The universe is getting bigger every second. Galaxies are flying apart from one another, clusters of galaxies are zooming away from other clusters, and the empty space between everything is growing wider and wider. This much was known since the 1920s, when observations by Edwin Hubble and others revealed that the cosmos is expanding. But more recently, astronomers found that the process is speeding up—the pace of the expansion of the universe is rising, so that galaxies are receding from one another faster now than they were a moment ago. This is the startling realization that one of us (Riess), along with collaborators he co-led with Brian Schmidt of the Australian National University, came to in 1998 through measurements of distant supernovae explosions.