eddington, who had been Lemaître’s teacher during a visit to the University of Cambridge. A good idea should stand on its merits, but the community has to know about it.

I draw a lesson on Lemaître’s character from the publication of an English translation of his paper in a widely-read journal, Monthly Notices of the Royal Astronomical Society, which Eddington generously arranged. The translation omits a footnote with comments on the relation between the recession velocity, v, of a galaxy and its distance, r,

v = H r. (1)

In the missing footnote Lemaître points out that astronomers’ attempts to find a relation between v and r had indicated at best only a very weak correlation; the distance measurements were too uncertain. That is, he was not offering an explanation for a relation that was already known, he was offering a prediction. Despite the large uncertainties in galaxy distances Lemaître used them to find a value of the constant of proportionality, H, in equation (1) that is quite close to Hubble’s measurement of H two years later. That seems surprising, but the footnote explains. In a sample of galaxies for which Hubble had published estimates of distances, Lemaître divided the mean value of the velocities by the mean value of the distances. The mean helps average out the large uncertainty in each distance measurement. And under the assumption that distance and velocity are related by equation (1) Lemaître’s procedure yields an unbiased estimate of H. Why was this interesting footnote not published in the English translation? The admirable study by Mario Livio (in the journal Nature, 10 November 2011, p 171) makes it clear that this was Lemaître’s decision, not a plot to deny him credit. My guess is that Lemaître removed his comment on the lack of clear evidence of the relation in equation (1) because it had become out of date with Hubble’s claimed observation of this relation. That was after publication of Lemaître’s original paper but before publication of the English translation. And I can imagine that, since Hubble directly measured H from the slope of the relation between v and r, Lemaître decided the explanation of his indirect method was no longer needed. By eliminating the footnote he removed arguably out-of-date comments. He also obscured credit for his prediction of the relation in equation (1), but Lemaître showed no interest in self-promotion.

Lemaître’s bold character is seen in this comment (in Nature, 9 May 1931, p. 706):

Sir Arthur Eddington states that, philosophically, the notion of a beginning of the present order of Nature is repugnant to him. I would rather be inclined to think that the present state of quantum theory suggests a beginning of the world very different from the present order of Nature.

Eddington’s feelings seem natural; surely it was difficult to imagine our universe evolved from a very different state. But Lemaître welcomed the idea as an opportunity to expand the reach of natural science. He saw that one should look for fossil remnants of an old order; his candidates were cosmic rays and galaxies. The fossils prove to include a sea of thermal radiation left from the hot dense Big Bang, light elements formed by thermonuclear reactions in this hot state, and, as Lemaître argued, the galaxies, which are fossil remnants of the growth of slight departures from an exactly homogeneous early universe. Lemaître’s analytic solution for the growth of a mass concentration such as a galaxy, under the convenient approximations of spherical symmetry and negligible pressure, reveals the spacetime curvature fluctuations that are now a carefully studied measure of the early universe. He conjectured that the early universe was cold; it proves to have been hot. But his interest in the role of quantum physics in the early universe is still very much on our minds. Einstein had adjusted his general relativity theory by adding a “cosmological constant,” Λ, that he expected would balance the attraction of gravity. When Einstein learned of the evidence that the universe is not unchanging he lost interest in Λ. Lemaître did not. His audacious idea was that Λ behaves as an energy density that surely has some physical significance. We know now that Λ is present and, in an appropriate though likely unintended tribute to Lemaître, it has been renamed “dark energy.” Its physical significance remains a puzzle.

Times select leaders. In the 1920s and 1930s Edwin Hubble had the qualities needed to lead the great observational advances in what he termed the “Realm of the Nebulae,” or galaxies. Georges Lemaître had the qualities needed to lead the great theoretical advances in what he termed the “Primeval Atom,” or Big Bang. Research has moved in directions Lemaître could not have anticipated, but we are moving along paths he explored. Consider the modern flavor of this comment (in my translation from G. Lemaître, Revue des Questions Scientifiques, XX (4^(e) série), 1931, p. 408):

The evolution of the world can be compared to a display of fireworks that has just ended. Some red wisps, ashes and smoke. Standing on a well-chilled cinder, we see the slow fading of the suns and seek to reconstruct the vanished burst of formation of the worlds.

