Lambda, the fifth foundational constant considered by Einstein

Abstract

The cosmological constant, usually referred to as Lambda, was introduced by Einstein in 1917 and then abandoned by him as his biggest ‘blunder’. It currently seems to have made a spectacular comeback in the framework of the new cosmological standard model. I will explain why, together with Planck’s constant, Boltzmann’s constant, the celerity of light, and Newton’s constant, also considered by Einstein, the cosmological constant may play a foundational role in the conceptual framework, and in the metrological framework in a role comparable to the one attributed to the Avogadro constant.