Volume: 02, Issue: 04 02/25/2004 
These images are from three of the most distant supernovae known, discovered using the Hubble Space Telesope. The stars exploded back when the universe was approximately half its current age.
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Other Articles in This Issue:
Scientists Watch Movie Of Neutron Star Explosion in Real Time
Mars Exploration Rovers Continue to Dazzle
View Two NASA Events Online This Thursday
NASA Updates Space Shuttle Return to Flight Plans

New Clues about The Nature of Dark Energy

The good news from NASA's Hubble Space Telescope is Einstein was right maybe.

A strange form of energy called "dark energy" is looking a little more like the repulsive force Einstein theorized in an attempt to balance the universe against its own gravity. Even if Einstein turns out to be wrong, the universe's dark energy probably won't destroy the universe any sooner than about 30 billion years from now, say Hubble researchers.

"Right now we're about twice as confident as before that Einstein's cosmological constant is real, or at least dark energy does not appear to be changing fast enough (if at all) to cause an end to the universe anytime soon," said Adam Riess of the Space Telescope Science Institute, Baltimore.

Riess used Hubble to find nature's own "weapons of mass destruction," very distant supernovae that exploded when the universe was less than half its current age. The apparent brightness of a certain type of supernova gives cosmologists a way to measure the expansion rate of the universe at different times in the past.

Riess and other astronomers transformed Hubble into a supernova search engine, discovering 42 new supernovae, including six of the seven most distant known.

Cosmologists understand almost nothing about dark energy, even though it appears to comprise about 70 percent of the universe. They are desperately seeking to uncover its two most fundamental properties: its strength and its permanence.

In a paper to be published in the Astrophysical Journal, Riess and his collaborators have made the first meaningful measurement of the second property, its permanence.

Currently, there are two leading interpretations for the dark energy, as well as many more exotic possibilities. It could be an energy percolating from empty space as Einstein's theorized "cosmological constant," an interpretation that predicts dark energy is unchanging and of a prescribed strength.

An alternative possibility is that dark energy is associated with a changing energy field dubbed "quintessence." This field would be causing the current acceleration - a milder version of the inflationary episode from which the early universe emerged.

When astronomers first realized the universe was accelerating, the conventional wisdom was that it would expand forever. However, until we better understand the nature of dark energy, other scenarios for the fate of the universe are possible.

If the repulsion from dark energy is or becomes stronger than Einstein's prediction, the universe may be torn apart by a future "big rip," during which the universe expands so violently that first the galaxies, then the stars, then planets and finally atoms come unglued in a catastrophic end of time. Currently this idea is very speculative but being pursued by theorists.

At the other extreme, a variable dark energy might fade away and then flip in force such that it pulls the universe together rather than pushing it apart. This would lead to a "big crunch" in which the universe ultimately implodes. But according to Reiss, "This looks like the least likely scenario at present."

Understanding dark energy and determining the universe's ultimate fate will require further observations. Hubble and future space telescopes capable of looking more than halfway across the universe will be needed to achieve the necessary precision. The determination of the properties of dark energy has become the key goal of astronomy and physics today.

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