Peering into the depths of the universe, NASA's Hubble Space Telescope has spotted the farthestand therefore the oldestsupernova ever seen. This exploding star, which resides some 10 billion light-years from Earth, suggests that "dark energy" is driving the expansion of the universe.

Researchers first detected the stellar explosion in 1997, but only recently determined its age and distance. Importantly, the supernova appears brighter than one would expect if the universe were expanding at a steady rate. "The supernova appears to be one of a special class of explosions that allows astronomers to understand how the universe's expansion has changed over time, much as a parent follows a child's growth spurts by marking a doorway," Adam Riess of the Space Telescope Science Institute explains. "This supernova shows us the universe is behaving like a driver who slows down approaching a red stoplight and then hits the accelerator when the light turns green."

Research conducted in 1998 suggested that the universe's light turned green when it reached half of its present age. The new findings bear that prediction out. "Long ago, when the light left this distant supernova, the universe may have been slowing down due to the mutual tug of all the mass in the universe," Riess remarks. "Billions of years later, when the light left more recent supernovas, the universe began accelerating, stretching the expanse between galaxies and making objects in them appear dimmer."

The source of the repulsive gravity tugging at the universe remains unclear, but it may resemble Einstein's so-called cosmological constantwhich is referred to as the energy of the "quantum vacuum." "While we don't know what dark energy is," says University of Chicago astrophysicist Michael Turner, "we are certain that understanding it will provide crucial clues in the quest to identify the forces and particles in the universe."