**Georges Lemaître and the Big Bang – When a priest corrected Einstein’s worldview**

The excitement was great when in February this year researchers at the international LIGO Virgo Collaboration announced that for the first time they had directly measured gravitational waves. Now they managed to do so for the second time. And again praises are dedicated to Albert Einstein, who in his greatest stroke of genius formulated the general theory of relativity, a theory that describesgravity not as the effect a spatially acting and temporally independent force, but as an action on the very structure of space-time itself, which allows wavelike distortions of space-time, namely those gravitational waves.

After the completion of his general theory of relativity in November 1915 Einstein quickly sensed that his theory not only represents a new theory of gravity, but also provides an insight into our universe as a whole. Can one possibly deduct from an assumed cosmic mass distribution the space-time structure of the entire universe? And would this not pave the way to a first unicosmological model along with its respective dynamics (i.e. its temporal evolution), and ultimately its beginning? Almost immediately after publishing his theory, Einstein took on this question. On theoretical grounds, he searched especially for solutions that describe a *finite* universe. But to his irritation, he had to recognize that nosuch universe could be found as a solution to his equations, which at the same time served a second condition that was as central to Einstein: His universe was supposed to be static, thatis not subject to any evolutionary development or changein time. He only found solutions of his equations that described contracting or expanding universes (which is essentially due to the fact that the gravitational forces actsonly as an attracting and never as a repelling force, as e.g. the electric force does). A global cosmos which inflates like a balloon or shrinks in itself, however, was to Einstein’s greatest disliking. The insolubility of his equations for a finite and at the same timestatic universe was such an essential problem for him that he found himself desperate enough to do something he was deeply reluctant to do: He added ad hoc an additional term to his equations such that it allowed a solution describinga finite and static universe (this term corresponds to a gravitational force of opposite, i.e. repelling nature).

But others had meanwhile begun to engage in the consideration of the universe as a whole on the basis of Einstein’s equations. Already in 1917 the Dutch mathematician Willem de Sitter had found a strange solution in which the universe expands (which, however, implied a universe without any matter), and in 1922 the Russian physicist Alexander Friedmann found solutions of the Einstein equations for a universe(with masses), which was not limited by Einstein’s condition to be static. A static „sphere like“ universe, as Friedmann recognized was extremely unlikely since it would be very fragile with respect to its boundary conditions.Smallest deviations from an ideal mass distribution would lead it to (depending on the sign of that perturbation) collapse or expand. Friedmann’s universe in contrast did expand and contract periodically. Einstein himself violently fought Friedmann’s calculation and even accused him of erroneous mathematics, but ultimately had to admitthe calculation was correct (sadly Friedmann passed away shortly thereafter, so that they could not continue their dispute).

From 1925 on, a young Belgian priests became interested in the Einstein equations. Georges Lemaître was a PhD in mathematics and graduate of the priest seminary (who however never took a post as chaplain and emphasized a strict separation between research and questions of faith). Lemaître was toresume Friedmann’s work (however apparently without knowing of it) and thus equallyfound himself inconfrontation with Albert Einstein. As Friedmann hefound solutions of Einstein’s equations in which the universe expands (plus he was able to correct an error inde Sitter’s calculations). Furthermore, Lemaître deduced directly from the general theory of relativity that the universe’s expansion follows a simple law which could be put to an empirical test: v = H⋅d. The velocity v with which two galaxies are moving away from each other by the expansion of space, is all the greater the greater the distance d between them is (H is a constant of proportionality, which was later called „Hubble constant“, see below). However, at first no one took notice ofhis publication from 1927 which appearedin the second-rate French-language journal *Annales de la Société Scientifique de Bruxelles*. In the same year Lemaître turned directly to Albert Einstein. As with Friedmann Einstein had nothing to counter the mathematical calculation of Lemaître. But for his belief system it was totally unacceptable. „Your calculations are correct, but your physics is awful!“, said Einstein in ending the conversation.

But Lemaître was persistent, not least because he had the impression that Einstein’s knowledge of the latest results in astronomy was rather limited. Because astronomers had actually already found preliminary evidence that some galaxies drift away from our Milky Way. As early as in 1915 Vesto Slipher had found that in 11 of the examined 15 galaxies so called „spectral redshifts“ (an indication of a movement away from the observer) can beidentified. But the data was generally not yet sufficient to draw definite conclusions. In 1929 came finally the breakthrough: The American astronomer Edwin Hubble found clear evidence that the galaxies move away from each other. Hubble’s observations were based on distance measurements of pulsating stars (so-called „Cepheid“) in galaxies outside the Milky Way for which he observed that the redshifts of the stars increase proportionally to their distance. This is exactly the relationship Lemaître had previously deducted! It is now called „Hubble’s Law“ (it is however only true up to a certain redshift or equivalently a certain distance). Hubble was not a theoretician and initially did not expressan opinionabout the meaning of the relationship he found. Ironically, for long he did not evenbelieve in an expanding universe, but sought the reasons for his law elsewhere.

When he heard about Hubble’s findings, Einstein called the introduction of the additional term in his equations and his insistence on a static universe „the greatest blunder of my life“. In 1931, he finally took it out and returned to the field equations in their original form. Calculating the observed expansion of the universe backwards one can conclude that at an earlier time the universe must have taken a much smaller space, comparable to a balloon before inflation. Was it not obvious to suppose that at its beginning the universe was concentrated in a single point, to be born in a violent explosion? This was precisely what Lemaître had indicated in his 1927work: a moment of cosmic origin in which space-time itself arose. The priest became the first champion of a physical theory of a cosmic beginning. Einstein himself in 1933 called this theory „the most beautiful and satisfactory explanation of the creation that I have ever heard“, and thereby he took back his criticism of Friedmann and Lemaître and declared himself an ardent advocate of Lemaître’stheory.

But the theory was initially still very controversial (to some degree probably because Lemaître was a priest). One of its most eccentric critics, the English physicist Fred Hoyle in a BBC program called once it ironically “big bang „. To Hoyle’s annoyance this term stuck firmly in both popular as well as scientific parlance. The „Big Bang theory“ is now the scientifically widely accepted theory of the origin of our universe. And on his sick bed, shortly before his death, a friend of Lemaître’s brought the issue of the journal *Astrophysical Journal*, in which the discovery of the cosmic background radiation was announced, which was the last empirical puzzle piece for the recognition of Lemaître’s theory. Georges Lemaître, the priest, who disproved Einstein’s cosmology and the first scientist proposing the Big Bang theory died 50 years ago, on 20 June 1966.He deserves far more fame and recognition than what is bestowed to him today.