Cosmologists and particle physicists have seldom felt so confused. Although our standard model of cosmology has been confirmed by recent observations, it still has a gaping hole: nobody knows why the expansion of the universe is accelerating. If you throw a stone straight up, the pull of Earth's gravity will cause it to slow down; it will not accelerate away from the planet. Similarly, distant galaxies, thrown apart by the big bang expansion, should pull on one another and slow down. Yet they are accelerating apart. Researchers commonly attribute the acceleration to some mysterious entity called dark energy, but there is little physics to back up these fine words. The only thing that is becoming clear is that at the largest observable distances, gravity behaves in a rather strange way, turning into a repulsive force.

The laws of physics say that gravity is generated by matter and energy, so they attribute a strange sort of gravity to a strange sort of matter or energy. That is the rationale for dark energy. But maybe the laws themselves need to be changed. Physicists have a precedent for such a change: the law of gravity that Newton formulated in the 17th century, which had various conceptual and experimental limitations, gave way to Einstein's general theory of relativity in 1915. Relativity, too, has limitations; in particular, it runs into trouble when applied to extremely short distances, which are the domain of quantum mechanics. Much as relativity subsumed Newtonian physics, a quantum theory of gravity will ultimately subsume relativity.

   
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