**« I am convinced that Mr. Friedmann's results are both correct and luminous.**

*. They demonstrate that in addition to a static solution of my equations, there are time-evolving solutions with a symmetric structure [of the Universe]”*(Albert Einstein, April 23, 1923).

**It is with these words that the greatest genius of all time admits his mistake**. Albert Einstein publishes this *mea culpa* in the most prestigious scientific journal of the time *Zeitschrift für Physik,* the same one which received on June 29, 1922 the article of an obscure professor of the University of Petrograd, Alexander Friedmann. In this work, the 34-year-old physicist establishes without any form of experimental proof, by sheer force of thought and great mathematical skill, that the new theory of gravitation developed by Einstein in 1915 naturally describes an expanding Universe. , thus opening the way to an origin of space and time. In order to understand how and why this revolutionary proposal caused the immediate rejection of the very designer of general relativity, let's go back 7 years.

**On November 18, 1915, a bomb exploded in the scientific field**. As war rages in Europe, a certain Albert Einstein proposes a new theory of gravitation where space and time are elastic quantities whose shape determines the motion of the planets. By replacing the concept of *attractive force* Newtonian the concept of *spacetime geometry*, he predicted effects impossible in Newton's theory, including the deflection of starlight by the mass of the Sun, a phenomenon that would be observed in 1919 by British astronomer Arthur Eddington during a solar eclipse. Meanwhile Einstein continues on his way by applying his theory to the entire Universe.

**In March 1917, he realized that his equations could not only describe the course of the planets in the solar system, but also the evolution of the Universe itself.**. It is important here to differentiate between *equations* of their) *solution(s)*. A *équation* can have several *solutions*. Take for example the equation x^2=1. x=+1 and x=-1 are two solutions of the same equation. In 1917, Einstein finds a solution of his equation corresponding to a static Universe, that is to say eternal, which corresponds perfectly to the astrophysical observations of the time.

**This is when Alexander Friedmann enters the scene.**. Born in 1888 to a ballet dancer father and a pianist mother, he experienced a troubled adolescence due to an unstable political context in Russia, before volunteering in 1914 as an airman specializing in ballistics targeting Prussian targets with diabolical precision. Several times decorated with the Croix de Guerre, he returned to the University after the Civil War, and worked on weather and turbulence prediction models. An outstanding mathematician since his earliest childhood, he knows how to play formal symbols like no other, expertise necessary in the context of his work where atmospheric disturbances require the resolution of extremely complex systems. It is therefore no surprise that when he discovered the equations described by Einstein in 1917, he set out to find the most general mathematical solutions. What is his surprise when he realizes that there is a solution that Einstein had not considered, which describes an expanding Universe that keeps growing over time. He sits at his desk and writes in 2 months this revolutionary proposal which he submits to the magazine *Zeitschrift für Physik* the 29 June 1922.

**At that time, this purely mathematical solution of Einstein's equations made no sense to the community of physicists.**, since no observation corroborates the hypothesis of an expanding Universe. Friedmann's model was therefore greeted with great skepticism, Einstein even writing a note to the journal, thinking he had discovered an error in the calculations... before retracting a little later. It was not until 1927 and 1929, with the work of Belgian canon Georges Lemaître, and the observations of astronomer Edwin Hubble, that the expansion of the Universe became a confirmed experimental fact and that the *solution* of Friedmann is finally recognized as that describing our Universe evolving from a singularity, called Big Bang, to its current form. Friedmann died tragically in 1925, of typhoid fever contracted during his honeymoon in the Crimea, without ever knowing how good his intuition was. And I admit that every year, when I teach his equations to master's students, I can't help but think of the posthumous tribute Einstein paid him when he said: " *Alexander Friedmann managed to read the book of the Universe, without having been influenced by observation*. "

**Janus Maat**

**source: YouTube**

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