The world we see, including ourselves, barely makes a dent in the universe or our understanding of it. The 4 percent of the universe that is visible matter fails to shine a light on how the universe evolved or why galaxies spin the way they do. Those answers lie instead just out of reach of our understanding in the dark patches of the cosmos: the so called dark matter.
But year by year during the last decade scientists have inched their flashlights closer to this dark matter threatening to uncover its constitutes and how it works.
"Most of the matter in our universe, about 85 percent, is not explained. It is not stars or planets or dust or gas," said Fermilab physicist Mike Crisler at the 2009 meeting of the American Association for the Advancement of Science in Chicago. "It is not made from atoms or molecules, protons, neutrons, or electrons. It does not absorb or emit light. We use the phrase "dark matter" in much the same way that ancient cartographers used the phrase ‘terra incognita.' We do not know its nature."
Even though we can't "see" dark matter, we infer its presence because of its gravitational pull, affecting the rotation speed of stars and galaxies. Stars are moving faster than they would if they were only influenced by the gravitational pull of the galaxy core, so the galaxy itself must be embedded in large clouds of dark matter exerting another pull.
The power of the dark side
Luminous matter accounts for only about 4 percent of the universe.
Scientists believe that without dark matter, galaxies would fly apart, the universe wouldn't have formed, and galaxies wouldn't cluster.
"Until we find this stuff, we can't really say that we understand gravity," said Dan Akerib, a physics professor from Case Western Reserve University in Ohio, at another AAAS talk on dark matter.
While the discovery of what makes up dark matter wouldn't change our everyday lives, it would change our perspective about the world, Crisler said.
"I think it would be a dramatic moment philosophically about the human role in the history of the universe," said Crisler.
Scientists across the globe are racing to make dark matter in the laboratory, see it in the sky, and catch it in deep underground caverns.
"This is a very active field. There are experiments being done in the United States, Europe, Asia," Akerib said. "During the last decade the sensitivity has increased by a factor of 100."
If that continues, the next five to 10 years could hold key discoveries, he added.
The AAAS talks highlighted upgrades in two of those experiments at Fermilab in Illinois that use temperature extremes to search for dark matter--the Chicagoland Observatory for Underground Particle Physics, COUPP; and the Cryogenic Dark Matter Search, CDMS.
COUPP bubble chamber
COUPP
Scientists first used superheated liquid in bubble chambers in the 1960s, but their use died out in the 1980s. COUPP revived and upgraded the bubble chamber and gave it a new use: the search for dark matter.
"Bubble chambers may be the next big thing in dark matter detection," said Crisler.
Bubble chambers effectively trace the interactions of weakly interacting massive particles, or WIMPs, with normal matter. Scientists believe the abundance of WIMPs left over from particle collisions just after the big bang may account for dark matter, Crisler said.
Mike Crisler adds a one-liter jar to the COUPP bubble chamber.
The liquid in a bubble chamber—typically hydrogen—is kept just above its normal boiling point, but under enough pressure that it will not boil unless disturbed. When a charged particle zips through the liquid it triggers boiling along its path, visible as a series of small bubbles. The biggest limitation for bubble chambers of the past was an inability to keep the liquid in a superheated state for an extended period of time. This required operators to time short blasts of particles from an accelerator to the few milliseconds when the temperature was just right. The COUPP collaborators got around this by finding a way to keep their liquid on the verge of boiling 80 percent of the experiment time, increasing the probability of catching dark matter particles.
Crisler and his colleagues recently upgraded a one-liter bubble chamber to block out more background noise and are almost finished constructing a 30-liter chamber, which will increase the probability of dark matter interactions. The current bell jar-shaped bubble chamber sits 350 feet underground at Fermilab and is filled with iodotrifluoromethane.
CDMS
At the opposite end of the temperature spectrum, CDMS uses cryogenics rather than super-heated liquid to pin-point WIMPs.
CDMS fridge and icebox in Soudan Mine.
When particles interact they can give off energy, which registers as heat. Scientists super cool the detector so that it conducts electricity without resistance. When energy is released from an outside particle colliding with a particle in the detector, it warms up part of the detector, briefly removing its superconductivity ability, and allowing scientists to see an electrical charge.
By measuring the difference in voltages associated with the temperature increases from an interaction, scientists can determine if the particle was a WIMP or a more common particle not associated with dark matter. WIMPs produce a low-charge yield while electron recoil from a photon has a large-charge yield.
ZIP detector in its mount.Silicon and germanium ZIPs, weighing 100 g and 250 g respectively, will be used in CDMS II.
The CDMS detector has been taking data since 2003 in the Soudan Mine in Minnesota with a 5 kg of active detector mass. An upgraded detector, with 25 kg of active detector mass and four times more sensitivity to particle collisions, is under construction. It will consist of seven stacks of six detectors, creating what Akerib calls the equivalent of a dark matter telescope underground.
Each type of technology used to search for dark matter has its strength. Which will prove the most useful remains to be seen.
CDMS and COUPP are racing other experiments in the United States, Europe, and Asia to find dark matter with accelerators, various types of underground detectors and telescopes.
“The search for dark matter is a horse race...," Crisler said. But many physicists see the finish line in sight.
With additional reporting by Kristine Crane.
