Distant lights. Peering at some of the oldest known supernovas has yielded insight into dark energy.

Dark Energy Stays the Course

The brightest objects in the universe are revealing the darkest mystery of the cosmos. In a press conference on 20 February, astronomers announced observations of distant supernovae that hint at the properties of the baffling "dark energy" force that pushes galaxies apart and stretches the very fabric of spacetime. The new observations, which include six of the seven most distant supernovae yet discovered, give the first glimpse of how a key property of dark energy is changing over time.

Adam Riess of the Space Telescope Science Institute (STScI) in Baltimore, Maryland, and colleagues used the Hubble Space Telescope to investigate the nature of dark energy by studying a certain class of exploding stars. Because their brightness is known, these supernovae act as cosmic signposts that tell researchers how far away distant galaxies are; meanwhile, their color reveals how fast the galaxies are speeding away. These two bits of information allow scientists to measure how fast the universe expanded during different eras of its 13.7-billion-year history, and that tells them how quickly that expansion is getting faster because of the push of dark energy.

The new group of supernovae, 16 in all, give a sense of how "squishy" the dark energy is--how much force it exerts at different pressures--and whether that squishiness has changed over time. That information is critical for determining which of several rival models best describes how the universe has evolved so far and where it is headed. "These are very crude measurements," Riess cautions. But he says that the observations are consistent with an unchanging dark energy force--a cosmological constant. "If it's changing, it's not changing very quickly," says Riess.

As important as these observations are, says STScI theorist Mario Livio, they're too preliminary to rule out theories such as quintessence, which posits a changing strength of dark energy, or something even more exotic. "There's still a lot of wiggle room for there to be a varying field there."

Related sites
Hubble Space Telescope
A brief history of dark energy