Since 1929, cosmologists have known that the universe is expanding. Until recently, they assumed that the expansion is slowing as the universe's own gravity tugs against it. In the late 1990s, astronomers set out to observe the slowing by measuring the distances to exploding stars known as supernovas. To their surprise, the farthest supernovas were much more distant than expected. These findings meant that the expansion of the universe is actually accelerating.

Cosmologists have proposed two explanations for the accelerated expansion of the universe, each one a challenge to the current conception of the universe. The first and more widely accepted theory posits a kind of "dark energy" that causes space itself to stretch. Recent studies of the afterglow of the big bang have bolstered the case for dark energy, suggesting that it might even account for a whopping two thirds of the universe's contents. The simplest version of the dark energy theory postulates that dark energy is an inherent part of the vacuum of space, and that it exerts a constant pressure. The basic notion dates back to 1917, when Einstein proposed such a quantity of pressure, which he labeled the "cosmological constant," in order to explain why the universe is not shrinking under its own gravity. He later abandoned the notion as unnecessary when astronomers found that the universe is in fact expanding, but Einstein's orphaned idea may in fact point to what drives the acceleration.

Some cosmologists postulate that the dark energy that drives the accelerated expansion of the universe is created by a class of "virtual" particles. Particle physicists have known for many decades that, thanks to quantum mechanics, the so-called "vacuum" roils with virtual subatomic particles popping in and out of existence, a process that generates a constant vacuum energy. According to the dark energy theorists, this vacuum energy constitutes the dark energy on which their theory depends. Unfortunately, however, there is a large discrepancy between the predicted value of the vacuum energy and the amount of energy needed to explain the accelerated expansion.

This discrepancy is not an issue for the second theory, as it does not invoke the concept of dark energy at all. Instead, it explains the accelerated expansion of the universe by positing that across very large distances—billions of light-years—gravity no longer works as Einstein's general theory of relativity predicts. Such a change to the theory of gravity would be quite radical, and it would presumably have significant implications for the way gravity works even at short distances. Because these implications could be tested in our own solar system, this might actually be an advantage for the radical theory, since it means that this theory may be easier to test than the dark energy theory.