Friday, 14 December 2007

Astrophysics Challenged By Dark Energy Finding

Added & Edited by:

Arip Nurahman
(Teacher and Professional Lecturer)
Department of Physics, Indonesian University of Education
Follower Open Course Ware at MIT-Harvard University, Massachusetts. U.S.A.

A strange repulsive force of "dark energy" pervades every nook and cranny of the universe, a team of scientists said earlier this month, but the force not only pushes against the master force of gravity -- it also has astrophysicists' heads spinning. While gravity gently binds planets, stars and galaxies together, dark energy tugs on the fabric of time and space, pushing galaxies apart ever faster and faster into the farthest reaches of the universe.

The evidence came in a recent Hubble space telescope observation of the most distant supernova (exploding star) ever detected. The record-breaking supernova appears brighter than it should if the universe had been expanding at a steady rate. The new finding suggests that a decelerating universe holds galaxies relatively close together and objects in them would have appeared brighter because they would be closer.

The trouble is that with this finding and others in the past three years, the universe is beginning to look like a complex witch's brew of dark energy, normal matter and dark matter (an invisible exotic form of matter). "This starts to look incredibly ugly and complicated," says Mario Livio of the Space Telescope Science Institute. "I even wonder if we are we asking right questions."
'New ocean of discovery'

For other scientists, the new finding holds promise for astrophysics. Morris Aizenman, a senior science associate with the National Science Foundation, was so moved by the finding that he likened it to a Keats poem about Cortez' first sighting of the Pacific Ocean.

"It's an entire new ocean of discovery we're about to set out on and I feel that it's an extraordinary opportunity for the physics and astronomy community as we really start probing everything from the structure of an atom to the entire structure of the universe," Aizenman said. "It's all part of the same question." The dark energy finding closes a loophole left by the second-greatest cosmological finding of the past century -- that the universe is not merely expanding (this is Edwin Hubble's finding of the 1920s, widely noted as the greatest cosmological finding of the past 100 years) but its expansion is accelerating.

Merely a year ago many astronomers were skeptical that the universe was accelerating, despite supernovae (exploded star) observations published in 1998 that suggested space is expanding faster today than long ago just the opposite of conventional cosmological wisdom. Now, the observation of the farthest supernova ever detected now puts a "big nail in the coffin of alternative theories," says astrophysicist Michael Turner of the University of Chicago.

Michael S. Turner


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The dark energy confirmation comes on the heels of other closed loopholes that have shaken up the astrophysics world and lent more support to the accelerating universe idea. For instance, in the past three years, scientists have discovered that the universe is basically flat. This finding and others are knocking down an explanation for variations in the brightness of supernovae -- that some type of cosmic dust simply absorbs the light.
Upheaval for physics

Saul Perlmutter, who headed up the first group to publish its findings on the accelerating universe three years ago, said the confirmation of dark energy will spark an entirely new brand of physics. "Dark energy is something we have no clue as to what is causing it, and it doesn't fit into current physics theories, and they have to develop new approaches to explain it," said Perlmutter, an astrophysicist at the Lawrence Berkeley National Laboratory. "That's exciting. It's rare that we get to do this."

In 1917, Albert Einstein first imagined a repulsive force pervading space, which later came to be called the "cosmological constant." Like a 20th-century version of the mythological Atlas, Einstein sought to shore up the universe by keeping stars from falling together under gravity. Einstein fudged his relativity equations to add a repulsive force under certain conditions in space. This would keep the universe eternally balanced at a "steady state."

When astronomer Edwin Hubble discovered that the universe wasnt static but expanding, Einstein abandoned his cosmological constant and called it his biggest blunder. Astronomers kept Einsteins "fudge factor" locked away as a closet skeleton for decades. "Now I have to take dark energy seriously whether I want to or not," said physicist Michael Dine of University of California, Santa Cruz. Livio agreed. "Dark energy now becomes very central to our ideas about the universe," he said.

Michael Dine
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Ph.D., Yale University, 1978


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Percolating energy and quintessence

Although physicists have yet to say what dark energy is, they have some ideas about where it came from. Some say it could percolate from the vacuum of space. Laboratory experiments show that seemingly empty space is actually seething with virtual particles that wink in and out of existence.

This perpetually bubbling vacuum provides energy that could take the form of a repulsive "negative gravity," some say. The problem is that this vacuum-energy as calculated would be so absurdly powerful that it would have blown apart the universe very long ago. One way out is to assume that the vacuum-energy vastly weakens over time and is not constant as imagined by Einstein. This leads to another idea called "quintessence" (for "fifth essence") which proposes a repulsive field embedded in space, not unlike a gravitational field or a magnetic field.

Under that hypothesis, the field was created in the early moments of the universe along with the other forces in nature, and now stretches across the universe like a spider web. As the universe expanded and cooled, gravity and quintessence were locked in an arm wrestle for dominance over space. Both fields weakened with the universes expansion. But ultimately quintessence won out over gravity and took control to push galaxies apart.

Laissez-faire astrophysics

Other astronomers simply say, "Why worry?" Dark energy is just a basic feature of the universe. Trying to explain it is as pointless as trying to explain why Earth was the right distance from the Sun for life to develop. It just turned out that way; if it didnt we wouldnt be here to ask the question.

Historically, successful scientific theories are assembled from the simplest ideas. In the case of dark energy the simpler explanation is that dark matter -- as-yet-unidentified particles that comprise the bulk of the universe's mass -- and dark energy are really manifestations of a new theory of gravity. Maybe gravity weakens over time. Or, maybe the gravitational force leaks into unknown dimensions of the universe, mimicking the effects of eerie "dark stuff" in space.
Astronomers plan to look at more distant supernovae to precisely track how the universes expansion rate has changed, and this hopefully will narrow the dark energy alternatives.

"The dark energy mystery may be answered only by precision astronomical observations and not in the physics lab," says Turner. "One of our goals is to test for dark energy and see if this is preposterous because we are just dead wrong, or find out that we really do live in a preposterous universe."

Arip Nurahman (Teacher and Professional Lecturer)

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