Georges Lemaître and the Age of the Universe

Georges Lemaître and the Age of the Universe

Georges Lemaître and the Age of the Universe

Jean Oostens


            How long has the world we live in existed?  This question has for thousands of years preoccupied philosophers.  In more recent times, answers on scientific grounds to this question have been attempted.  And those scientific discoveries have presented us with a universe that is older than the wildest speculations the philosophers could imagine.  It is my goal to present briefly how twentieth-century scientists have reconstituted that vast history.


            The most recent account of how the Universe began is to be traced to an 1927 article published in French by the young Belgian priest Georges Lemaître (1894-1966) in Annales de la Société Scientifique de Bruxelles, a journal where many young scientists would try their hand for their first formal publication.  It took several years before Lemaître's work would be recognized through a translation in English, at the request of Arthur Eddington.  Many great names took over the seminal idea, and it is only recently that the general literature gave this pioneer full credit to "Le Père du Big Bang" (Father of the Big Bang), as Lemaître is referred to in the title of one of his first biographies in French.1  Additional biographies have more recently appeared in English.2

Early Attempts

            Periodic universes were part of the culture of many non- European civilizations.  The Aztec universe had a period of 2,500 years.  The Chinese considered a period of 129,600 years called the Yuan.  Indian mythology assumed even longer time for a new start of the world: 4.3 billions of years (this is about the age of the Earth by present standards).

            Aristotle (384-322 B.C.) and other Greek philosophers considered an unchanging universe.  This conception permeated the Christian world as Aristotle's ideas were integrated into theology by Thomas Aquinas (1225-1274) after being retrieved from oblivion by the Islamic world.  Hebrews and Christians viewed a one-time world created by God.  Some literalist interpreters of the Bible estimated the time since creation by studying the genealogy of the patriarchs mentioned in the Bible.  They place the beginning at about 2,000 B.C., not counting the time Adam and Eve would have spent before the Fall.

            Until the twentieth century, even the age of the Earth (younger than the universe reasonably) could not be accurately determined.  Geologists classified the epoch of their excavations according to their depth and the fossils they contained.  In general, in an excavation, the deeper layers could be attributed to more ancient events or civilizations.  Geologists used names going from Holocene, Pleistocene, to Cambrian and even pre-Cambrian according to the depth of their excavations.  But no absolute value for the age of those eras could be ascertained.

            Accurate dating had to wait for modern methods using radioactivity: Carbon-14 (5,730 years half life) for life forms and Potassium-40/Argon-40 (1.28 billion years half life) for geological times.  From those methods, our planet is currently estimated to have been formed 4.5 billion years ago.

            During the Renaissance, ideas about the universe evolved with the advent of the telescope.  Besides the abandonment of the geocentric view, suggestions for a larger universe were advanced.  For example, Galileo assumed stars were similar to our sun. Seen from Earth, the angular diameter of the Sun is 0.5 degree or 1,800 seconds of arc.  The brightest stars seen in Galileo's telescope appeared as dots of 5 seconds of arc.  From this he calculated a distance of (1800"/5")x(distance to the Sun) = 360 Astronomical Units, where the distance to the Sun, called Astronomical Unit (AU) is 150,000,000 Km.  This places stars at least 360 AU or 54,000,000,000 Km from us.  This is about 0.0055 light-year.  Even though the Sun qualifies as an average star, Galileo's estimate had an inherent flaw: the size of a point source is affected by the resolution of the telescope, due to diffraction.3  In fact, the closest star, Alpha Centauri lies about 4.2 light-years away.

Modern Quantitative Estimates

            As bigger and more powerful telescopes were built, the Solar System began to be seen as part of a larger cluster of stars forming the Milky Way.  It was coined "Galaxy" after the Greek word Γαλα, meaning milk.  Anthropocentrism assumed we were at the center of this cluster of billions of stars, but further observations led to relegate the Earth to the periphery of the Galaxy.  Even larger telescopes revealed the existence, amongst the point-like stars, of luminous blurbs, called Nebulae. Messier (1730- 1817) catalogued them not for their own sake, but to avoid confusing them with comets, which were his real point of interest.

            Some nebulae were found to be so far away as to be outside the Milky Way.  Just as in our own Galaxy, they were made of billions of stars that at first could not be resolved.  Reviving an idea proposed by Kant in 1755, their similarity with our Galaxy led some to call them "Island Universes."4

Einstein's Cosmology

            On May 29, 1919, the bending of light in the strong gravitational field of the Sun as observed during the solar eclipse in Brazil validated Einstein's Theory of General Relativity.  Using General Relativity, Einstein had proposed a static model of the universe.  His equations had room for an extra term, later called the Cosmological Constant λ (presently noted Λ to avoid confusion with wavelength).  Newton's law of attraction of masses had successfully predicted the behavior of planets, moons, comets, asteroids and other celestial bodies.  The Cosmological Constant introduced an extra force at very large distance.  The sign chosen for λ made masses to repel each other at very large distances.  Its magnitude, in Einstein's view, would balance the Newtonian attraction and prevent a large-scale collapse of the Universe.  It would thus always have been that way and would last forever!

