Dark fingerprints: hubble sheds light on cosmic expansion.

The mysterious cosmic push that's tearing apart the universe began revving up about 5 billion years ago. But a new study reveals that several billion years earlier, the bizarre, elastic substance that fuels this push was lurking in the shadows and already beginning to fight gravity's tendency to pull things together.

The new report, based on 24 distant stellar explosions recorded by the Hubble Space Telescope, indicates that the substance, dark energy, was present in the universe 9 billion years ago. The observations also hint that dark energy, which pervades all space, might emanate from the cosmic vacuum and have a constant density. If so, dark energy would resemble the cosmological constant, which Albert Einstein conceived shortly after he developed his theory of gravitation more than 90 years ago.

At press time, Adam Riess of the Space Telescope Science Institute in Baltimore and his colleagues were scheduled to hold a Nov. 16 briefing to unveil the new results, which document dark energy farther back in time than ever before.

Deciphering the nature of dark energy, which turns gravity from attractive to repulsive, is the most elemental riddle in all of physics and cosmology, many researchers say. The new study, "is a very significant first step" in examining the early history of dark energy and its effect on the universe, says theorist Andy Albrecht of the University of California, Davis.

"These new results help extend our map of the cosmic expansion farther into the past," adds Eric Linder of the Lawrence Berkeley (Calif.) National Laboratory.

Astronomers first discovered the handiwork of dark energy in 1998. That's when studies of exploding stars called type la supernovas revealed that despite the mutual gravitational tug of all the matter in the universe, the cosmos is expanding faster and faster (SN: 1/21/06, p. 35). The finding indicated that some 70 percent of all the energy and mass in the universe is made of dark energy.

Because all type la supernovas have about the same intrinsic brightness, their appearance in the sky indicates how far away they lie. These distances, combined with measurements of how rapidly each supernova's home galaxy is receding from Earth, enable researchers to measure past rates of cosmic expansion.

Previous Hubble studies had found that dark energy 5 billion years ago won the cosmic tug of war, turning the overall gravitational force from a pull to a push and revving up the universe's expansion. The new Hubble observations, which reveal dark energy when gravity's tug still dominated, could begin to constrain theories about the origin of this mysterious substance, says Riess.

The researchers also found hints that type la supernovas haven't changed over time. Those observations boost scientists' confidence that the explosions can be used to study cosmic expansion, says Riess.

It will be fascinating to compare Riess' supernova findings with other Hubble data now being analyzed, which were recorded from equally distant supernovas, notes Linder. The latter supernovas originate in regions that contain little dust, so the data aren't confounded by dust's dimming effects.