We are much more certain what dark matter is not that we are what it is.. First, it is dark, meaning that it is not in the form of stars and planets that we see. Observations show that there is far too little visible matter in the Universe to make up the 25% required by the observations. Second, it is not in the form of dark clouds of normal matter, matter made up of particles called baryons. We know this because we would be able to detect baryonic clouds by their absorption of radiation passing through them. Third, dark matter is not antimatter, because we do not see the unique gamma rays that are produced when antimatter annihilates with matter. Finally, we can rule out large galaxy-sized black holes on the basis of how many gravitational lenses we see. High concentrations of matter bend light passing near them from objects further away, but we do not see enough lensing events to suggest that such objects to make up the required 25% dark matter contribution.
However, at this point, there are still a few dark matter possibilities that are viable. Baryonic matter could still make up the dark matter if it were all tied up in brown dwarfs or in small, dense chunks of heavy elements. These possibilities are known as massive compact halo objects, or “MACHOs“. But the most common view is that dark matter is not baryonic at all, but that it is made up of other, more exotic particles like axions or WIMPS (Weakly Interacting Massive Particles)
Could physics’ next biggest mystery be solved in Sudbury?
The hottest thing in science today is cold. It’s also invisible, though it still manages to be heavy.
Dark matter — the mysterious stuff that physicists believe makes up a quarter of the universe but which no one has been able to directly detect — is having what the style world would call “a moment.” At this year’s American Association for the Advancement of Science (AAAS) annual meeting, the Fashion Week of science, dark matter talks were the Marc Jacobs fall collection: devotees crammed themselves into darkened rooms to get a glimpse of the Next Big Thing.
“I’ve been saying for a couple years now that the 2010s will be the dark matter decade,” says Sean Carroll, a theoretical physicist at the California Institute of Technology. With the discovery last summer of what is almost certainly the Higgs Boson, dark matter is the next big mystery in physics — and experiments designed to detect it are just beginning to show fruit.
Some of the most exciting are sitting in a mine shaft two kilometres below Sudbury, Ont.
By 2014, the SNOLAB underground laboratory will have five different experiments searching for what physicists believe dark matter is made of: WIMPs, or Weakly Interacting Massive Particles.
“Either we’re right or we’re wrong, but either way we’ll know because the experiments have enough sensitivity,” says Tony Noble, a SNOLAB scientist and physics professor at Queen’s University.
There are also dark-matter-hunting projects underway inside Italian mountain ranges, at the South Pole, and on the International Space Station. Many physicists are optimistic that these will confirm our theories of dark matter within the next couple years — or show they are wrong, which would be even more interesting.
“We’re hopeful that we’re close — and it’s just a hope, I don’t want to get to out of control here — but we’re hopeful that we’re close to directly detecting it,” says Carroll.
The first theories of dark matter came in the 1930s from Fritz Zwicky, an oddball Swiss-born astronomer. Zwicky convinced Caltech, where he worked, to build a telescope that let him measure the velocity of galaxies in a distant region called the Coma Cluster. He used the velocity to figure out the galaxies’ mass and discovered they were much more massive than they should be based on the stars visible inside them, the matter emitting light. There had to be more mass somewhere — “dark matter.”
The idea was mostly ignored until the 1970s when astronomers like Vera Rubin began examining individual galaxies in closer detail. Rubin, who had once been rejected from Princeton because their graduate astronomy program didn’t accept women, showed that galaxies seemed to be “missing” matter, putting the problem back on physicists’ agendas.
Scientists now believe all the matter we can see — everything on Earth and all the stars and planets in every other galaxy — accounts for only four per cent of the content of the universe. A full 25 per cent is dark matter, and the rest is dark energy (a force so mysterious we don’t even know how to design experiments to study it.)
Theoretical physicists have come up with a number of candidates for what dark matter might be. Neutrinos were a top contender, until their properties were better understood (thanks in large part to SNOLAB’s former incarnation as a solar neutrino observatory).
Some physicists think dark matter is made of axions. Some think it’s made of MACHOs, Massive Compact Halo Objects. Some physicists don’t think dark matter exists at all.
But most think dark matter is real and that it is made of WIMPs, a hypothetical particle that doesn’t interact with the electromagnetic force, so doesn’t emit light. That, and other qualities, makes them exceptionally hard to detect.
So experimental physicists and astronomers are hunting for dark matter in three different ways. They are using particle accelerators, like the Large Hadron Collider where the Higgs-like particle was discovered. They are using indirect detection methods that search for clues dark matter might leave behind, like the experiment mounted on the International Space Station. And they are trying to directly detect it in special laboratories deep below the earth’s surface.
“It’s remarkable that all the different ways we’re looking for dark matter are maturing at similar times,” says Carroll.
All of the different techniques complement each other — and some might reveal exciting things soon. At the AAAS meeting in Boston last month, Samuel C.C. Ting, the senior scientist of the space station experiment and a Nobel laureate, said his first results would be announced in March and hinted they would be important (prompting some private eye-rolling from colleagues).
But many physicists not involved in the experiments say they are most excited about direct detection projects. And SNOLAB will soon have one of the biggest: DEAP-3600, a massive experiment being assembled underground right now.
DEAP will contain 3,600 kg of liquid argon, making it the largest experiment of its kind in the world once it is operational a year from now, according to Noble, one of the lead scientists on the project. The idea is that the wind of dark matter passing through the Earth will result in a WIMP bumping into an argon atom, causing it to emit a flash of light that can be picked up by sensors. DEAP and other direct-detection experiments are buried underground in order to cut down on background noise on the Earth’s surface, like low-level radiation and the cosmic rays constantly bombarding the atmosphere.
SNOLAB also hosts PICASSO, another direct-detection experiment that consists of tubes containing tiny droplets of superheated Freon. The theory is that a WIMP will bump a Freon droplet, causing it to boil, which emits a noise that can be picked up by super-sensitive microphones.
As these experiments get bigger and more sensitive, they are ruling out places we thought we’d find dark matter, bringing us ever closer to either a major discovery — or a rash of new questions.
“How close are we to saying that it’s more surprising to say we haven’t seen it than that we have? The answer is very, very close.”