Figure 36

Even during Einstein’s lifetime, there were a few scientists who did not want to give up on the cosmological constant. Physicist Richard C. Tolman, for instance, wrote to Einstein in 1931, “A definite assignment of Λ = 0, in the absence of experimental determination of its magnitude, seems arbitrary and not necessarily correct.” Lemaître, in addition to his general sentiment that Λ should not be rejected only because it had been introduced for the wrong reasons, had two other main motivations for wanting to keep the cosmological constant alive. First, it offered a potential solution to the discrepancy between the perceived young age of the universe (as Hubble’s observations seemed to imply) and geological timescales. In some of Lemaître’s models, a universe with a cosmological constant could linger for a long time in a coasting state, thus prolonging the age of the cosmos. Lemaître’s second reason for championing Λ had to do with his ideas about the formation of galaxies. He conjectured that regions of higher density would be amplified and grow into protogalaxies during that coasting phase. While this particular idea was shown not to work in the late 1960s, it did help to keep the cosmological constant on the back burner for a while.

Arthur Eddington was another strong supporter of the cosmological constant. So much so, in fact, that at one point he declared defiantly, “Return to the earlier view [without the cosmological constant] is unthinkable. I would as soon think of reverting to Newtonian theory as of dropping the cosmical constant.” The main rationale for Eddington’s advocacy was that he thought that the repulsive gravity was the true explanation for the observed expansion of the universe. In his words:

There are only two ways of accounting for large receding velocities of the nebulae: (1) they have been produced by an outward directed force as we have supposed, or (2) as large or larger velocities have existed from the beginning of the present order of things. Several rival explanations of the recession of the nebulae, which do not accept it as evidence of a repulsive force, have been put forward. These necessarily adopt the second alternative, and postulate that the large velocities have existed from the beginning. This might be true; but it can scarcely be called an explanation of the large velocities.

In other words, Eddington recognized that even without the cosmological constant, general relativity allowed for an expanding universe solution. However, this solution had to assume that the cosmos started with large velocities, without providing an explanation for those particular initial conditions. The inflationary model—the idea that the universe underwent a stupendous expansion when it was only a fraction of a second old—was born out of a similar dissatisfaction with having to rely on specific initial conditions as a cause for observed cosmic effects. For instance, inflation is assumed to have puffed up the universe’s fabric so much that it flattened the cosmic geometry. At the same time, inflation is believed to have been the agent that took quantum fluctuations of subatomic size in the density of matter and inflated them to cosmological scales. These were the density enhancements that later became the seeds for the formation of cosmic structure.