A Dark Matter Breakthrough?
By LAWRENCE KRAUSS
In early December, the Cold Dark Matter Search (CDMS) experiment located in the deep Soudan Mine in northern Minnesota leaked a tantalizing hint that they may have discovered something remarkable. The experiment is designed to directly detect new elementary particles that might make up the dark matter known to dominate our own Milky Way galaxy, all galaxies, and indeed all mass in the universe—so news of a possible breakthrough was thrilling.
The actual result? Two pulses were detected over the course of almost a year that might have been due to dark matter, CDMS announced on Dec. 17. However, there is a 25% chance that the pulses were actually caused by background radioactivity in and around the detector.
Physicists remain fascinated by the possibility that the events at CDMS, reported on the back pages of the world's newspapers, might nevertheless be real. If they are, they will represent the culmination of one of the most incredible detective stories in the history of science.
Beginning in the 1970s, evidence began to accumulate that there was much more mass out there than meets the eye. Scientists, mostly by observing the speed of rotation of our galaxy, estimated that there was perhaps 10 times as much dark matter as visible material.
At around the same time, independent computer calculations following the possible gravitational formation of galaxies supported this idea. The calculations suggested that only some new type of material that didn't interact as normal matter does could account for the structures we see.
Meanwhile, in the completely separate field of elementary particle physics, my colleagues and I had concluded that in order to understand what we see, it is quite likely that a host of new elementary particles may exist at a scale beyond what accelerators at the time could detect. This is one of the reasons there is such excitement about the new Large Hadron Collider in Geneva, Switzerland. Last month, it finally began to produce collisions, and it might eventually directly produce these new particles.
Theorists who had proposed the existence of such particles realized that they could have been produced during the earliest moments of the fiery Big Bang in numbers that could account for the inferred abundance of dark matter today. Moreover, these new particles would have exactly the properties needed for such material. They would interact so weakly with normal matter that they could go through the Earth without a single interaction.
Emboldened by all of these arguments, a brave set of experimentalists began to devise techniques by which they might observe such particles. This required building detectors deep underground, far from the reach of most cosmic rays that would overwhelm any sensitive detector, and in clean rooms with no radioactivity that could produce a false signal.
So when the physics community heard rumors that one of these experiments had detected something, we all waited with eager anticipation. A convincing observation would vindicate almost half a century of carefully developed, if fragile, arguments suggesting a whole new invisible world waiting to be discovered.
For the theorist working at his desk alone at night, it seems almost unfathomable that nature might actually obey the delicate theories you develop on pieces of paper. This is especially true when the theories involve ideas from so many different areas of science and require leaps of imagination.
Alas, to celebrate would be premature: The reported results are intriguing, but less than convincing. Yet if the two pulses observed last week in Minnesota are followed by more signals as bigger detectors turn on in the coming year or two, it will provide serious vindication of the power of human imagination. Combined with rigorous logical inference and technological wizardry—all the things that make science worth celebrating—scientists' creativity will have uncovered hidden worlds that a century ago could not have been conceived.
If, on the other hand, the events turn out to have been mere background radioactivity, physicists will not give up. It will only force us to be more clever and more energetic as we try to unravel nature's mysteries.
—Mr. Krauss is director of the Origins Institute at Arizona State University, and a theoretical physicist who has been involved in the search for dark matter for 30 years. His newest book, "Quantum Man," will appear in 2010.Printed in The Wall Street Journal, page A
Scientists shine light on dark matter: International workshop set in Cleveland
Posted by dsims March 09, 2009 05:53AM
Scientists believe that rotating galaxies like this one, called M64, are embedded in a cloud of dark matter whose gravitational tug keeps them from flying apart. The galaxy was photographed with the Hubble Space Telescope.Astronomers and physicists may be close to cracking one of science's biggest mysteries -- what makes up most of the universe.
It's not ordinary matter, the stuff that constitutes planets, stars, galaxies and humans. The bulk of the cosmos is some strange, unseen something. Scientists call it "dark matter," because they can't see it with telescopes or probe it with other conventional means.
No dark matter particles have yet been detected; they're likely so wispy that they can breeze through solid rock. But there are compelling clues that dark matter, whatever it is, exists. Otherwise, whirling galaxies like our Milky Way would fly apart of their own momentum.
Several developments are quickening the pace of the search, giving researchers hope that they are closing in on dark matter's identity. A space telescope put in orbit last summer, and powerful new detectors being sunk deep in mines and beneath polar ice, may soon capture dark matter's tracks. Computer models are pinning down how dark matter has sculpted immense clusters of galaxies. And the operators of a giant underground atom-smasher in Switzerland are poised to try to make "artificial" dark matter.
"All these pictures are beginning to come together now," said Case Western Reserve University astrophysicist Chris Mihos, one of the organizers of an international dark matter workshop in Cleveland this week.
For your information: Cosmologist Carlos Frenk, director of the Institute for Computational Cosmology at England's Durham University, will give a free public lecture, "The Great Cosmic Gamble: Making Galaxies from Nothing," at 8 p.m. Thursday, March 12, at the Cleveland Museum of Natural History. The museum is at 1 Wade Oval Dr. in University Circle. No tickets or reservations are required. For more information, call 216-231-4600 or go to www.cmnh.org
The gathering coincides with a public lecture here by Carlos Frenk, director of Britain's Institute for Computational Cosmology and one of the pioneers of dark matter research.
