Using DNA in the hunt for 'dark matter', the glue of galaxies
Dark matter amid elliptical galaxies. Photo: AFP
That wonder molecule of life on Earth, DNA, is now being enlisted in the search for an exotic species zooming through the cosmos: dark matter.
As far back as the 1930s, astronomers watching distant galaxies saw that something was missing: there were not enough stars to account for the heavy gravity needed to whirl galaxies so quickly or smash them together so swiftly.
Something else must surround and suffuse every galaxy, some kind of gravitational glue.
Cosmologists dubbed it "dark matter", as it sheds no light. And, they say, it far outweighs all the ordinary matter – stars and planets – that they can account for.
The leading candidate for this mystery substance: subatomic particles called WIMPs, or "weakly interacting massive particles". They can't be seen, but they should be nearly everywhere (at least in our galactic neighbourhood). If true, every once in a great while, a zooming WIMP will by chance smack the nucleus of an atom like a well-struck cue against an eight ball.
For two decades, physicists have built detectors crammed with dense crystals and other heavy materials to try to catch WIMPs in this act. The results have been largely equivocal. There's no smoking WIMP signal yet – although hints have appeared.
Proposals for the next generation of dark matter detectors run into the tens or hundreds of millions of dollars. One such project would require an empty mine filled with a cubic kilometre of gas.
But now, a group of big-name theoretical physicists and biologists has proposed a radical new type of detector that dangles DNA as dark-matter bait. At coffee-table size, it would be much less expensive than other proposed detectors, they say.
Call it the ultimate mashup of biology and cosmology.
"For the very first time, an important problem in physics can be solved by techniques from another science," said Andrzej Drukier, the physicist-turned-biologist who struck on the idea.
Dr Drukier proposed a DNA detector in a 2010 talk at UCLA. Soon after, Katherine Freese, a University of Michigan theoretical physicist, joined him. The pair are well-placed to leapfrog the hunt for dark matter: in 1986, they laid out the theoretical rationale that led to the current generation of detectors.
This US spring, Dr Drukier and Dr Freese drove to the San Diego home of Charles Cantor, a pioneer of the Human Genome Project who built a successful DNA technology company, Sequenom.
They eased past Dr Cantor's Maserati, Dr Freese recalled, and sat around his pool overlooking the Pacific, batting around ideas. A challenge immediately appeared.
"When you're trying to think across such a vast range of disciplines, finding a common language is difficult," Dr Cantor said. "So we scribbled lots of pictures on pieces of paper – pictures instead of equations."
From those scratchings, a rudimentary design emerged, a cube about three feet on a side.
In it, thousands of strands of DNA hang from sheets of gold, like row upon row of beaded curtains. A WIMP zooms in – or so they hope – and hits a gold nucleus, which shakes free and tears through the DNA curtains, slicing them. (Gold, with its relatively heavy nucleus, makes a good choice for a material to rip through the DNA.) The severed DNA drops to the bottom of the detector, where it's collected. Standard DNA-reading machines – off-the-shelf technology – then reconstruct the path of the WIMP through the curtains.
By tracing the direction of the WIMP, the DNA detector offers crucial information that current detectors cannot provide: it tells you where in the sky the WIMP came from.
That's essential for testing dark-matter theories. As our solar system circles the centre of the Milky Way, it is moving towards constellation Cygnus. That means that the dark matter in our galaxy, which should be more or less stationary, will appear to be streaming in from that constellation.
Or, as Dr Freese said, "We're moving into this headwind of WIMPs." The DNA detector – the trail of broken DNA – will be able to sense this wind like a hand stuck out the window of a moving car.
Dr Freese said that if the detector works, finding evidence of just 30 WIMPs will be enough to prove that these elusive particles do, in fact, exist. Shazam: an 80-year cosmic mystery solved.
Early on, Dr Drukier passed the idea to other top DNA scientists, including Takeshi Sano of Japan and Harvard's George Church.
"I said, 'I have this crazy idea; kill it,' " Dr Drukier said. "They tried, but they couldn't."
Instead, both signed on as collaborators.
"This is exactly my kind of challenge," Professor Church said. In October, Dr Drukier spent a week in Professor Church's lab "hammering out details".
Professor Church calls the project payback. For two decades, biology has stolen talent, ideas and technology from physics, he said. Both he and Dr Drukier, in fact, began as physicists. The time has come, Professor Church said, for biologists to aid the quest to understand the universe.
In June, the collaborators unveiled the concept in the arXiv, an online repository of physics papers. The response, Dr Freese said, was immediate and positive. Invitations for talks in Paris and New York arrived; recently, she peddled the idea to a standing-room-only crowd at CERN, the European particle physics laboratory.
Optimistically, Dr Freese said, they can build the device within five years for perhaps $US100,000 ($95,500), as the price of manufactured DNA continues to drop. She recently won a small grant from the University of Michigan for pilot studies.
"We can do this," Dr Cantor said. "It all comes down to cost."
Professor Church is similarly optimistic. He said most of the DNA technology needed for the project already exists. Soon, he said, a gram of manufactured DNA will cost less than a gram of gold, now about $US55. Dr Freese estimates they will need a kilogram of each.
A theoretical physicist not involved with the project, Itay Yavin of the Perimeter Institute in Waterloo, Ontario, called it "a very cute idea", then added, "but whether it's actually possible in practice, that's hard to tell".
So first, Dr Freese will test a crucial aspect of the concept: how, exactly, will the nucleus from a gold atom slice through strands of DNA? What pattern might it create? She expects to start such tests within a few months at a small particle accelerator that her university rents for $US50 an hour. By the standards of most physics experiments, she said, "that's like free".
The Washington Post