SCIENCE : Dark Energy's Fingerprint Found in Distant Galaxies

This image was produced by combining a dozen NASA Chandra X-ray Observatory observations made of a 130 light-year region in the center of the Milky Way. When microwaves created by the Big Bang pass through large galaxies, they appear to pick up energy. The reason, say astronomers, is dark energy. Photo courtesy of NASA.
'May be the clearest detection of dark energy to date'
By Irene Klotz / August 5, 2008

Scientists don't know much about the mysterious phenomenon known as dark energy, but they do have a picture of what it's doing to the universe, namely, driving it apart.

In what may be the clearest detection of dark energy to date, astronomers at the University of Hawaii looked at microwaves left over from the beginning of the universe some 13.7 billion years ago. The scientists grouped the rays depending on whether they had passed through massive clusters of galaxies or bee-lined to Earthly detectors through areas largely lacking galactic real estate.

The premise was that these "background" microwaves would pick up a little energy as they entered the clusters, urged on by the forces of gravity. But if gravity had a monopoly on the game, the rays would lose that snap as they pressed through the other side, rendering the energy gain a transitory phenomenon.

So how to explain that the microwaves passing through super-sized galaxy clusters got to keep a bit of unearned gain?

István Szapudi and colleagues believe it is because dark energy, which sometimes is referred to as anti-gravity or vacuum energy, had spread out the galaxy clusters, as it is doing to all space. By the time the microwaves exited, there was proportionally a bit less mass to deal with, leaving the rays with a slight energy advantage.

"It's kind of like if you have a car on a hill," explained Gary Hinshaw of NASA's Goddard Space Flight Center in Greenbelt, Md. "You pick up energy as you're going down, but you give it all back up again when you go up.

"What is happening here," he added, "is that the acceleration of the universe is forcing the gravity wells to be less strong...so over the time it takes for energy to cross the cluster, the strength of the cluster has diminished."

The measurements are difficult to make because tiny variations in the Big Bang remnant waves are larger than the observable effects of intervening galaxy clusters and voids. But by grouping together data from background radiation maps of the 50 largest galaxy clusters and the 50 largest voids, researchers were able to come up with a finding they say has only a one in 200,000 chance of being a statistical fluke.

The data was taken from Sloan Digital Sky Survey, which has mapped the distribution of galaxies in about 25 percent of the sky.

Previous dark energy studies have a one in 20 chance of being statistically coincidental, Szapudi said.

Dark energy was discovered about 10 years ago and is considered the leading outstanding puzzle in cosmology today.

"In the last six or seven billion years, the expansion of the universe has switched over from slowing down to speeding up, meaning that dark energy is becoming more dominant in controlling the growth of the universe," Hinshaw said. "As the universe expands, matter gets more and more dilute and dark energy gets more and more dominate."

Szapudi's research will be published in an upcoming issue of The Astrophysical Journal.

Source / Discovery News

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Fourth of July Fireworks

Celestial Stripe. This image of a delicate ribbon of gas, taken by NASA's Hubble Space Telescope, reveals a very thin section of a supernova remnant caused by a stellar explosion that occurred more than 1,000 years ago. Photo courtesy of NASA/ESA/Hubble Heritage Team.
Celestial Stars and Stripe Revealed in Hubble Image
Irene Klotz / July 3, 2008

About 700 years before the birth of America, a dying star exploded, creating a shock wave that blasted through space at nearly 20 million m.p.h. for the next thousand years.

Initially, the burst of light was so bright that it could be seen in daylight on Earth, nearly 7,000 light-years away in a constellation known as Lupus.

Radio telescopes picked up its trail in the 1960s with the discovery of a nearly circular ring of material in the general area of where the supernova had occurred. It wasn't until 1976 that astronomers had a powerful enough observatory in the southern hemisphere and the good luck to pick up another visual.

On the northwest edge of the radio ring, the shock wave had reached a part of space sprinkled with hydrogen atoms, causing them to radiate in visual light.

"It's kind of like a sonic boom," said astronomer Frank Winkler with Middlebury College in Vermont. "Your ears clearly detect that as a change in pressure. In the case of the supernova, this shock wave has been propagating outward from the site of the explosion for a little more than 1,000 years now.

"One of really interesting things that happens is that behind the shock wave, some of these hydrogen atoms, which are essentially bare nuclei, are really fast-moving -- a few thousand kilometers per second -- so if you have a collision between one of these and an unsuspecting neutral hydrogen atom that suddenly finds itself right behind the shock wave, they can trade electrons," he said. "The fast-moving one gloms on to the other's electron and that leads to the emission of a photon."

The Hubble Space Telescope picked up the trail 30 years later with a series of observations, culminating in the release this week of a picture to mark Independence Day.

The celestial version of the stars and (a) stripe show orange-hued points of light that are background galaxies and white dots which are background and foreground stars in our own Milky Way.

The bold red ribbon of light is a tiny portion of the tenuous hydrogen gas being heated by the supernova blast wave. The bright spots are areas where the shock wave is edge-on to our line of sight. Hydrogen's glow is mostly in a deeper red hue so the Hubble team shaded it a bit more orange to make it easier to see.

The supernova, known as SN 1006, is 60 light-years in diameter and still growing at a rate of about 6 million m.p.h.

Source. / Discovery News

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