I conclude by offering what I consider a profoundly valuable lesson found in G. Lemaître, “La culture catholique et les sciences positives” (Actes du VI^(e) Congrès catholiques de Malines, Bruxelles, 1936, p. 70, in the translation from O. Godart and M. Heller, Cosmology of Lemaître, Tucson (Arizona), Pachart Publishing House, 1985, p. 173) on his life in the Roman Catholic Church and his work in natural science:

In a sense, the researcher makes an abstraction of his faith in his researches. He does this not because his faith could involve him in difficulties, but because it has directly nothing in common with his scientific activity. After all, a Christian does not act differently from any non-believer as far as walking, or running, or swimming is concerned.Chapter I

From Louvain to Namur

1. The reed and the Universe

On Sunday 23 June 1963, a number of the members of the association of ‘Amis de l’Université de Louvain’, as well as journalists and people from various circles in Wallonia and Brussels, met at the Namur Bourse du Commerce in order to participate in an event called “Louvain at work: scientific research from recent years”. This event, organized by Philippe le Hodey, president of the ‘Amis de l’Université de Louvain’, aimed to illustrate the scientific vitality of their alma mater. Among the eight workshops conducted on that day, one was the responsibility of Msgr. Georges Lemaître.^() Without a doubt, Lemaître was the best ambassador to promote the vitality of research at the Faculty of Sciences at the University Louvain University (UCL). The audience attentively listened to the lecture delivered under the provocative title: “Univers et Atome” (The Universe and the Atom)^() in which Lemaître summarized and commented upon his famous hypothesis about the ‘primeval atom’ in relation to the purely natural beginning of the Universe. Members of the audience, perhaps somewhat overwhelmed by the technical details of Lemaître’s exposition, were unaware that they were witnessing the culmination of Lemaître’s defense of a hypothesis which was first formulated thirty years ago, an idea that constitutes today the standard cosmological model, or in other words, the theory of the Big Bang. The public was also unaware of how far the scientific contributions of the Louvain professor had gone beyond the field of relativistic cosmology, a field that he had partially forsaken after 1933, except in popular writing. Thus, what captured the attention of the people who had the chance to attend Msgr. Lemaître’s workshop was the audaciousness and optimism of his representation of the physical world as well as his trust in Reason. At the end of the academic year in 1963, he testified to his hope of obtaining the confirmation of a ‘primeval radiation’ that would be “analogous to fossils which testified to the primitive forms of life”. His allusion to a “fossil” radiation was prescient, as in 1965, merely two years later, Penzias and Wilson would discover precisely that, although in a somewhat different form than he envisioned. The discourse of Msgr. Lemaître therefore appeared not only as a model of scientific audacity, but also of a philosophical and religious one. Several members of the audience were even astonished by the departure made by the prelate, who was also the President of the Pontifical Academy of Sciences, from the theology of creation to his representation of the Universe’s origin, going as far as to see his hypothesis of the primeval atom as the antithesis of the world’s creation.^() This audacity is correlative of the confidence in the capacity of human intelligence. Far from being crushed by the immensity of the Universe evoked by Pascal in his Pensées, this intelligence can be found perfectly in the proportions of the physical dimensions of the cosmos. Man is indeed the Pascalian ‘thinking reed’, but – as asserted by George Lemaître at the end of his Namur’s conference – “the thinking reed (can) really dominate the Universe by recognizing it in its entirety”^().

The questions that the ‘Amis de Louvain’ could have asked themselves at the conclusion of the Namur conference are still ours nowadays. Considering the priest-scientist that Msgr. Lemaître was, how can one explain the distance he preserved between his physical cosmology and the theology of creation? Was there in fact a fracture or at least a fissure between Msgr. Lemaître’s scientific ideals and his religious ones? From where did the ‘anti-Pascalian’ character of his implicit philosophy come from? Why did he never put into equations his hypothesis of the primeval atom? What was his preference in terms of mathematical research? Was cosmology the real focus of his theoretical concerns? In order to answer all of these questions, it is first and foremost necessary to understand the genesis and unity of the scientific thought of Lemaître, as well as fathoming its scope and magnitude. It is also necessary to understand the way in which faith and science coexisted at the heart of his public and private life. The present book aims to provide the reader with a few elements essential to answering the questions that one may ask about the life and work of Msgr. Lemaître. In doing so, we need to retrace the thread of George Lemaître’s physical-mathematical interests, to examine anew his sacerdotal life, and to map out the contours of Lemaître’s formulation of the science-theology landscape.

