Einstein was aware of the fact that without the cosmological constant, Hubble’s measured rate of expansion produced an age for the universe that was uncomfortably short compared with estimated stellar ages, but he was initially of the opinion that the problem might be with the latter. The largest contribution to the error in the observationally determined cosmic expansion rate was corrected only in the 1960s, but uncertainties of a factor of about two in the rate continued to linger until the advent of the Hubble Space Telescope. Surprisingly, however, the banished cosmological constant did return with a bang in 1998.

You’ll notice that the language used by Einstein and de Sitter regarding the cosmological constant is benign; they note merely that in an expanding universe, it is not needed. Yet, if you read almost any account of the history of the cosmological constant, you will invariably find the story that Einstein denounced the introduction of this constant into his equations as his “biggest blunder.” Did Einstein actually say this, and if so, why?

After scrutinizing all the available documents, I first confirmed something that a few historians of science have already suspected: The tale of Einstein calling the cosmological constant his biggest blunder originated from a single source: the colorful George Gamow. Recall that Gamow was responsible for the idea of big bang nucleosynthesis, as well as for some of the early thinking about the genetic code. James Watson, the codiscoverer of the structure of DNA, once said about Gamow that he was “so very often a step ahead of everybody.” Gamow told the “biggest blunder” story in two places. In an article entitled “The Evolutionary Universe,” published in the September 1956 issue of Scientific American, Gamow wrote, “Einstein remarked to me many years ago that the cosmic repulsion idea was the biggest blunder he had made in his entire life.” He repeated the same story [and for some reason, most accounts of the history of the cosmological constant are aware only of this source] in his autobiographical book My World Line, which was published posthumously in 1970: “Thus, Einstein’s original gravity equation was correct, and changing it [to introduce the cosmological constant] was a mistake. Much later, when I was discussing cosmological problems with Einstein, he remarked that the introduction of the cosmological term was the biggest blunder he ever made in his life.”

Since Gamow was known, however, to embellish many of his anecdotes (his first wife said once, “In more than twenty years together, Geo has never been happier than when perpetuating a practical joke”), I decided to dig a bit deeper in an attempt to establish the authenticity of this account. My motivation to investigate this particular quote was enhanced by the fact that the recent resurrection of the cosmological constant has turned “biggest blunder” into one of Einstein’s most cited phrases. The last time I checked, there were more than a half million Google pages containing “Einstein” and “biggest blunder”!

I started by trying to ascertain whether Gamow was purporting to actually quote Einstein directly. Unfortunately, each of the two citations presented above appears insufficient, as it stands, to determine whether Gamow was claiming that Einstein himself had used the words “biggest blunder” that he ever made in his life, or whether Gamow was merely reporting the spirit of the conversation. However, in My World Line, Gamow continued to say, “But this ‘blunder,’ rejected by Einstein, is still sometimes used by cosmologists even today, and the cosmological constant denoted by the Greek letter ‘Λ’ rears its ugly head again and again.” The use of quotation marks around the word “blunder” seems at least to suggest that Gamow meant to imply an authentic quote. The fact that Gamow used precisely the same language twice also indicates that he was trying to give the impression, at least, that he was quoting Einstein directly. Note also that Gamow reveals here his own prejudice concerning the cosmological constant, through the phrase “its ugly head.”