I should note that Einstein himself made one interesting attempt to connect the cosmological constant to elementary particles. In what could be regarded as his first foray into the arena of trying to unify gravity and electromagnetism, Einstein proposed in 1919 that perhaps electrically charged particles are being held together by gravitational forces. This led him to an electromagnetic constraint on the value of the cosmological constant. Apart, however, from one additional short note on the subject in 1927, Einstein never returned to this topic.

The idea that the vacuum is not empty but, rather, could contain a vast amount of energy is not really new. It was first proposed by the German physical chemist Walther Nernst in 1916, but since he was interested primarily in chemistry, Nernst did not consider the implications of his idea for cosmology. The practitioners of quantum mechanics in the 1920s, Wolfgang Pauli in particular, did discuss the fact that in the quantum domain the lowest possible energy of any field is not zero. This so-called zero-point energy is a consequence of the wavelike nature of quantum mechanical systems, which causes them to undergo jittery fluctuations even in their ground state. However, even Pauli’s conclusions did not propagate into cosmological considerations. The first person to specifically connect the cosmological constant to the energy of empty space was Lemaître. In a paper published in 1934, not long after he had met with Einstein, Lemaître wrote, “Everything happens as though the energy in vacuo would be different from zero.” He then went on to say that the energy density of the vacuum must be associated with a negative pressure, and that “this is essentially the meaning of the cosmological constant Λ.” Figure 37 shows Einstein and Lemaître meeting in Pasadena in January 1933.

Figure 37

As perceptive as Lemaître’s comments were, the subject lay dormant for more than three decades until a brief revival of interest in the cosmological constant attracted the attention of the versatile Jewish Belarusian physicist Yakov Zeldovich. In 1967 Zeldovich made the first genuine attempt to calculate the contribution of vacuum jitters to the value of the cosmological constant. Unfortunately, along the way, he made some ad hoc assumptions without articulating his reasoning. In particular, Zeldovich assumed that most of the zero-point energies somehow cancel out, leaving only the gravitational interaction between the virtual particles in the vacuum. Even with this unjustified omission, the value that he obtained was totally unacceptable; it was about a billion times larger than the energy density of all the matter and radiation in the observable universe.