2. Sources

Assimilating and synthesizing all the available sources dealing with Lemaître’s multidimensional life and work is a formidable task. The author was blessed with the proverbial embarrassment of riches given the sheer volume of chronicles and archives of this remarkable man. The present biography is a supplement to several other biographies, and to which the present book owes a great debt. An exhaustive list of works prior to 1996, dedicated to the life and work of George Lemaître was compiled by Jean-François Stoffel.^() We highly recommend it to the serious Lemaître scholar. If one includes a few articles that were intended for popularization in newspapers, one may classify those works into three great periods that shows the progressive increase in interest for the man himself as well as for the works of the Louvain cosmologist during the thirty years between his demise and Stoffel’s compilation.

The first period (1966-1972) that immediately followed the death of Lemaître is marked by eulogies and official ‘notices’ from academies.^()

The second period (1972-1983) is characterized by the pioneering works of Odon Godart^() and Michael Heller^() who, starting from the first-hand documents, attempted to delineate the specific contributions of Lemaître to cosmology and, at the same time, to clarify his approach regarding the relationship between the sciences and faith.^() This period began with the republication of the L’hypothèse de l’atome primitif (1946e)^() as well as of certain bibliographical notes from the Pontifical Academy of Sciences on the occasion of the fifth anniversary of Lemaître’s death.^() The impact of the works of Godart and Heller can be measured easily by consulting, for instance, the fundamental work of Helge Kragh, professor of the history of sciences at the University of Aarhus^(): Cosmology and Controversy. The Historical Development of Two Theories of the Universe. One may assert that before 1983, more or less, the contribution of Lemaître was neglected in academic literature and in the general works of scientific popularization, especially in the Anglo-Saxon world. Obviously, there are some exceptions, for instance, the now-classic work of P.J.E. Peebles The Large-Scale Structure of the Universe.^()

The third period (1983-1994) was initiated by the conference ‘The Big Bang and George Lemaître’ organized in 1983, in Louvain-la-Neuve, by André Berger and l’Institut d’astronomie et de géophysique Georges Lemaître de l’UCL, on the occasion of the fiftieth anniversary of the publication of the seminal article of the Louvain cosmologist on the expansion of Universe.^() This conference would result in a major stimulus to the study of cosmology in Louvain. The conference would also lead, notably, to what could be considered the first critical biography of Lemaître by André Deprit^(), Lemaître’s student and successor. The book is based on unpublished documents and testimonies gathered by André Deprit and his wife, Andrée Bartholomé, who was Lemaître’s assistant. These documents have been placed at my disposal.^() The hundredth anniversary of the birth of Msgr. Lemaître, 1994, was marked by two important conferences held in Louvain-la-Neuve. The first was organized by André Berger^() and the second by Patricia Radelet and Jean-François Stoffel from the Centre interfacultaire d’étude en histoire des sciences de l’UCL^() These conferences not only highlighted but helped lead the rediscovery of the central and essential role Lemaître played in the development of contemporary cosmology by the general scientific community.^() Both conferences also attracted, beyond the scientific community, the interest of a large public for the life and the work of this famous professor of Louvain.^() More recently, a general biography of Lemaître was published by John Farrell, The Day Without Yesterday – Lemaître, Einstein, and the Birth of Modern Cosmology^() and a similarly important conference was organized by Rodney D. Holder and Simon Mitton, in Cambridge in 2011, Georges Lemaître: Life, Science and Legacy^(). This conference underscored the fact that Lemaître was the first to compute in 1927 the value of what is called now the “Hubble constant” and to explain the law that would only be published by Hubble two years later in 1929.

