ABSTRACT: The cosmological constant was proposed 100 years ago in order to make the model of static Universe, imagined then by most scientists, possible. Today it is the main candidate for the physical essence causing the observed accelerated expansion of our Universe. But, as well as a hundred years ago, its nature is unknown. This paper is devoted to the story of invention of Λ by Albert Einstein in 1917, rejection of it by him in 1931 and returning of it into the science by other scientists during the century.

**Einstein’s denial of λ**

In the paper *On the cosmological problem of general relativity* published in 1931 Einstein notes that the static solution is obtained from the Friedmann equations for R which is constant in time and space. *However with the help of these equations, one can show that this solution is unstable. It means, that any solution, which at some moment of time is slightly different from the static, will over time be more and more different from it. Already for this reason, without telling as for the results of Hubble’s observations, I do not consider it possible to attribute the physical meaning to my previous solution. In this connection one can ask whether it is possible to describe the observations without introducing a λ-term that is clearly contradictory from the theoretical point of view*. In this paper and in the next one with de Sitter published in 1932 he showed that it is possible.

In addition to the second edition of *The Essence of the Theory of Relativity* published in 1945 Einstein wrote: *The introduction into the gravitational equations of the cosmological term is possible, albeit in terms of the theory of relativity is not logically necessary. As Friedmann showed for the first time, the density of matter that is finite everywhere can be reconciled with the first form of equations of gravity, assuming that the metric distance between two material points change over time. Already one requirement of spatial isotropy of the Universe leads to the scheme of Friedmann. There is no doubt that this is the most general scheme that gives a solution to the cosmological problem*.

In a letter to Lemaitre, written on 26 August 1947, we find: *Since I introduced this constant, I was accompanied by a sense of unclean consciece. … I believe that it is actually very ugly … and I can not believe that such ugly thing could have been realised in nature*.

However, the history of the λ-term did not end there. It was destined to have a long and interesting “life”. Steven Weinberg noted in his book *Cosmology* published in 2008: *Einstein’s mistake was not that he introduced the cosmological constant – it was that he thought it was a mistake*.