Author Topic: Dark Energy?  (Read 2376 times)

Devious Viper
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Dark Energy?
« on: April 28, 2006, 02:36:09 PM »
It is one of the most famous, and most embarrassing, problems in physics. In 1998, astronomers discovered that the universe is expanding at ever faster speeds. It's an effect still searching for a cause - until then, everyone thought the universe's expansion was slowing down after the big bang. "Theorists are still floundering around, looking for a sensible explanation," says cosmologist Katherine Freese of the University of Michigan, Ann Arbor. "We're all hoping that upcoming observations of supernovae, of clusters of galaxies and so on will give us more clues."

One suggestion is that some property of empty space is responsible - cosmologists call it dark energy. But all attempts to pin it down have fallen woefully short. It's also possible that Einstein's theory of general relativity may not add up when applied to the very largest scales of the universe. "The field is still wide open," Freese says.
« Last Edit: August 22, 2006, 11:48:20 AM by Devious Viper »

Devious Viper
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Re: Dark Energy?
« Reply #1 on: May 25, 2006, 07:13:13 AM »
Dark Matter

Take our best understanding of gravity, apply it to the way galaxies spin, and you'll quickly see the problem: the galaxies should be falling apart. Galactic matter orbits around a central point because its mutual gravitational attraction creates centripetal forces. But there is not enough mass in the galaxies to produce the observed spin.

Vera Rubin, an astronomer working at the Carnegie Institution's department of terrestrial magnetism in Washington DC, spotted this anomaly in the late 1970s. The best response from physicists was to suggest there is more stuff out there than we can see. The trouble was, nobody could explain what this "dark matter" was.

And they still can't. Although researchers have made many suggestions about what kind of particles might make up dark matter, there is no consensus. It's an embarrassing hole in our understanding. Astronomical observations suggest that dark matter must make up about 90 per cent of the mass in the universe, yet we are astonishingly ignorant what that 90 per cent is.

Maybe we can't work out what dark matter is because it doesn't actually exist. That's certainly the way Rubin would like it to turn out. "If I could have my pick, I would like to learn that Newton's laws must be modified in order to correctly describe gravitational interactions at large distances," she says. "That's more appealing than a universe filled with a new kind of sub-nuclear particle."

Devious Viper
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Proof of dark matter?
« Reply #2 on: August 22, 2006, 11:47:20 AM »
Team finds 'proof' of dark matter
by Paul Rincon, Science reporter, BBC News

US astronomers say they have found the first direct evidence for the mysterious stuff called dark matter.

Dark matter - which does not emit or reflect enough light to be "seen" - is thought to make up 25% of the Universe.

By contrast, the ordinary matter we can see is believed to make up no more than about 5% of our Universe.

Until now, astronomers have only been able to infer the existence of this dark material through the gravitational effects it has on ordinary matter.

The researchers have discovered what is effectively the gravitational signature of dark matter.

This signature was created by dark matter and ordinary matter being wrenched apart by the immense collision of two large galaxy clusters.

"The kinetic energy of this collision is...enough to completely evaporate and pulverise planet Earth ten trillion trillion times over," said team member Maxim Markevitch of the Harvard-Smithsonian Center for Astrophysics in Cambridge, US.

Study leader Doug Clowe, from the University of Arizona, said: "This provides the first direct proof that dark matter must exist and that it must make up the majority of the matter in the Universe."

Astronomers have known since the 1930s that these galaxy clusters have far too much gravity to be explained by the amount of visible matter in them alone.

This extra gravity has two possible explanations. One is that most matter in the clusters is in a form we cannot see, because it does not absorb or emit light.

A second explanation is that gravity does not behave the same way in galaxy clusters light years in size as it does on Earth.

Usually, the gas and the galaxies in the clusters are held close together in space by gravity.

But in the cosmic smash-up known to astronomers as the Bullet Cluster, these components have been pulled apart. The astronomers were lucky enough to catch the collision just 100 million years after it occurred - the blink of an eye in cosmic time.

The researchers could see that the hot gas in the collision had been slowed down by a drag force, similar to air resistance. Meanwhile, the galaxies themselves continued speeding through space, leaving the gas behind.

Dark matter particles should not slow down in the same way as the gas; they do not interact directly with themselves or the gas except through gravity. Instead, dark matter should behave in a similar way to the galaxies.

If dark matter did exist, the astronomers expected to find the majority of mass in clusters residing around the galaxies.

But if dark matter did not exist, most of the galaxy cluster's mass would be in its diffuse hot gas. This is because galaxy clusters typically contain 10 times as much ordinary mass in gas as in stars.

The researchers found most of the mass was located near the galaxies - ahead of the gas clouds - showing the dark matter really was there.

The majority of the Universe - some 70% - is composed of dark energy, an equally mysterious quantity which exerts negative pressure.

"Dark matter and dark energy are not what anyone would have expected starting from the perspective of what the Universe should be like," said Sean Carroll, a cosmologist at the University of Chicago, who was not involved with the study, "but we're trying to understand why it's like that and this result puts us on that path."

In order to locate the mass in the clusters, researchers used the Chandra and Hubble space telescopes, along with the Very Large Telescope and Magellan optical telescopes in Chile.

This was done by measuring the effect of gravitational lensing, where gravity from the clusters distorts light from background galaxies, as predicted by Einstein's theory of general relativity.


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Re: Dark Energy?
« Reply #3 on: August 23, 2006, 10:09:30 AM »
I think the neatest thing is that the more we learn about the universe the more we discover we don't know Jack!  I now wonder how the evidence of dark matter will effect other scientific theories?  Will dark matter be a conduit in which worm holes tunnel through? 
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