Frenk is willing to bet that scientists will identify dark matter within five years. "Technology has improved and now there is genuine expectation that discovery may be just around the corner," he said in a telephone interview. "There's a feeling in the air."
A lot is riding on the outcome, including professional reputations, millions of dollars being spent on dark matter detectors, and a likely Nobel Prize.
CWRU is well-positioned in the dark matter search. Mihos and astronomy department colleagues Heather Morrison and Idit Zehavi are using computer simulations to probe dark matter's impact on the evolution of galaxies. Theoretical physicists Glenn Starkman and Tanmay Vachaspati are studying the role of dark matter particles in the overall theory of how the universe works. And experimental physicists Tom Shuttand Dan Akerib are helping build new, ultra-sensitive dark matter detectors.
But why does dark matter matter? Why should anyone care about something so fleeting it can't be seen, felt, touched or tasted? After all, dark matter, as it's currently imagined, is streaming through everyone and everything on the planet without notice or apparent consequence.
The reason is that dark matter is the glue that holds the cosmos together. Its immense gravitational power binds star systems and shapes clusters of galaxies, the largest structures in the universe. And its quixotic behavior falls far outside the rules governing normal matter, like electrons, protons and neutrons.
"We're interested in this because we want to understand how [dark matter] fits into the bigger picture . . . how our universe works and why it works that way. It's the grandest of all the ambitions," said Dan Hooper, a particle astrophysicist at the Fermi National Accelerator Lab and author of "Dark Cosmos: In Search of Our Universe's Missing Mass and Energy."
Fritz Zwicky, a quirky, brilliant Caltech astronomer, was the first to suggest in the 1930s that the universe contained large amounts of unseen material, but it would take decades to find convincing proof.
In a 1970 research paper based on years of measuring the velocity of stars moving within galaxies, and of galaxies rotating in clusters, astronomer Vera Rubin made the formal case for dark matter. She showed there wasn't enough visible matter in the nearby Andromeda Galaxy to account for the speed at which its stars whirled around the galactic center. The stars ought to have been flung loose like gravel from a spinning tire - unless the gravitational tug from lots of unseen matter kept them in place.
More definitive, though still indirect, proof of dark matter came from telescope images in the 1980s. Astronomers saw that light from faraway galaxies was being warped in telltale ways, a phenomenon called gravitational lensing. [jma: http://astro.berkeley.edu/~jcohn/lens.html : ]The culprit was something invisible, and with colossal mass, lying between the galaxy and the telescope.
Ordinary matter, if there's enough of it, can bend light, and for a time scientists thought dark matter might be normal stuff like stars and planets that was just too distant and faint to detect. They gave such stuff the whimsical acronym MACHOs, for massive compact halo objects.
But calculations showed the Big Bang that created the universe nearly 14 billion years ago didn't produce enough protons, neutrons and other components of ordinary matter to account for dark matter's gravitational heft. MACHOs weren't macho enough.
So physicists turned to WIMPs instead. So-called weakly interacting massive particles are a weird, mostly hypothetical, standoffish kind of matter that barely mingles with ordinary stuff. The only WIMP ever to be detected is a stealthy particle called the neutrino. Neutrinos were a leading dark matter candidate for a while, until researchers figured out that they move too fast, at nearly the speed of light. Such fast-moving particles are too "hot," too energetic, to have settled down yet into the clumps that are the gravitational framework for galaxies.
Frenk has run computer simulations programmed to build a universe based on hot dark matter, like neutrinos, "but when we looked in detail, it really didn't make universes like our own."
So the question of what makes up dark matter is still open. The new favorite suspect is an even stranger WIMP called a neutralino. Its existence is predicted by a theory known as supersymmetry, suggests that ordinary particles like electrons and photons have a slew of exotic cousins, or super-partners.
Calculations show neutralinos should be light, slow and "cold" enough to be capable of shaping a universe that looks like ours. But they would be maddeningly hard to spot. Research teams are readying electronic dark matter sentries in Minnesota and Italy, buried far underground to shield them from interference. Like burglar alarms, they'll look for tiny shivers of energy when dark matter WIMPs rustle atoms in the detectors.
"The new generation of detectors will be able to say not only, 'I've gotten hit by a particle of dark matter,' but 'I've gotten hit by a particle coming from that direction,'" said Mihos. Knowing the direction of strike will help scientists learn about dark matter's distribution in the cosmos.
NASA's new Fermi space telescope is on dark matter watch, too, scanning for characteristic patterns of gamma rays that should spew from the collision of dark matter particles. Frenk said recent computer simulations have shown that dark matter might be packed densely enough in the center of the Milky Way to cause such collisions.
Finally, scientists expect that the new Large Hadron Collider, a 17-mile-long particle accelerator buried under the Franco-Swiss border, will slam together particles hard enough to briefly form the WIMPy, supersymmetric "super-particles" that may be dark matter's ingredients.
The visiting researchers will discuss all these dark matter scenarios at this week's Cleveland workshop. In relaxed moments, they might even kick around the heretical idea, endorsed by a few contrarian scientists, that dark matter doesn't exist and its galaxy-shaping effects are caused by some unknown property of gravity.
"My bias is I think dark matter is out there," Mihos said. "I'm trying to keep an open mind. If it's that we don't understand gravity, all those [dark matter] models are out the door. I would love that. The universe is even quirkier than we believe? That would be really cool."
tremonster says...
I'm in the dark on this one...LOL