For decades, physicists had assumed the expansion of the universe was getting ever-slower.  

The Hubble Space Telescope has spied the most distant supernova ever, an 11-billion-year-old exploding star whose existence suggests that a repulsive 'dark energy' is spurring the expansion of the universe. (NASA)

But when Saul Perlmutter, Brian Schmidt and Adam Riess in the 1990s discovered that light from exploding stars was far weaker than expected, they came to a startling realization — the expansion of the universe is constantly accelerating.

The discovery likely means a bleak future for the universe, what scientists describe as a dark, “very cold and lonely place,” covered entirely by ice. It also means a Nobel Prize in physics for the three U.S.-born scientists, who won the award Tuesday.

Behind this discovery lies a great deal of uncertainty, as the acceleration of the expanding universe is driven by one of the great mysteries of the universe, a cosmic force scientists call “dark energy.”

While more is unknown than known about dark energy, NASA can say one thing for certain — roughly 70 percent of the universe is dark energy.

“Other than that, it is a complete mystery,” NASA writes on its Web site.

Because so little is known, scientists have struggled to come up with a consistent definition of the term: “A hypothetical form of energy that permeates all of space and has strong negative pressure, counteracting gravity.”

ThinkQuest: “An invisible force believed to be pulling the universe apart.” “A form of energy... It is unknown whether it is constant throughout the universe or whether it varies in space and time.”

Northern Arizona University Department of Physics: “Form of energy that is gravitationally repulsive, due to a negative effective pressure.” “A mysterious force that seems to work opposite to that of gravity and makes the universe expand at a faster pace.”

To come to a more complete definition, NASA says we need “more data, better data.” Until then, scientists must live with the knowledge that because the Earth’s expansion is accelerating, the universe will likely one day end in ice.