Early Symptoms of Expansion

            In a spectrum, each element is identified with specific "lines" at well established wavelengths.  A systematic displacement toward longer wavelengths (called red shift) of the lines seen in the spectrum of a star is an indication, via the Doppler effect, that it moves away from the observer.5  A "blue shift," where the lines are shifted to shorter wavelengths (i.e. the blue end), indicates the source is moving towards the observer.  Such line displacements in either direction (blue shifts and red shifts) had been documented since 1912 by Vesto Slipher, but as both red-shifted and  blue- shifted cases were observed, it was interpreted as due to the random motion of the stars with respect to the solar system.  Only later was it found that the farthest stars had only red shifts (i.e., were going away from us).  The amount of red shift was found proportional to the distance.

Alternate Models

            A different solution to General Relativity was proposed by the Dutch astronomer De Sitter.  His model of the Universe was also static but assumed the absence of matter in it!  An empty universe was just a pointless exercise in mathematics, but De Sitter's model had one feature that could explain the red-shift of spectral lines found in very distant stars.  Mathematically this was caused by a slowing down of time as one went away from the center of coordinates.  Of course, having a "center" negated the concept of a Universe homogeneous on large scale.  Also, if two masses were introduced in De Sitter's Universe, they would repel each other to the edge of the Universe.  Einstein refuted this theory as irrelevant.

            The unchanging appearance of the starry sky over the recorded history tended to support the eternal existence of the universe.  It may be that insisting on a static model reflected the mental habit of their proponents: atheists could more easily do away with the existence of a Creator.  A century earlier, a similar point was made by Pierre-Simon Laplace.  Napoleon asked him about the role of God in his model explaining how the Solar System evolved from the rotation of the proto-sun that spewed out material as it shrunk by its own gravitational pull.  Laplace's answer was: "We had no need for that hypothesis."6

            In Russia, Alexander Friedmann proposed an equation allowing a time dependence of the size of the Universe.7  His model was presented as just a mathematical feature, without any connection to actual data.  Under Soviet rule, he may have been reticent to assume a universe with a beginning.  His death from illness in 1925 prevented him from having a significant impact.

Lemaître's Expanding Universe

            Lemaître had considered priesthood while in high school, however, after graduation, on his father‘s advice, he enrolled in the engineering program ("Ecole des Mines") at the Catholic University of Louvain, where he added an extra course in philosophy to keep his options open.  In 1914 came the First World War, where he served in the artillery in the Belgian army.  In 1919 he resumed his studies, switching to physics while pursuing his priestly vocation.  He was ordained priest in 1923.  Then, making good on his previous experience, he went to Cambridge, England, for graduate studies with Arthur Eddington (1882-1944), then a leading figure in astronomy.  In 1925 he obtained a grant to travel to the United States, and visited Harvard College Observatory and the Massachusetts Institute of Technology.  There he became acquainted with the red shift data collected by Slipher and later by Harlow Shapley and Edwin Hubble.

            Back in Belgium as a professor at the Catholic University in Louvain, Lemaître came up with a model of the universe that had a definite beginning.  His result drew on the red shift-distance relationship to posit a universe increasing in size with the passage of time.  He published his idea under the title "Un univers homogène de masse constante et de rayon croissant rendant compte de la vitesse radiale des nébuleuses extra-galactiques" (A Homogeneous Universe of Constant Mass and Increasing Radius Accounting for the Radial Velocity of Extra-galactic Nebulae).8

            Here is how a layman can visualize Lemaître's "Atome Primitif" (Primeval Atom), as it was later called.  Imagine a rubber balloon.   Mark several points A, B, C, etc., on the surface, each representing one galaxy.  As more and more air is blown into the balloon, simulating the passage of time, the distance between each point will increase, and a virtual observer in any of the galaxies will see all the other ones moving away.9  Quantitatively, let the distance AB be twice the distance AC.  Blowing more air in the balloon will increase the distance between A and B by twice as much than between A and C.  An observer in A will deduce that point B is fleeing twice as fast as point C.  During the same time interval, B will have moved twice as far from A as point C.  An observer in A will deduce that point B is receding at twice the speed of C.

            More generally, each point will be seen as going away at a speed proportional to its distance.  This simplified example has only two space dimensions, and time is simulated by the amount of air blown into the balloon.  Mathematicians use non-Euclidian geometry to represent a space in three dimensions that is closed, just as the Spherical Earth concept replaced "Flat Earth" when Magellan circumnavigated the globe.

