Lemaitre and a renewal of interest in cosmology

In 1930, the cosmological stalemate over conflicting theoretical models of the universe came to an end in a most surprising way, with the revival of a ''forgotten'' paper by a Belgian cosmologist. The flurry of activity generated by this paper swept Hubble's research into the limelight.

Georges Lemaître, an unusual man who was both an ordained priest and a physicist, belonged to Hubble's generation. After serving as a soldier in the Belgian army during World War I, he returned to the Catholic University of Louvain, where he had earlier begun his studies. He obtained a PhD in mathematics and physics in 1920, and was ordained as an abbé in 1923. He then taught himself Einstein's relativity theory. In 1923, the Belgian government and the Belgian American Educational Foundation supported his post-doctoral stint at Cambridge University, where he deepened his understanding at Eddington's side and began to focus on the cosmological implications of relativity theory. The following year he began research with Harlow Shapley at Harvard and also at the Massachusetts Institute of Technology, working on a second PhD.

Lemaître attended the 1924-1925 meeting at which Russell read Hubble's epoch-making paper on observations of Cepheids in spiral nebulae. Lemaître realized then that Einstein and de Sitter's cosmologies, which had been conceived at a time when the Milky Way galaxy was the known universe, must be tested against observations of a universe in which galaxies, not individual stars, marked the contours of space. He set out to try to incorporate observational data, particularly Slipher's and Hubble's observations of receding nebulae, in a new cosmology.

Lemaître never saw any problem reconciling his religious faith with the rationalism of a scientist, and he demonstrated his willingness to contemplate the mysteries of the universe at their deepest level, from the perspective of a physicist. In 1927, he published in the Annals of thé Brussels Scientific Society a remarkable paper on ''A Homogeneous Universe of Constant Mass and Increasing Radius, Accounting for the Radial Velocities of Extra-Galactic Nebulae.'' He presented a concept that was radically new to theoretical cosmology: the physical expansion of the universe.

Not all the characteristics of Lemaître's proposed universe were radical. Like others before him, he envisioned a universe that was finite rather than infinite, but boundless, or without an edge. The surface of a balloon is often cited as an example of a finite but boundless space: the surface area of a balloon can be measured (even if it is expanding) but a line drawn on the surface can be extended indefinitely without encountering any edge.

Three-dimensional space may be finite but boundless like the two-dimensional surface of a balloon; it is just more difficult to visualize.

Lemaitre showed that Einstein's static universe solution to the field equations of general relativity had a flaw: the solution was not physically realistic. Einstein described matter as uniformly distributed, but slight variations in this theoretical uniformity would cause the universe to lose its equilibrium and, rather than remain static, it would expand or contract. Thus, static solutions simply couldn't correspond to reality, Lemaitre argued. His new perspective on the problem called for cosmologists to consider non-static solutions. In particular, he urged theorists to choose among the mathematically possible alternative solutions based on the kind of radial-velocity data provided by Slipher, Hubble and Humason—data that might indicate an expanding universe.

His own solution to Einstein's equations was an expanding universe that included both matter (unlike de Sitter's empty universe) and redshifts (unlike Einstein's static universe). He even predicted the linear velocity-distance relation that Hubble was soon to find among the extra-galactic nebulae, due to the expansion of space.

Cosmologists and astronomers did not, apparently, take any notice of Lemaitre's paper when it first appeared; in their defense it could be noted that the Annals of the Brussels Scientific Society was a rather obscure journal. Lemaitre himself did not promote his theory through personal contacts until he heard that Einstein and de Sitter had become dissatisfied with their own solutions to Einstein's field equations and had discussed the problem at a 1930 meeting of the Royal Astronomical Society in London. Then he gently reminded Eddington, secretary of the society, of his contribution to the field.

Eddington was embarrassed at having forgotten his former student's important analysis, which he found ''brilliant.''41 He arranged for an English translation of the 1927 paper to appear in 1931 in the Monthly Notices of the Royal Astronomical Society, and attached a commentary of his own. Soon other ''overlooked'' papers began to be discussed, including some by the Russian Alexander Friedmann. Friedmann had also written about expanding space, although without relating his work to astronomical observations. After a period of stagnation in the 1920s, when theoretical cosmologists and observational astronomers knew little of each other's work, the 1930s began with an opening of communication and a surge of fresh ideas.

Even as Eddington prepared a translation of Lemaitre's 1927 paper, Lemaitre was tackling an even more challenging problem than solving Einstein's field equations. He was contemplating the origin and possible end of the universe itself. The expansion of the universe does not necessarily imply a beginning of the universe; for example, the universe might expand and contract repeatedly and indefinitely. Indeed, most astronomers and cosmologists recoiled at the thought of a singular moment in the history of the universe, when all came into being. But Lemaitre ventured to contemplate such a moment, reflecting on the implications of an expanding universe for earlier and earlier times.

With what one historian of science called an ''audacious notion,'' Lemaiitre speculated on the origin of the universe from what he called a primeval atom or super-atom, a nugget containing all that was or ever could be.42 His primeval atom ''decomposed'' into a multitude of particles, like a radioactive atom decomposing into lighter atoms and elementary particles. The particles flew apart in an explosive moment of creation. For this reason Lemaiitre is sometimes known as the father of Big Bang cosmology, although the term ''Big Bang'' for the origin of the universe was coined later, after the theory had matured and had been elaborated on by others.

