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.
(I mean that affectionately!) Thank you for the information!
Neutrinos have an extremely low rate of interaction with matter; see reply #20.
I sure hope there isn't a quiz on this next period.
;-)
Not wanting to start anything but, why do you say what you do in your tag line? Do you think this is all some sort of grand accident?
Thanks for the ping!
I think we tend to get in trouble when we use one hypothetical particle to explain the existence of another hypothetical particle. I agree that the Higgs should exist, but for now it is purely hypothetical.
LOL & residual spasmodic chuckling. Anyway, I'm as open to the idea of other dimensions as an amateur can be...
I was asked for real examples of collisionless particles. Z particles are NOT hypothetical. In theory, the collisions of Z particles will be mediated by the Higgs. If there is no Higgs, then my example is even better.
Thanks always. As Hamlet's Laertes, "I will the effects of this good lesson keep."
Thanks for the ping.
The first too-simple answer is that they can't...and the second too-simple answer is that--to a certain extent--they do!
Suppose you had a point-like collection of many of these dark matter, and another one comes tooling along. Of course the incoming particle will be attracted to the larger mass. Regular matter would stick together and clump up, but not these. Even if the incoming particle falls dead-center, it'll pass right through and keep going.
If it has enough energy, it'll escape. But if it doesn't, it'll get to some maximum distance, fall back through, reach the same maximum distance in the opposite direction, and fall back through again, repeating forever, the perfect harmonic oscillator.
But that's a bit contrived. In the real world, what you'd have isn't a single, large clump with some test particles falling directly at it, but a diffuse gas of particles falling every which-way. If there happend to be a local area of higher density (and for this purpose a clump of regular matter will do), the nearby dark matter particles will bend their trajectories towards it, and eventually, the combined effect of all the purely elastic collisions will be to condense the dark matter. On a large scale, it actually will form clumps, and you can see that that has happened in the pictures from this group.
But in order to condense, a gas must somehow lose its gravitational potential energy. If all collisions are elastic, all they do is to redistribute the energy, and not shed it...so how can this be? Well, don't forget that the dark matter particles also undergo these extremely weak, elastic collisions with regular matter as well. If the dark matter is hotter than the regular matter, then on average, the dark matter will be losing kinetic energy to the matter, which will in turn radiate some of it away as photons. In this way, the dark matter slowly cools and condenses.
One more thing. If a collection of dark matter does, by itself, form a black hole, as could certainly happen, it will be indistinguishable from one formed by regular matter, except that it won't have a magnetic field. But the main features--the Schwartzschild radius, the event horizon, the lensing--will be the same.
Bump!
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