Scientists say they have proof of dark matter
Two galaxies' collision studied
By Marc Kaufman, Washington Post | August 22, 2006
WASHINGTON -- For decades, many scientists have theorized that the universe is made up of nearly undetectable mysterious substances called dark matter and dark energy. But until yesterday there was no proof that the subatomic matter exists.
After studying data from a long-ago collision of two giant galaxies, researchers now say they are certain dark matter exists and plays a central role in creating and defining gravity throughout the universe.
While the scientists are not sure exactly what dark matter is, since they have yet to identify it in a laboratory, they said the workings of the universe cannot be explained without it.
The finding will have potentially great impact on an active debate among physicists and cosmologists about not only dark matter, but the workings of gravity. Indeed, the theory of dark matter evolved largely to explain the finding several decades ago that there was not enough visible matter in the universe to produce and account for the gravity needed to keep galaxies from flying apart.
``A universe that's dominated by dark stuff seems preposterous, so we wanted to test whether there were any basic flaws in our thinking," said Doug Clowe of the University of Arizona in Tucson, leader of the NASA-Harvard University study. ``These results are direct proof that dark matter exists."
The breakthrough came by using data from NASA's orbiting Chandra X-ray Observatory, and involved information from what researchers called the most massive releases of detected energy in the universe since the Big Bang.
Scientists said a collision between the enormous ``bullet" cluster of galaxies more than 3 billion light years away and another smaller galaxy cluster proved the existence of dark matter by, in effect, stripping the dark matter away from visible matter.
Once stripped, dark matter was clearly identified by the strong gravitational pull that it exerted.
``We now have direct evidence" of dark matter, said Sean Carroll, a cosmologist in the Physics Department of the University of Chicago, who did not participate in the study. ``There is no way to explain the observations without dark matter."
The Chandra X-Ray Center, linked to the Smithsonian Astrophysical Observatory, is in Cambridge.
While the theoretical existence of dark matter has been broadly embraced for years -- and has now been further endorsed by some of the most prominent researchers and institutions in the field -- a strong counter theory has grown contending that the laws of gravity established by Newton and Einstein need modification.
This group says a relatively limited tweaking of those laws, especially as they pertain to the massive nature of faraway galaxies, could explain the missing gravity better than undetectable dark matter does.
Stacy McGaugh, an astrophysicist at the University of Maryland, has been one of the dark matter skeptics, and he said yesterday that he remained unconvinced.
``I've been aware of this result some time, and I agree that it is interesting, and may make more sense in terms of dark matter than alternative gravity," he said. ``However, it is premature to say so."
He said a definitive detection of dark matter particles would mean ``grabbing them in the laboratory, not just inferring that their effects can be the only possible explanation for an observation before the alternatives have actually been checked."
The NASA-affiliated team that announced its findings yesterday said the next step in trying to understand dark matter (and related dark energy) is, in fact, to identify it in a laboratory.
That task has proved difficult, they said, because dark matter leaves no detectable traces, except to create a gravitational pull.
``This finding doesn't tell us where dark matter comes from," said Carroll. ``It tells us that dark matter exists, but it doesn't say what it is, or why there's so much of it. The real adventure is ahead of us."
for some odd reason, while the word "Ganya" was still just a thought-dropping in my head, I thought it'd only be four letters. But apparently it's five. yep.
By MATT CRENSON
The Associated Press
Thursday, November 16, 2006; 6:41 PM
NEW YORK -- The Hubble Space Telescope has shown that a mysterious form of energy first conceived by Albert Einstein, then rejected by the famous physicist as his "greatest blunder," appears to have been fueling the expansion of the universe for most of its history.
This so-called "dark energy" has been pushing the universe outward for at least 9 billion years, astronomers said Thursday.
"This is the first time we have significant, discrete data from back then," said Adam Riess, a professor of astronomy at Johns Hopkins University and researcher at NASA's Space Telescope Science Institute.
He and several colleagues used the Hubble to observe 23 supernovae _ exploding white dwarf stars _ so distant that their light took more than half the history of the universe to reach the orbiting telescope. That means the supernovae existed when the universe was less than half its current age of approximately 13.7 billion years.
Because the physics of supernova explosions is extremely well-known, it is possible for the astronomers to gauge not just their distance, but how fast the universe was expanding at the time they went off.
"This finding continues to validate the use of these supernovae as cosmic probes," Riess said.
He and his colleagues describe their research in a paper that is scheduled for publication in the Feb. 10 issue of Astrophysical Journal.
