Hot on the trail of unviverses darkest secret

Originally sourced from The Age, November 2013: (link no longer works).



Hot on the trail of universes darkest secret

With physicists awash in tantalising data about dark matter, one of the greatest mysteries of modern physics may be solved within the next few years, some physicists say.

They now have the tools to find dark matter, mysterious stuff that makes up about a quarter of the universe. It’s streaming to them from a variety of sources, including the International Space Station, the world’s most powerful atom smasher in Switzerland as well as a number of exotic instruments buried deep within mine.

With all this data at hand, they believe the answer is within their grasp.

“I think this is the most exciting time of my life,” said Bhaskar Dutta, a theoretical physicist at Texas A&M University. “Before the last few years we had nothing to look with. Now we’re looking everywhere.”

As galaxies rotate they are held together by gravity. The faster the spin, the more gravity needed to hold a galaxy together. And galaxies spin rather quickly.

But when astronomers look to the heavens and count all the stars in our galaxy and others, they simply don’t see enough “normal” matter made out of atoms to hold galaxies together by the force of gravity. The galaxies should be flying apart.

Since they’re not, and for other reasons, astrophysicists believe large amounts of hidden matter – dark matter – must exist.

Scientists have been looking for dark matter for decades, but it appears to interact very weakly with normal matter, making it very hard to detect.

That doesn’t mean it’s impossible, and scientists believe there are ultimately several ways to spot dark matter. They can look in space for the leftovers from collisions between dark matter particles, they can try to create them in a large particle accelerator, or they can try to catch the elusive particles passing through the Earth.

Each of these methods required better tools, which are now coming online.

In 2011, after 15 years of development, space shuttle Endeavour carried the Alpha Magnetic Spectrometer to the International Space Station to study cosmic rays for evidence of dark matter collisions.

The experiment produced its first results in April, finding an interesting excess of positrons – a form of antimatter that is the counterpart of the electron. But the results were inconclusive.

“Over the coming months, our experiment will be able to tell us conclusively whether these positrons are a signal for dark matter, or whether they have some other origin,” said Sam Ting, the experiment’s chief scientist, at the time.

Initial efforts to make dark matter in a particle accelerator have also failed. At the Large Hadron Collider in Switzerland, while some physicists were finding the Higgs boson, others were unsuccessfully trying to make dark matter particles using the powerful particle collider. Physicists are hopeful that, after upgrades double the power to their experiment, they might succeed in creating dark matter in 2015.

“It’s a little frustrating that we haven’t found it yet, but we know it’s there,” said Paul Padley, a physicist at Rice University who works on experiments at the Switzerland-based collider.

A final way to detect dark matter is building elaborate detectors filled with exotic elements, such as Xenon and Germanium, and burying them deep in mines. Physicists believe that every once in awhile a dark matter particle will collide with one of these elements.

More than two dozen such experiments have been set up.

Taking data at an old iron mine in Minnesota, a team of physicists published a research paper in April with the spectacular conclusion that they were 99.8 per cent sure they had observed dark matter.

This was a truly exciting result – until late October when another buried in a gold mine experiment in South Dakota released results after taking data for three months.

Whereas the Minnesota experiment found three candidate dark matter particles, the much larger $US15 million LUX detector in South Dakota should have found 1550 events, or one every 80 minutes during the experiment’s initial run.

It found not a single one.

In essence, then, the newest and best experiment to detect dark matter didn’t confirm earlier experiments that claimed to have found it.

“They’re just not there,” said Richard Gaitskell, a Brown University physicist who is one of the leaders of the LUX experiment. “We are just not consistent with the observations that a number of other dark matter experiments have made. With our detector it’s much more difficult to be fooled.”

If this all sounds like terrible news, then you’re not a physicist.

The truth is, because scientists don’t know precisely how large dark matter particles are, nor what they’re like, they only have theories about what they are. And so they only have a few clues where to look. As more experiments probe dark matter and try to find it, physicists are narrowing down the mass dark matter particles can have.

“Dark matter is running out of places to hide,” said Rupak Mahapatra, a Texas A&M University physicist involved with the Minnesota mine experiment.

In other words, knowing what dark matter isn’t, is almost as useful as knowing what it is.


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