This is the snapshot of the computer simulation of the dark matter Universe. These filamentary structures are called "cosmic webs" of dark matter.
Posted on 12/10/2005 11:49:52 AM PST by PatrickHenry
Clues revealed by the recently sharpened view of the Hubble Space Telescope have allowed astronomers to map the location of invisible "dark matter" in unprecedented detail in two very young galaxy clusters.
A Johns Hopkins University-Space Telescope Science Institute team reports its findings in the December issue of Astrophysical Journal. (Other, less-detailed observations appeared in the January 2005 issue of that publication.)
The team's results lend credence to the theory that the galaxies we can see form at the densest regions of "cosmic webs" of invisible dark matter, just as froth gathers on top of ocean waves, said study co-author Myungkook James Jee, assistant research scientist in the Henry A. Rowland Department of Physics and Astronomy in Johns Hopkins' Krieger School of Arts and Sciences.
"Advances in computer technology now allow us to simulate the entire universe and to follow the coalescence of matter into stars, galaxies, clusters of galaxies and enormously long filaments of matter from the first hundred thousand years to the present," Jee said. "However, it is very challenging to verify the simulation results observationally, because dark matter does not emit light."
Jee said the team measured the subtle gravitational "lensing" apparent in Hubble images — that is, the small distortions of galaxies' shapes caused by gravity from unseen dark matter — to produce its detailed dark matter maps. They conducted their observations in two clusters of galaxies that were forming when the universe was about half its present age.
"The images we took show clearly that the cluster galaxies are located at the densest regions of the dark matter haloes, which are rendered in purple in our images," Jee said.
The work buttresses the theory that dark matter — which constitutes 90 percent of matter in the universe — and visible matter should coalesce at the same places because gravity pulls them together, Jee said. Concentrations of dark matter should attract visible matter, and as a result, assist in the formation of luminous stars, galaxies and galaxy clusters.
Dark matter presents one of the most puzzling problems in modern cosmology. Invisible, yet undoubtedly there — scientists can measure its effects — its exact characteristics remain elusive. Previous attempts to map dark matter in detail with ground-based telescopes were handicapped by turbulence in the Earth's atmosphere, which blurred the resulting images.
"Observing through the atmosphere is like trying to see the details of a picture at the bottom of a swimming pool full of waves," said Holland Ford, one of the paper's co-authors and a professor of physics and astronomy at Johns Hopkins.
The Johns Hopkins-STScI team was able to overcome the atmospheric obstacle through the use of the space-based Hubble telescope. The installation of the Advanced Camera for Surveys in the Hubble three years ago was an additional boon, increasing the discovery efficiency of the previous HST by a factor of 10.
The team concentrated on two galaxy clusters (each containing more than 400 galaxies) in the southern sky.
"These images were actually intended mainly to study the galaxies in the clusters, and not the lensing of the background galaxies," said co-author Richard White, a STScI astronomer who also is head of the Hubble data archive for STScI. "But the sharpness and sensitivity of the images made them ideal for this project. That's the real beauty of Hubble images: they will be used for years for new scientific investigations."
The result of the team's analysis is a series of vividly detailed, computer-simulated images illustrating the dark matter's location. According to Jee, these images provide researchers with an unprecedented opportunity to infer dark matter's properties.
The clumped structure of dark matter around the cluster galaxies is consistent with the current belief that dark matter particles are "collision-less," Jee said. Unlike normal matter particles, physicists believe, they do not collide and scatter like billiard balls but rather simply pass through each other.
"Collision-less particles do not bombard one another, the way two hydrogen atoms do. If dark matter particles were collisional, we would observe a much smoother distribution of dark matter, without any small-scale clumpy structures," Jee said.
Ford said this study demonstrates that the ACS is uniquely advantageous for gravitational lensing studies and will, over time, substantially enhance understanding of the formation and evolution of the cosmic structure, as well as of dark matter.
"I am enormously gratified that the seven years of hard work by so many talented scientists and engineers to make the Advanced Camera for Surveys is providing all of humanity with deeper images and understandings of the origins of our marvelous universe," said Ford, who is principal investigator for ACS and a leader of the science team.
The ACS science and engineering team is concentrated at the Johns Hopkins University and the Space Telescope Science Institute on the university's Homewood campus in Baltimore. It also includes scientists from other major universities in the United States and Europe. ACS was developed by the team under NASA contract NAS5-32865 and this research was supported by NASA grant NAG5-7697.
High resolution images also are available, as are digital photos of Jee, Ford and White. Contact Lisa De Nike at Lde@jhu.edu.
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Have they decided on the fate of Hubble yet?
In a coarsely gridded sort of way.
Don't be such a spoilsport.
Griffin wants to send up the Shuttle for Hubble repairs, but the Shuttle schedule is rapidly evaporating. It may become impossible.
I post; you decide.
Another story about the same dumb ol' universe. Doesn't anyone ever broaden out a little?
The universe -- love it or leave it!
Of course they "interact with other matter" -- that's the point of the article: the gravitational interaction of dark matter with other matter (including dark matter) in the universe. The simulation of that predicted gravitational interaction produces a distribution of matter that matches what we see.
I'm not aware of any theoretical requirement for dark matter to be massless in order to be "collisionless," but since I am neither a particle physicist nor play one on TV, I'm pinging someone who is, in case he can shed more light on the subject.
"it is very challenging to verify the simulation results observationally"
Really? (Not surprisingly!)
"Dark matter [is] invisible, yet undoubtedly there"
You can count me as a doubter. Dark matter is a contrivance to fill the gaps in otherwise sensible theories.
I find it much easier to believe the fine structure constant and light velocity have changed over the history of the universe.
To me this is a more rational explanation of measurements that seem to indicate an accelerating universe and gravity with no apparent matter.
While we're waiting for the pros to turn up, another amateur opinion.
Dark matter has to have some kind of effective mass or it wouldn't be invoked to explain what it explains. It just doesn't have a gas pressure. Gas pressure (the effect of particles colliding) works against clumping. If particles that had gas pressure were all there is, space would look a certain way.
Having some of the mass in a space unable to bump into other particles, even its own kind of particles, helps stuff (including the stuff that does have collisions) clump up gravitationally better. Space looks a little different.
Which is why dark matter was hypothesized in the first place. The gravity wells of galaxies and eventually other spaces were deeper than we could explain without something invisible and very different from normal matter made of protons and neutrons.
Meanwhile, most of the mass-energy of the universe is something called "dark energy." That's a different can of worms yet.
Anyone know if the property of "collisionless" has been postulated or observed for anything other than dark matter? I guess this is the first I've heard of the term.
Unfortunately, Z's and Higgs bosons are extremely unstable, so collecting a gas of same is a tall order.
Neutrinos are stable and are practically non-interacting, but they are also practically massless.
In practice, all the other particles we have measured carry some other kind of charge, be it electromagnetic, weak or strong, but nothing in principle prevents there from being another type (or even class) of particles that don't interact except gravitationally. The universe could be brimming with them, but there's no way to detect them in the laboratory, just because of the fact that they carry no Standard Model charges. The only way to detect them would be through gravitational means.
[Geek alert: If there are extra dimensions, the possibility exists that a high-energy electron-positron collider could produce massive Kaluza-Klein "tower" graviton states. These in turn would couple to the gravitation-only dark matter states. You wouldn't see those dark-matter-producing collisions directly, as the final-state dark matter particles would escape, undetected as usual, but you could see a faint impression of them through Bremsstrahlung radiation from the initial-state electron and positron. This would manifest itself as events with a single, hard gamma ray, the distribution of which would be peaked in the forward direction (along the beam axis).]
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