The idea of dark energy was first proposed by Einstein as a means of explaining how the universe could resist collapsing under the pull of gravity. But then Edwin Hubble _ the astronomer for whom the NASA telescope is named _ demonstrated in 1929 that the universe is expanding, not a constant size. That led to the big-bang theory, and Einstein tossed his notion on science's scrap heap.
There it languished until 1998, when astronomers who were using supernova explosions to gauge the expansion of the universe made a shocking observation. It appeared that older supernovae, whose light had traveled a greater distance across space to reach the Hubble telescope, were receding from Earth more slowly than simple big-bang theory would predict. Nearby supernovae were receding more quickly than expected. That could only be true if some mysterious force were causing the expansion of the universe to accelerate over time.
Cosmologists dubbed the force "dark energy," and ever since they've been trying to figure out what it is.
"Dark energy makes us nervous," said Sean Carroll, a theoretical physicist at the California Institute of Technology who was not involved in the supernova study. "It fits the data, but it's not what we really expected."
Answers may come once NASA upgrades the Hubble Space Telescope in a space shuttle mission scheduled for 2008. NASA and the Department of Energy are also planning to launch an orbiting observatory specifically designed to address the mystery in 2011.
Dark energy could be some property of space itself, which is what Einstein was thinking of when he proposed it. Or it could be something akin to an electromagnetic field pushing on the universe. And then there's the possibility that the whole thing is caused by some hitherto undiscovered wrinkle in the laws of gravity
The data comes from a three-year study of exploding stars
Dark energy - the mysterious force that is speeding up the expansion of the Universe - has been a part of space for at least nine billion years.
That is the conclusion of astronomers who presented results from a three-year study using the Hubble Space Telescope.
The finding may rule out some competing theories that predict the strength of dark energy changes over time.
Dark energy makes up about 70% of the Universe; the rest is dark matter (25%) and normal matter (5%).
Understanding the nature of dark energy is arguably the biggest problem physics is facing today
Mario Livio, Space Telescope Science Institute
"It appears this dark energy was already boosting the expansion of the Universe as much as nine billion years ago," said co-investigator Adam Riess from the Space Telescope Science Institute in Baltimore, US.
"That's out of a Universe which we think is about 13.7 billion years old - most of the way back."
The findings are consistent with the idea of dark energy behaving like Albert Einstein's cosmological constant. The cosmological constant describes the idea that there is a density and pressure associated with "empty" space.
In this scenario, dark energy never changes; it has the same properties across the age of the Universe.
Repulsive force
Einstein first conceived of the notion of a repulsive force in space in his attempt to balance the Universe against the inward pull of its own gravity, which he thought would ultimately cause the Universe to implode.
His cosmological constant remained a curious hypothesis until 1998, when astronomers used observations of supernovae from ground-based telescopes and Hubble to show that the expansion of space was accelerating.
These findings suggested there really was a repulsive form of gravity in space, a force that was shortly dubbed "dark energy".
There have been many attempts to explain the nature of dark energy.
One of these is that it behaves like the cosmological constant. Another is that dark energy behaves like a field that changes over time. The third proposes changes to our theories of gravity to explain the mysterious force.
The latest data from Hubble contradict theories that dark energy might have behaved differently billions of years ago to how it behaves now, or might not even have been present. Some astronomers had thought that dark energy might mimic whatever was the dominant force in the Universe at the time, such as matter for example.
Previous Hubble observations of the most distant supernovae known revealed that the early Universe was dominated by matter whose gravity was slowing down the Universe's expansion rate.
The observations also confirmed that the expansion rate of the cosmos began speeding up about five to six billion years ago. That is when astronomers believe that dark energy's repulsive force took over from that of gravity.
'Tug of war'
"Imagine that you were having a tug of war and the other end of the rope disappears behind a curtain. Somebody else is tugging on the other end; we'll call that dark energy," said Dr Riess.
"In 1998, we saw that the thing behind the curtain was winning, it was pulling harder and the Universe was accelerating.
"In 2004, we showed that was not always the case. There was a time when you - ordinary matter - were winning. The Universe was decelerating. Now, we have shown that, even at that time, the thing on the other end of the rope was beginning to pull."
The discovery comes from observations of 23 exploding stars, or supernovae. Using Hubble to peer far across the Universe, the astronomers were able see back to a time when the cosmos was less than half its present size.
"These supernovae provide cosmic mile-markers that allow us to measure the growth rate of the Universe about nine billion years ago," said Adam Riess.
Mario Livio, of the Space Telescope Science Institute, added: "Understanding the nature of dark energy is arguably the biggest problem physics is facing today."
In October, the US space agency (Nasa) said that shuttle astronauts would be sent to service the Hubble Space Telescope, which will fail within two or three years without running repairs.
08-23-2006 01:26 AM
