In principle, one could imagine “counterfactual” universes that are not conducive for the appearance of complexity. For instance, consider a universe harboring the same laws of nature as ours, and the same values of all the “constants of nature” but one. That is, the strengths of the gravitational, electromagnetic, and nuclear forces are identical to those in our universe, as are the ratios of the masses of all the elementary particles. However, the value of one parameter—the cosmological constant—is a thousand times higher in this hypothetical universe. In such a universe, the repulsive force associated with the cosmological constant would have resulted in such a rapid expansion that no galaxies could have ever formed.

As we have seen, the question we have inherited from Einstein was this: Why should there be a cosmological constant at all? Modern physics transformed that question into: Why should empty space exert a repulsive force? However, owing to the discovery of accelerating expansion, we now ask: Why is the cosmological constant (or the force exerted by the vacuum) so small? In 1987, in the wake of all the previous failed attempts to put a cap on the energy of empty space, physicist Steven Weinberg came up with a bold “What if?” question. What if the cosmological constant is not truly fundamental—explicable within the framework of a “theory of everything”—but accidental? That is, imagine that there exists a vast ensemble of universes—a “multiverse”—and that the cosmological constant may assume different values in different universes. Some universes, such as the counterfactual one we discussed with a thousandfold larger lambda, would not have developed complexity and life. We humans find ourselves in one of those universes that are “biophilic.” In such a case, no grand unified theory of the basic forces would fix the value of the cosmological constant. Rather, the value would be determined by the simple requirement that it should fall within the range that would allow humans to evolve. In a universe with too large a cosmological constant, there would be no one to ask the question about its value. Physicist Brandon Carter, who first presented this type of argument in the 1970s, dubbed it the “anthropic principle.” The attempts to delineate the “pro-life” domains are accordingly described as anthropic reasoning. Under what conditions can we even attempt to apply this type of reasoning to explain the value of the cosmological constant?

In order to make any sense at all, anthropic reasoning has to rely on three basic assumptions:

1. Observations are subjected to a “selection bias”—filtering of physical reality—even merely by the fact that they are executed by humans.

2. Some of the nominal “constants of nature” are accidental rather than fundamental.