Their conclusions are based on observing dark energy with the aid of the Chandra X-ray Observatory, the researchers explained, unveiling their work before journalists at NASA headquarters here.

"The universe really is accelerating, we have obtained direct confirmation, the results provide important new clues about the nature of dark energy," said Steve Allen, an astrophysicist at the Institute of Astronomy in Cambridge, England.

"This has important implications for the destiny of the universe many billions of years from now," he added.

Researchers as yet are unsure what dark energy is composed of, but believe that its density determines the rhythm of expansion of the universe.

"Dark energy is pushing the universe appart and accelerating its expansion," Allen said, noting research that used Einstein's model of the constant density of dark energy.

Three scenarios exist relating to the density of dark energy.

"If dark energy remains constant (in density) then the universe will continue to accelerate and in a mere 100 billion years, the sky -- instead of being filled with billions of galaxies -- will be down to a few hundred," said Paul Hertz, a senior NASA scientist. "It will be a very lonely place."

"It's still possible that the dark energy might be decreasing with time and if it decreases sufficiently, then the universe will slow down and can recollapse to a very spectacular Big Crunch," he added.

"The third possibility -- the most radical -- comes into play if the dark energy density increases with time and, in that case, the increasing expansion of the universe ends up ripping apart the atoms that make us up," Hertz said.

University of Chicago and National Science Foundation independent researcher Michael Turner warned that until better understanding of cosmic acceleration, and the nature of dark energy, exists, "We cannot hope to understand the destiny of the universe."

Researchers who led the study estimate that the universe is composed of 75 percent dark energy, 21 percent dark matter, and only four percent of normal matter such as that making up the Earth.

The scientists studied 26 clusters of galaxies some eight billion light years away -- a span that runs from shortly after the Big Bang when the expansion of the universe began to slow, through to its acceleration due to the effect of being repulsed by dark energy.

SPACE.WIRE