Summary
In this quote, the speaker expresses remorse over the cosmological constant, which refers to a term in Einstein's field equations of general relativity that was introduced by Einstein himself to achieve a static universe. However, it turned out to be unnecessary as Edwin Hubble's observations confirmed the universe's expansion. This quote suggests that the speaker acknowledges the introduction of the cosmological constant as their greatest mistake, as it represented an erroneous assumption about the nature of the universe.