theories unpalatable, theorists sought a way of balancing out this excess with a factor that contained a negative cosmological constant. Such considerations led various researchers, including Sundrum and physicist Lisa Randall, to postulate the background geometry of the cosmos as a type of manifold called an “anti-de Sitter space.” Such a space is warped (along a fifth dimension) rather than strictly flat. With deep connections to new versions of superstring theory, particularly the approach known as M-theory, the Randall-Sundrum model represents a popular new means of addressing the dark-energy dilemma.
Many field-theoretical approaches (including Randall-Sundrum) postulate that gravity takes a different form on various scales, which would naturally explain why gravity seems strictly attractive on the local level (the solar system, say) but harbors a repulsive component much farther out. Physicists like Eric Adelberger of the Eöt-Wash group have engaged in high-precision torsion balance testing of this hypothesis but have found no deviation so far from the standard law of gravity. Nevertheless, theorists have pressed on with a variety of alternative gravitational models.
If antigravity turns out to be a dynamic property of the universe, one of the most frightening possibilities is it snowballing beyond control. The scenario unfolds like this. In the early universe, matter dominated dark energy. As the universe expanded, dark energy caught up and eventually slightly tipped the balance. But suppose this growth is far from over, releasing new reservoirs of repulsive force. Over time, gravity would increasingly cower before its towering competitor. Large structures such as clusters, then galaxies and smaller entities, would break apart. Ultimately, this would lead to the complete decimation of every shred of material in the universe— in other words, a “Big Rip.”