The current work, which was prompted by the aforementioned conferences, owes much regarding its scientific dimension to the largely unpublished records which are held in the ‘Archives of Lemaître’ (referred from now as ‘AL’) located inside the UCL at Louvain-laNeuve. The AL contains the scientific library, correspondence, manuscripts, notes and notebooks of Georges Lemaître^() as well as a part of the furniture of his office and calculating machines, which he used (including the first computer the Burrough E101, from the Laboratory of Numerical Research of UCL). How has the AL been constituted? In October 1966, the landlord, wishing to rent the apartment of Msgr. Lemaître^() asked the family to take back what had belonged to Lemaître. Due to the loyalty of Odon Godart, and perhaps also a consequence of Lemaître’s will^(), the Lemaître family entrusted him with the scientific documents.^() Godart transferred them first to his office at the “Institut de Physique” at Héverlé (Heverlee), where a first curation was done with the help of René Dejaiffe (1966-1967). When the Francophones left the Institute of Physics of Heverlee, in order to establish one at Louvain-la-Neuve, Odon Godart moved the archives to his own home at Bousval, near Louvain-la-Neuve where they were once more investigated and classified thanks to the contribution of Michael Heller. The documents later were returned to the Institute of Astronomy and Geophysics of UCL where the work of classification continued under the direction of André Berger, thanks to the unreserved devotion and skill of Liliane Moens, as well as the collaboration of Gilbert Lemaître^(), physicist and computer scientist as well as nephew and godson of Msgr. Lemaître.

The study of the sacerdotal life of Msgr. Lemaître was largely based on the analyses of the archives of the “Fraternité sacerdotale des Amis de Jésus” (these archives of the “Sacerdotal Fraternity of the Friends of Jesus”, will be referred hereafter as AFSAJ), to which Lemaître remained faithful all his life, following the suggestion of Abbé Maurits Lenaerts. These archives were held in the retreat house of Regina Pacis in Schilde, close to Antwerpen, from 1934^() to September 1996, after which the house was sold and partially demolished.^() The history of the Fraternity was documented by Abbé Willocx, a collaborator of its Founder, Cardinal Mercier. These documents were given to the “Fond Mercier” of the UCL Archives by Professor Jean Ladrière who obtained them from F. Willocx before his death. For an analysis of the religious life of Lemaître, I have consulted some of the files which belonged to Canon Fernand Van Steenberghen^(), famous professor at the Higher Institute of Philosophy (Institut supérieur de philosophie) of UCL and who for many years was also a member of the same Fraternity. Finally, I also had access to the documents that belonged to Msgr. Cammaert^(), who was in charge of the Fraternity beginning in 1954.

I also investigated the archives of the Saint-André Abbey near Bruges for information concerning the apostolate of Lemaître within the Chinese student community;^() these documents are complemented by the archives of Fr. Lebbe, held in Louvain-la-Neuve. Professor Claude Soetens, from the UCL Faculty of Theology who studied these archives in depth provided the first detailed account of Lemaître’s dedication to the Chinese students of Louvain.^()

These two parallel sources provide the specific theme of this biography, namely his twofold unity of thought and life. On the one hand, his unity of scientific thought insofar as the documents from the AL identify the thread that structures an important part of Lemaître’s scholarly work and extricating it from the limits of the cosmology itself. On the other hand, a unity of life, as illuminated by the AFSAJ, by which Lemaître achieved a synthesis harmonizing a deep interior life with an absolute commitment to the very forefront of scientific inquiry. The conclusions of this work will thus be found in distinct contrast to traditional analyses of Lemaître which portray, superficially in the author’s opinion, a sort of duality in his approach to natural and human realities, i.e. a man who followed two paths only intersecting at infinity.

The studies of these two sources together allowed some previous gaps in the author’s previous research to be filled in. For example, there had been insufficient attention to situating Lemaître’s cosmological research^() within the historical development of the expanding universe. Similarly, the studies dedicated to the science-faith relationship in Lemaître’s works^() did not properly take into account two important and linked components of his sacerdotal life, namely his membership in the Amis de Jesus and his engagement with the Chinese student community as underscored by Fr. Lebbe. Thus, the full dimension of Lemaître’s interior life could have appeared to certain authors as marginal^() when compared to his scientific life, which was obviously not the case.

3. Guide to this work

Readers who do not possess any scientific background or those who want to discover the aspects of the life of Msgr. Lemaître that are not related to mathematics or physics may read first chapters 1, 2, 3, 4 (except section 4), 7, 9, 12 (except section 3), 13, 15, 16, 17, 18 (except section 2), 19 and 20. Readers with a minimal scientific background may read without difficulty chapters 4 (section 4), 5, 6, 8, 11 and 18 (section 2). Only chapter 10 requires the reader to be conversant in algebra, along with a keen interest in this topic.