            Lemaître‘s model explained well what Hubble had found: the radial velocity deduced from the red shifts in the galaxies is proportional to their distance.  The data pointed to a moment in the past when all matter was lying in just a small place: the Atome Primitif.  The data available in the 1920s gave a figure close to 2 billion years for the age of the Universe.  For a while this seemed to be in conflict with the age of the solar system, estimated to about twice as much (cfr. supra).

            The discrepancy faded out when more accurate distance measurement methods were developed.  First came a class of variable stars called Cepheids that vary in intensity with periods of several days.  There is a known relationship between their period and their absolute magnitude.  They serve as "Standard Candles": their observed magnitude relative to their absolute magnitude tells us their distance, pretty much as seeing how bright a car's headlights are tells us how far the car is.  Even further improvements came when exploding stars called "Type Ia Supernovae" played the same role of "Standard Candles."  These more reliable Standard Candles gave the current value of 14 billion years for the age of the Universe,

            The Annales de la Société Scientifique de Bruxelles, where Lemaître's article was published in French, was not widely read in the scientific world.  It was run by the Jesuit motherhouse in Louvain, and often published articles by recent graduates from the University.  In 1931, thanks to his mentor Eddington, the 1927 paper was published in English.  Lemaître updated the new version with several references, including the one about Friedmann's work, which he was not aware of when he published the original paper in French.10

A Return of a Static Universe

            A competing theory, called Steady State came around for people who did not like a universe implying a beginning.  It recognized an increase in the size of the Universe, with constant creation of an immeasurably small amount of additional matter to keep the density constant.  Fred Hoyle, a proponent of the Steady State theory coined the term Big Bang as a derisive term for the Primeval Atom model.  This name stayed but lost its derogatory character when further evidence confirmed Lemaître's original idea (cfr. infra).

Lemaître's Further Interests

            In the 1930s, Lemaître stayed active in the field of cosmology as a professor at the Université Catholique de Louvain.  He traveled to many conferences and was invited to lecture in a number of American universities.  Cosmic rays, which he suspected to be a remnant of the Big Bang, attracted his interest for many years.

            Early in 1933, when the Nazis took power in Germany, Albert Einstein decided to give up his German citizenship and resigned from the Kaiser Wilhem Academy. Lemaître lost no time in organizing a series of conferences by the exiled colleague.  He also arranged for Einstein to stay in the quiet resort of De Haan (Le Coq) on the Belgian coast until arrangements for emigrating to the United States could be finalized.  Einstein returned the favor by recommending Lemaître for the Prix Franqui in 1934.

            In 1940, World War II and the German occupation of Belgium interrupted Lemaître's contacts with his colleagues.  After he was bombed out of his apartment in Louvain in a disastrous 1944 air raid that missed the train station, he moved to Brussels to take care of his mother, a widow since the death of his father in 1942.  He commuted to Louvain by train to teach his classes in mechanics, astronomy, and relativity.  He became interested in numerical computation and started a group using mechanical calculators.  In 1956, after years of chugging along with mechanical machines that could not even perform divisions, he used some of his own funds to purchase a Burroughs 101 computer, the first electronic calculator for academic research in Belgium.

Personal Reminiscences

            As a professor, Georges Lemaître was a very jovial person.  In an academic setting where other professors taught from a pedestal, not condescending to mingle with the freshman or sophomore classes, Lemaître was sprinkling his lectures with humor and funny plays on words to clarify a concept or drive his point home.  With an infectious laughter, he liked to speak of extra-galactic galaxies (cfr. supra).  Or, whenever a fraction needed to be inverted in a calculation, he jokingly invoked the Belgian Waffle Method (Le Principe du Fer à Galette)!11

            My own early contacts with Lemaître as a commuting freshman were on the platform of the train station in Louvain, where one late afternoon he asked me why I attended both his first year mechanics class and, sitting in the back, his second year astronomy lecture.  My explanation was that I had no class during that Tuesday 10 a.m. slot, and could not wait until my second year to learn about astronomy.

            At the end of the astronomy course, Lemaître invited the students on a tour of the Uccle Observatory in Brussels.  With a friend who had a larger car, we had the honor of giving him a ride from Louvain to the Observatory.  After seeing the telescopes and the mercury pool used to determine the exact time of the day from the stars, the tour was concluded with a visit to a nearby café where our beloved teacher treated the whole class to good chat around refreshments.

            Professor Lemaître was also a skilled piano player.  When at the keyboard, he would forget the passage of time, and arrive late for his class.  He would probably have a big laugh if someone would say: "Le piano est son violon d'Ingres," a French expression referring to the famous painter Dominique Ingres (1780-1867) who was also a talented violin player.

