Two teams of astronomers studying distant supernovae stunned the physics world in 1998 when they announced evidence that the expansion of the universe is speeding up.1 That remarkable discovery, since confirmed by other observations, was awarded the 2011 Nobel Prize in Physics (see Physics Today, December 2011, page 14). In the 15 years since the discovery, astronomers have taken a number of approaches to more precisely measure the parameters that define that acceleration and other aspects of the now-standard concordance model of cosmology.
Yet the question of why the expansion of the universe is accelerating remains a mystery. According to general relativity (GR), gravity’s attraction should slow the expansion over time, just as it slows the upward motion of a ball thrown from the surface of Earth. To explain cosmic acceleration, physicists are faced with two possibilities: Either 70% of the universe exists in an exotic form—now...
