With Friedmann's return to Petrograd, the last and exceptionally eventful and productive period of his scientific activity began. His talent reached maturity, gained confidence in itself and full strength; yet his desire for knowledge, his hunger for creative effort seemed to be insatiable. Friedmann would tell his relatives and friends: “No, I'm an ignoramus, I know nothing, I should sleep less and should not do anything outside science, because all this so-called ‘life’ is a mere waste of time.” He was on the threshold of the most important accomplishment in his life – he was soon to develop his cosmological theory.

Friedmann's long-time and close friend, the mathematician (and later Professor) Alexander Felixovich Gavrilov, wrote: “The last five years in the life of this amazing person were full of virtually selfless work in new areas, with tremendous achievements.”

Studying relativity

Friedmann set out to study the general theory of relativity with unusual diligence and at the same time with great interest and zeal. At that time this theory was referred to as the strong relativity principle. That was by no means his only passion at that time, but the theory of relativity undoubtedly overwhelmed him with its broad scope, its simple and clear theoretical basis, its elegant mathematical apparatus. The structure of the Universe as a whole had for the first time become the object of exact scientific study. The nature of space and time was linked in the new theory with the distribution and motion of gravitating masses in the Universe.

The general theory of relativity was developed by Albert Einstein in 1914–16.