Assignment 5: Georges Lemaitre


Discoverer of Expanding Universe


Georges Lemaître, (17 July 1894 – 20 June 1966) was a Belgian Catholic Priest, astronomer, and professor of physics at the Catholic University of Leuven.[1]

                                                 Graph 1 George Lemaitre                                                          

His study in civil engineering was interrupted by World War 1 when he joined the army. Coming back from the battlefield, he studied Mathematics and physics. He began his astronomy study in 1923, as a graduate student at University of Cambridge. Two years later he went back to Belgium and worked as a part-time lecturer at the Catholic University of Leuven.

Contribution to the idea of an expanding university and the Big Bang theory in the theoretical aspect is the most important part of his cosmological works. In 1927, he discovered a family of solutions to Einstein's field equations of relativity that described not a static universe, but an expanding universe.[2] He published the results with the name of "A Homogeneous Universe of Constant Mass and Increasing Radius accounting for the Radial Velocity of Extra-Galactic Nebulae."[3], which caught the world’s attention later in 1931.

We can’t fully understand Lemaitre’s theory without the knowledge regarding Einstein’s new general theory of relativity. In 1917, Einstein applied this theory to the universe as a whole and surprisingly discovered that the universe is either expanding or contracting. Since he couldn’t find out which is the true situation, Einstein assumed the universe is eternally static (He called this “the greatest mistake of my life” late).[4] 

Graph 2 Einstein Field Equation
Einstein called Lemaitre’s theory “the most beautiful and satisfactory explanation of creation to which I have ever listened.”[5]

Now let’s take a closer look at Lemaitre’s model. He assumed a linear relationship between galaxy’s distance and its redshift. Using published redshift and galaxy distance data, he tested and proved such correlation in his paper. He tried to give a reasonable explanation to such relation. Lemaitre reasoned that given the universe is expanding, a galaxy’s light will be stretched in frequency during its journey. Longer the journey is (greater galaxy distance), greater the stretch, or to say redshift.[6]  

Graph 3 the Expanding Universe

George Lemaitre’s work provided theoretical proof for another scientists’ study, such Edwin Hubble. Lemaitre’s theoretical findings that the universe is expanding was observationally confirmed by Edwin Hubble in 1929. Hubble spent nearly a decade to observe the universe and finally reached a conclusion that the redshift in light coming from distant galaxies is proportional to their distance.[7] Lemaitre was the first to derive what is now known as Hubble's law and made the first estimation of what is now called the Hubble constant, in his 1927 report.[8]

Lemaitre was not the only one who raised the idea of expanding the universe. In 1922, Alexander Friedmann, a Russian cosmology, also worked out the solution to Einstein’s field equation, which theoretically underpins the idea of expanding the universe and big bang theory.[9] Lemaitre and Friedmann discovered the solutions independently. Actually, I didn’t find out significant differences between their solutions. When Lemaitre talked to Einstein about his solutions in 1927, the latter pointed out that Friedmann had proposed the similar solution 5 years ago.[10]
 
Graph 4 Alexander Friedmann

As we have mentioned before, Lemaitre is not only a cosmology but also a priest. His career in religion brought some questions to his scientific study. In 1931, he raised an even more radical idea based on his expanding theory --- the universe has a finite beginning, a “single quantum” or “Cosmic Egg”.[11] Some physicists argued that Lemaitre was trying to prove the Genesis story described in Bible. However, I believed that being a religious man didn’t compromise the fact that Lemaitre is a great scientist.




Work Sited:
[1] [8] [10]https://en.wikipedia.org/wiki/Georges_Lema%C3%AEtre
[2] https://www.physicsoftheuniverse.com/scientists_lemaitre.html
[3] https://gizmodo.com/georges-lemaitre-the-greatest-scientist-you-ve-never-h-1519769080
[4] [5] [6] [11]https://www.decodedscience.org/georges-lemaitre-discovered-the-expansion-of-the-universe/5588 
[7] https://www.physicsoftheuniverse.com/scientists_lemaitre.html
[9] https://www.physicsoftheuniverse.com/scientists_friedmann.html

Picture Sited:
https://www.physicsoftheuniverse.com/scientists_lemaitre.html
http://www.bbc.com/earth/story/20160120-the-most-beautiful-equation-is-einsteins-field-equation
https://www.physicsoftheuniverse.com/scientists_friedmann.html


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