            His scientific work was recognized by the Catholic Church: in the late fifties his church status rose from Chanoine to the rank of Pontifical Prelate with the title of Monseigneur, and in 1960 he was appointed by John XXIII to the presidency of the Pontifical Academy of Science, an organization of which he had been part since 1936.12

The 3 Kelvin Background Radiation

            In 1948, a remnant of the Big Bang had been predicted by George Gamov (1904-1968). Early in the Big Bang, Gamov said, the density was such that radiation and matter were in thermal equilibrium.  But, as the universe expanded, the density of matter became so thin that radiation did not have much chance to interact with anything, and cooled on its own, while matter condensed into galaxies, stars and planets.  Gamov predicted that such residual radiation could be seen all over the universe.

            Gamov's idea was experimentally confirmed in 1965 by pure serendipity.  Arno Penzias and Robert Wilson worked on space communication and radio astronomy at Bell Labs in Princeton, New Jersey.  Their equipment displayed an unwanted and unexplained noise coming from all directions in the sky.  The continuous spectrum in the microwave range pointed to a "Black Body Radiation" with a temperature of three degree above absolute zero.  The explanation was just across the road at Princeton University.  There two cosmologists, Robert H. Dicke and Philip Peeble, recognized in this effect the remnant radiation anticipated by Gamov.

            That confirmation of the Big Bang prediction meant the end of the Steady State model.  The news of this vindication reached Lemaître a short time before his death on June 20, 1966.

            Penzias and Wilson were awarded the Nobel Prize in Physics in 1978, leaving Dicke, Peeble and Gamov out of  the


            Already in the 1930s, Lemaître was recognized for  his work in cosmology in his native country as well as in the international scientific community.  On March 14, 1934, he was honored with the Prix Franqui, an annual prize provided with a monetary award second only to the Nobel Prize.  He was received as a private guest by King Albert I.  In 1955, his native town of Charleroi commissioned a bronze bust in his likeness by Charles Leplae.

            After Monseigneur Lemaître's death, a square in the newly built University of Louvain-la-Neuve was named after him.  At that University, the Institut d'Astronomie et de Géophysique Georges Lemaître was established and houses a rich collection of documents for historians of science.  In 1994, on the centennial of his birth, the Belgian Post Office issued a 16-franc stamp with his likeness in the Europa collection, of which six million were printed.  On this occasion, a bronze medal engraved by Paul Huybrechts was also made available to the subscribers of the Fondation Mgr. Georges Lemaître.  This foundation was endowed to provide an annual prize, the first of which was attributed to Philip Peebles in 1994.



1 Valérie de Rath, Georges Lemaître, Le Père du Big Bang (Brussels: Editions Labor, 1994) and Jean-Pierre Luminet, L'Invention du Big Bang (Paris: Editions du Seuil, 1997).

2 John Farrell, The Day Without Yesterday: Lemaître, Einstein and the Birth of Modern Cosmology (New York: Thunder Mouth Press, 2005) and Harry Nussbauer and Lydia Bieri, Discovering the Expanding Universe (Cambridge, England: Cambridge University Press, 2009).

3 Christopher M. Graney, "Objects in the Telescope are Farther than They Appear: How Diffraction Tricked Galileo into Mismeasuring Distances to the Stars," The Physics Teacher 47:362, September 2009.

4 In his astronomy class, Lemaître dubbed them "Extra-Galactic Galaxies." This was one of his favorite terms to bring a smile to his students.

5 This is the equivalent for light to the change of pitch heard for example on the passage of a loud motorcycle running at a constant speed: an approaching motorcycle will be heard as a higher frequency sound.  The increase will be proportional to its speed. The opposite situation will occur as the motorcycle moves away from the observer.

6 Nussbauer, op. cit., p. 24.

7 Friedmann (1888-1925) was professor at the University of Petrograd (Saint Petersburg), USSR, during the early 1920s when he published this model, known today as Friedmann‘s Equation.

8 Georges Lemaître, «Un Univers homogène de masse constant et de rayon croissant redant compte de la vitesse radiale des nébuleuses extra-galactiques,» Annales de la Société Scientifique de Bruxelles XLVII:49, 25 avril 1927.

9 Lemaître chose a soap bubble in a popularization of his model. Physicists prefer a rubber balloon that lends itself to actual measurements of the distances as the balloon is inflated.

10 Georges Lemaître, "A Homogeneous Universe of Constant Mass and Increasing Radius Accounting for the Radial Velocity of Extra- Galactic Nebulae," Monthly Notices of the Royal Astronomical Society 91:483-90 (1931).

11 Belgian waffles are baked in a waffle iron shaped as a book. When opened, it reveals two opposite molds. The batter is poured into one of them. After folding the top over, the iron is placed on the fire. After a while, the cook turns the iron over to bake the other side.

12 The Pontifical Academy was founded in 1936 under Pope Pius XI. In an early recognition of Ecumenism, it selected its members without regard to religious affiliation.