Eddington had earlier voiced the opinion of many scientists when he said, ''Philosophically the notion of a beginning of Nature is repugnant to me.''43 But Lemaitre's vision eventually persuaded cosmologists to consider seriously the physical state of the universe at early times. For example, if all the matter in the universe were more compact, the universe must have been much hotter in the past than it is now. In contrast to Jeans' suggestion that nebulae evolved from diffuse, amorphous states to condensed, organized bodies, Lemaitre suggested that the evolution of physical bodies — whether of nebulae or of the universe itself—proceeded from an organized initial condition and led to a more fragmented, diffuse, low-energy state. ''The origin of the world does not lie, apparently, in a primordial nebula,'' wrote Lemaitre, ''but rather in a sort of primordial atom whose products of disintegration form the actual world.''44

At the moment of creation, in Lemaitre's cosmology, the simple primordial atom unleashed a fiery torrent of particles, which subsequently dispersed and cooled as the universe expanded. Most vivid was Lemaitre's description of the cos-mologist trying to piece together what happened. He wrote, ''The evolution of the world can be compared to a display of fireworks that has just ended. A few red wisps, ashes and smoke. Astride one of the more cooled-off cinders, we watch as suns fade out, and we seek to reconstruct the vanished brilliance of the formation of the worlds.''45

His paper ended with a speculation on the eventual end of the universe: ''It is likely that the expansion has already passed a critical point and will not be followed by a contraction. In that case we cannot expect anything too exciting: the suns will cool, the nebulae will recede, ashes and smoke from the original fireworks will finish cooling off and dispersing.''46

All of these new visions of the cosmos — theoretical formulations of an expanding universe, observations of redshifts of extra-galactic nebulae in all directions—caught the public's imagination in 1931, during the course of Einstein's visit to Pasadena and the frenzy of associated media reports.

Einstein and his wife Elsa stayed in January and February, allowing Einstein to discuss recent developments in relativity theory, astronomy and cosmology with Caltech faculty members and Mount Wilson staff members. Richard Tolman, a leading cosmologist on campus, was one with whom he had much to discuss. On the observational side, he particularly wished to meet Hubble.

On 4 February, Einstein spoke to a crowd assembled in the library of the Mount Wilson Observatory offices in Pasadena. There the one known as ''the smartest man in the world'' admitted that he had changed his mind about the state of the universe, and that he should not have constrained himself to a static solution to the field equations. Hubble's observations had played a key role in his review of the problem.

''New observations by Hubble and Humason [...] concerning the redshift of light in the distant nebulae make the presumption near that the general structure of the universe is not static,'' Einstein asserted. He added that the theories of Lemaitre and Tolman ''show a view that fits well into the general theory of relativity.''47

Hubble enjoyed an unprecedented degree of media attention during Einstein's visit, worming his way to Einstein's side during photo opportunities and responding at length to reporters' questions about his own work. Curiously, amid the excitement over Einstein's revelations, he remained cautious about the interpretation of the redshifts as indications of the expansion of space. To the puzzlement of his friends and colleagues, he stayed clear of explicitly endorsing any cosmological theory.48 In interviews with reports, as in his published papers, he did not wax philosophical about the expansion of the universe or the amazing speed of extra-galactic nebulae rushing away in all directions. His graduate student Allan Sandage, who started working with him in the 1950s, once remarked that Hubble was ''of a poetic nature, he was an intellectual of a most profound type,'' but that he ''didn't really open up'' when it came to philosophical discussions.49

Some of the media attention may have gone to Hubble's head. In the 1930s, he was to have chaired the IAU's Commission on Nebulae and Star Clusters at a meeting in Cambridge, Massachusetts. But he failed to prepare a report or even to attend the meeting, leaving Shapley to assume the role. Other behavior that rankled with Mount Wilson Observatory director Adams included the fact that he treated his colleagues with something very like disdain, ignoring mail to be answered, and took extended vacations in Europe with full pay and with Grace in tow. Hubble's desire not to be bothered with his professional obligations and to shape his own public image seems even to have resulted in his breaking off contact with his family. When his mother, who had been cared for by his brother Bill, died in 1934, he failed to attend the funeral.

Adding to Hubble's difficulties maintaining good relations with his colleagues and family, the old conflict with van Maanen resurfaced in the mid-1930s. Hubble was determined to sweep away van Maanen's rotation measurements, a small but persistent thorn in his side. As early as 1925, Shapley had written to Seares that Hubble placed too much importance on being publicly vindicated. He wrote, ''Hubble's attitude toward van Maanen disturbs me a little, because of my friendship for the latter: Hubble can so well afford to be generous as he has nothing to lose.''50

Matters came to a head when Hubble examined his own photographic plates of the four major spirals van Maanen had studied and found no consistent signs of rotation at the level van Maanen had claimed to see. He wrote up his findings for publication, but observatory director Adams and editor Seares were aghast at the hostile way he expressed himself, and refused to publish the paper in the observatory's own series, Contributions of the Mount Wilson Observatory. In the end, Hubble published a report of his investigation in the Astrophysical Journal. Immediately following Hubble's paper, the journal published a note by van Maanen conceding that his earlier results might have been affected by systematic errors.51 Hubble had succeeded in laying to rest the last serious argument against the spirals as extra-galactic star systems at great distances—but the victory added nothing to his reputation. As for van Maanen, his results were definitely in error, but to this day it is not clear how such a careful and well-intentioned observer could have been led so far astray.

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