But if Einstein and Gamow were not close, isn’t it surprising that Einstein would use such strong language (“biggest blunder” in his “entire life”) concerning the cosmological constant with Gamow, and not with any other of his more intimate friends and colleagues? To explore this point further, I perused Einstein’s papers, books, and personal correspondence written later than 1932, for any other mention of the cosmological constant. I used 1932 as the starting point because that was the year in which Einstein and de Sitter declared the cosmological constant unnecessary.

Einstein’s writings leave no doubt that following the discovery of the cosmic expansion, he was unhappy with having introduced the cosmological constant in the first place. For instance, in 1942 his assistant and collaborator physicist Peter Bergmann published a book entitled Introduction to the Theory of Relativity, which included a foreword by Einstein, who later reviewed the work. The book does not even mention the cosmological constant. However, in the second edition of his own book The Meaning of Relativity, Einstein added an appendix in which he did remark on the cosmological term:

The introduction of the “cosmological member” into the equations of gravity, though possible from the point of view of relativity, is to be rejected from the point of view of logical economy. As Friedman[n] was the first to show one can reconcile an everywhere finite density of matter with the original form of the equations of gravity if one admits the time variability of the metric distance of two mass points.

In other words, Einstein recognized that the principles of general relativity allowed the addition of the cosmological repulsion term to the equations, but since it was not needed, he invoked mathematical simplicity to reject it. He then supplemented this comment with a footnote: