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'Dark Energy'  Causing Universe to Expand, say Scientists - 2004-02-26

U.S. astronomers say their Hubble Space Telescope observations make them more confident that a mysterious, repulsive "dark energy" that counters gravity will keep the universe expanding, but not so fast as to tear it apart.

If astronomers at the Space Telescope Science Institute in Baltimore are right, the universe will continue to expand steadily. If they are wrong, it will either collapse onto itself or expand so rapidly that it flies apart.

All three options depend on the strength of a poorly understood force scientists call dark energy, which is pushing the universe apart. Whether it continues to do so and how rapidly depends on whether it stays the same, increases, or dissipates.

The Space Telescope Science Institute researchers, led by Adam Riess, have measured the force and believe it to be stable.

"If it's growing very rapidly, it has the ability to fill all systems with enough dark energy to overcome their attraction and rip them apart," said Mr. Riess. "Likewise, if dark energy is weakening, it can weaken so much that it can dissipate altogether and cause the universe to recollapse. Our measurement of dark energy indicates it's not changing rapidly enough to cause a cosmic doomsday by one of these mechanisms in the next tens of billions of years."

Dark energy is the modern version of a concept German physicist Albert Einstein called his cosmological constant. He created the notion in 1917 to explain why the universe was not collapsing from gravitational pull exerted by galaxies and other celestial objects. However, he developed it at a time the universe was thought to be static. When U.S. astronomer Edwin Hubble showed in the 1920s that it was expanding, Einstein abandoned his cosmological constant, calling it his greatest blunder.

But six years ago, astronomers resurrected the notion when they discovered that the universe was expanding at a constantly accelerating rate. They called it dark energy.

Yet, despite the theory and a name for it, scientists are otherwise ignorant about dark energy and why it expands the universe. Space Telescope Science Institute theorist Mario Livio makes an analogy to the oceans.

"Dark energy is about 70 percent of the energy of the universe and we don't have a clue what it is," said Mr. Livio. "About two-thirds the surface of the Earth is covered with water. Imagine we didn't have a clue what water was. This is the situation in which we currently are."

To measure dark energy, Adam Riess' team measured the expansion rate of the universe it was causing at different times in the past. They used the Hubble Telescope to observe the light just arriving at Earth from ancient star explosions called supernovae. In their sky survey, the astronomers discovered 42 new supernovae in far-off galaxies, including six of the seven most distant known.

Mr. Riess says they measured two properties of each supernova to learn how fast the universe was expanding at the time the light left them.

"The way we measure the expansion rate is we measure its distance and we measure the rate at which it appears to be moving away from us," he said. "The ratio of those two numbers gives us a velocity and that is what tells us the expansion rate of the universe in the past."

Since the light left each supernova at different times over billions of years depending upon its distance, the astronomers could calculate the expansion rate of the cosmos at those times and, hence, the strength of dark energy. This is what tells Mr. Riess that the expansion rate and dark energy have been relatively stable.

"The dark energy looks at least semi-permanent, if not permanent, and in the other property, its strength, it is also relatively consistent with the cosmological constant, Einstein's theory,' explained Mr. Riess. "This is still a very crude measurement. We can't rule out that the dark energy is only semi-permanent and it is changing, but if that is the case, it is not changing very rapidly."

Mr. Riess adds that, under this scenario, the universe will expand forever, with the rate of acceleration always increasing.

"Eventually it will be expanding so fast that there won't even be time for light to reach us from distant galaxies," he said. "So each galaxy will become an island and it will be a quiet, cold, dark fate for the universe."

But he adds that if he is wrong about dark energy and it does destroy the universe, that event is 30 billion years away.