Since the initial discovery in 1998, more pieces of this puzzle have emerged, all corroborating the fact that some new form of a smoothly distributed energy is producing a repulsive gravity that is pushing the universe to accelerate. First, the sample of supernovae has increased significantly and now covers a wide range of distances, putting the findings on a much firmer basis. Second, Riess and his collaborators have shown by subsequent observations that an earlier epoch of deceleration preceded the current six-billion-year-long accelerating phase in the cosmic evolution. A beautifully compelling picture emerges: When the universe was smaller and much denser, gravity had the upper hand and was slowing the expansion. Recall, however, that the cosmological constant, as its name implies, does not dilute; the energy density of the vacuum is constant. The densities of matter and radiation, on the other hand, were enormously high in the very early universe, but they have decreased continuously as the universe expanded. Once the energy density of matter dropped below that of the vacuum (about six billion years ago), acceleration ensued.

The most convincing evidence for the accelerating universe came from combining detailed observations of the fluctuations in the cosmic microwave background by the Wilkinson Microwave Anisotropy Probe (WMAP) with those of supernovae, and supplementing those observations with separate measurements of the current expansion rate (the Hubble constant). Putting all of the observational constraints together, astronomers were able to determine precisely the current contribution of the putative vacuum energy to the total cosmic energy budget. The observations revealed that matter (ordinary and dark together) contributes only about 27 percent of the universe’s energy density, while “dark energy”—the name given to the smooth component that is consistent with being the vacuum energy—contributes about 73 percent. In other words, Einstein’s diehard cosmological constant, or something very much like its contemporary “flavor”—the energy of empty space—is currently the dominant energy form in the universe!

To be clear, the measured value of the energy density associated with the cosmological constant is still some 53 to 123 orders of magnitude smaller than what naïve calculations of the energy of the vacuum produce, but the fact that it is definitely not zero has frustrated much wishful thinking on the part of many theoretical physicists. Recall that given the incredible discordance between any reasonable value for the cosmological constant—one that the universe could accommodate without bursting at the seams—and the theoretical expectations, physicists were anticipating that some yet-undiscovered symmetry would lead to the complete cancellation of the cosmological constant. That is, they hoped that the different contributions of the various zero-point energies, as large as they might be individually, would come in pairs of opposite signs so that the net result would be zero.

Some of these expectations were hung on concepts such as supersymmetry: particle physicists predict that every particle we know and love, such as electrons and quarks (the constituents of protons and neutrons), should have yet-to-be-found supersymmetric partners that have the same charges (for example, electrical and nuclear), but spins removed by a half quantum mechanical unit. For instance, the electron has a spin of 1/2, and its “shadow” supersymmetric partner is supposed to have spin of 0. If all superpartners were also to have the same mass as their known partners, then the theory predicts that the contribution of each such pair would indeed cancel out. Unfortunately, we know that the superpartners of the electron, the quark, and the elusive neutrino cannot have the same mass, respectively, as the electron, quark, and neutrino, or they would have been discovered already. When this fact is taken into account, the total contribution to the vacuum energy is larger than the observed one by some 53 orders of magnitude. One might still have hoped that another, yet-unthought-of symmetry would produce the desired cancellation. However, the breakthrough measurement of the cosmic acceleration has shown that this is not very likely. The exceedingly small but nonzero value of the cosmological constant has convinced many theorists that it is hopeless to seek an explanation relying on symmetry arguments. After all, how can you reduce a number to 0.00000000000000000000000000000000000000000000000000001 of its original value without canceling it out altogether? This remedy seems to require a level of fine-tuning that most physicists are unwilling to accept. It would have been much easier, in principle, to imagine a hypothetical scenario that would make the vacuum energy precisely zero than one that would set it to the observed minuscule value. So, is there a way out? In desperation, some physicists have taken to relying on one of the most controversial concepts in the history of science—anthropic reasoning—a line of thought in which the mere existence of human observers is assumed to be part of the explanation. Einstein himself had nothing to do with this development, but it was the cosmological constant—Einstein’s brainchild or “blunder”—that has convinced quite a few of today’s leading theorists to consider this condition seriously. Here is a concise explanation of what the fuss is all about.

Almost everybody would agree that the question “Does extraterrestrial intelligent life exist?” is one of the most intriguing questions in science today. That this is a reasonable question to ask stems from an important truth: The properties of our universe, and the laws governing it, have allowed complex life to emerge. Obviously, the precise biological peculiarities of humans depend crucially on the Earth’s properties and its history, but some basic requirements would seem necessary for any form of intelligent life to materialize. For instance, galaxies composed of stars, and planets orbiting at least some of those stars, appear to be reasonably generic. Similarly, nucleosynthesis in stellar interiors had to forge the building blocks of life: atoms such as carbon, oxygen, and iron. The universe also had to provide for a sufficiently hospitable environment—for a long enough time—that these atoms could combine and form the complex molecules of life, enabling primitive life to evolve to its “intelligent” phase.