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Hubble images of supernovae
NASA - STScI
These are Hubble images of three of the most distant supernovae known. By tracking these exploding stars, astronomers can trace the expansion rate of the universe, and determine how it is affected by dark energy.
By Senior science writer
updated 2/23/2004 11:30:55 AM ET 2004-02-23T16:30:55

Recent Hubble Space Telescope images of distant exploding stars add further confirmation to the permanence of a mysterious, repulsive force called dark energy that appears to dominate the universe.

While scientists are not ready to close the case, they said today that dark energy, which is thought to permeate the cosmos and work in opposition to gravity, does appear to be a constant presence as predicted.

The results bolster a theory that the universe won't end soon. But they leave researchers no more informed about the actual nature of dark energy.

"We still have almost no clue what it is," said study leader Adam Riess of the Space Telescope Science Institute (STScI) in Baltimore.

Dark energy was conjured to explain a phenomenal discovery in 1998: Nearly all galaxies in the universe are receding from each other at an ever-faster pace.

Gravity is losing some unknown battle, cosmologists admit. They theorize that about 70 percent of the universe is made up of dark energy, while most of the rest is another mysterious thing called dark matter and only a small fraction is real matter like stars, planets and living entities.

Albert Einstein was the first to consider something similar, which he called a cosmological constant. He said even the emptiest space would have some of this strange stuff in it.

But when Edwin Hubble discovered the expansion of the universe in the 1920s, Einstein called his cosmological constant his greatest blunder.

Einstein is back
With the more recent finding that the expansion is accelerating, Einstein's idea was revived.

The new findings support Einstein's cosmological constant, which modern cosmologists say implies that dark energy should not characteristically change over time. If that's right, the universe will continue to expand at an accelerating forever.

The new results suggest that even if Einstein and modern dark energy theory are both wrong, dark energy will not destroy the universe for at least 30 billion years, Riess and his colleagues say.

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"Right now we're about twice as confident than 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," Riess said.

The universe is presently 13.7 billion years old.

Riess' team uses Hubble to find stars that exploded when the universe was about half its present age. A certain type of these supernovas, as they are called, shine with a known brightness. So examining the light that reaches Hubble tells astronomers how far away each one is and the rate at which the universe was expanding when that star exploded.

That rate of expansion has changed over time, other studies have shown. The initial expansion, after a theoretical Big Bang, was the most rapid and was called inflation. Then things leveled off before another round of acceleration, which is apparently underway now.

Riess' team has now observed 42 of the very distant supernovas -- including 16 in the new work -- in its Great Observatories Origins Deep Survey (GOODS) program.

The data was first presented last fall but has only now been fully analyzed. The results were discussed in a teleconference with reporters Friday and will be published in the Astrophysical Journal.

What's going on
There are two main ideas for the source of dark energy. It might percolate from empty space, as Einstein theorized, and is unchanging and of a fixed strength. The other holds that dark energy is associated with a changing energy field called "quintessence," something akin to a magnetic field. In that scenario, the field causes the current acceleration of the universe.

Another research team recently theorized that if the repulsion from dark energy gets stronger than Einstein's prediction, the universe could expand so incredibly that it would end in a Big Rip. All matter — galaxies, then stars, then planets, and everything right down to the atomic level — would be torn apart.

If dark energy can change, it might also one day reverse course and pull the universe back together in a Big Crunch. "This looks like the least likely scenario at present," Riess said.

There are two initial questions scientists are trying to answer: What is the strength of dark energy today, and does it grow or decay with time?

The new data show that if the repulsive force is changing, "it is not changing very rapidly," Riess said.

There is a lot of work ahead.

"Determining these two properties still leaves us very, very far from understanding what dark energy is," said Mario Livio, a theorist who heads the science division at the STScI. But until these first two parameters are determined, a fundamental understanding of the cosmos will remain elusive. It remains possible, for example, that our understanding of gravity "is completely lacking," Livio said.

There are other methods for probing dark energy, but none are as developed as the supernova observations. So in the near term, progress toward understanding dark energy will rely heavily on more observations of exploded stars that are even farther away and deeper in time.

Astronomers worry, however, that if Hubble stops working by around 2007, which would be the case under NASA's current plan, they would lose their primary tool in the hunt for distant supernovas.

And with NASA's new human spaceflight plans, other useful projects are in jeopardy. Many missions under NASA's Beyond Einstein initiative, including proposed missions to study dark energy, have taken a budgetary back seat to programs that will help get humans back on the Moon and on to Mars.

Whatever methods are applied, Anne Kinney, director of NASA's Astronomy and Physics Division, cautioned that final answers on the nature of dark energy will not likely come for a very long time. Science can sometimes be much like art, she said: "You approach, you don't arrive."

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