Figure 35

If Hubble’s expansion had been discovered at the time of the creation of the general theory of relativity, the cosmological member would never have been introduced. It seems now so much less justified to introduce such a member into the field equations, since its introduction loses its sole original justification—that of lending to a natural solution of the cosmological problem.

In appendix 4 to his popular book Relativity: The Special and General Theory, Einstein also noted that the cosmological term “was not required by the theory as such nor did it seem natural from a theoretical point of view.” Similarly, the revised, 1958 edition of Nobel laureate Wolfgang Pauli’s book Theory of Relativity included a supplementary footnote referring to the fact that Einstein was fully aware of the Friedmann and Lemaître solutions and of Hubble’s discovery. According to the author, a member of Einstein’s inner circle, Einstein subsequently rejected the cosmological term as “superfluous and no longer justified.” Pauli commented further that he himself fully accepted Einstein’s new standpoint. Nowhere, however, is there any allusion to “biggest blunder.”

An analysis of Einstein’s entire record about the cosmological constant makes it absolutely clear that he denounced it on two grounds only: an aesthetically motivated simplicity, and the regret over the wrong motivation for its introduction. As I noted in chapter 2, simplicity in terms of the principles involved is considered one of the hallmarks of a beautiful theory. To Einstein, simplicity was more than that—it was almost a criterion of reality: “Our experience up to date justifies us in feeling sure that in nature is actualized the ideal of mathematical simplicity.” Einstein’s experience during the development of general relativity had only enhanced his trust in mathematical principles. When he tried to follow what he thought were the physical constraints, he got nowhere, whereas following the most natural equations from a mathematical perspective opened the door to a “theory of incomparable beauty,” in his words. Adding another constant (the cosmological constant) to the equations did not convey reductionistic beauty to Einstein, but he was willing to live with it for as long as it appeared to be imposed by what he perceived as a static reality. Once the cosmos was found to be dynamically expanding, Einstein was delighted to rid his theory of what he now regarded as excess baggage. He articulated his feelings in a letter he wrote to Georges Lemaître on September 26, 1947. This was a reply to a letter that the Belgian cosmologist had sent to Einstein on July 30 of the same year. In that letter (and in an article by Lemaître that followed), Lemaître did his best to persuade Einstein that the cosmological constant was actually necessary to explain a number of cosmic facts, including the age of the universe.

Einstein first admitted that “the introduction of the Λ term offers a possibility” to avoid contradiction with geological ages. Recall that the age of the universe implied by Hubble’s original observations was much shorter than the age of the Earth. Lemaître thought that he could resolve this conflict if the equations included the cosmological constant. However, Einstein repeated his reductionist arguments to justify his continuing reluctance to accept the cosmological constant. He wrote: