Perhaps the greatest and most fiercely contested race in modern science is the search for dark matter.
Physicists cannot see this stuff, hence the name. However, they infer its existence because they can see its gravitational influence on the structure of galaxies and clusters of galaxies. It implies that the universe is filled with dark matter, much more of it than the visible matter we can see
If they’re right, dark matter must fill our galaxy and our Solar System. At this very instant, we ought to be ploughing our way through a dense sea of dark matter as the Sun moves towards the constellation of Cygnus as it orbits the galactic centre.
That’s why various groups are racing to detect this stuff using expensive detectors in deep underground caverns, which shield them from radiation that would otherwise swamp the signal.
These experiments are looking for the unique signature that dark matter is thought to produce as a result of the Earth’s passage around the Sun. During one half of the year, the dark matter forms headwind as the Earth ploughs into it; for the other half of the year, it forms a tailwind.
Indeed, a couple of groups claim to have found exactly this diurnal signature, although the results are highly controversial and seem to be in direct conflict with other groups who say they have not seen it.
There’s a a straightforward way to make better observations that should solve this conundrum. The dark matter signal should vary, not just over the course of a year, but throughout the day as the Earth rotates.
The dark matter headwind should be coming from the direction of Cygnus, so a suitable detector should see the direction change as the Earth rotates each day.
There’s a problem, however: nobody has built a directional dark matter detector.
That’s why a revolutionary new idea from an unlikely collaboration of physicists and biologists looks rather exciting. The group brings together diverse people, such as Katherine Freese at the University of Michigan in Ann Arbor, an astrophysicist and one of the leading thinkers in the area of dark matter, and George Church at Harvard University in Cambridge, a geneticist and a pioneer in the area of genome sequencing.
These guys say they can overcome the problems with conventional dark matter detection by using DNA to spot dark matter particles.
Their detector is unconventional, to say the least. Its basic detecting unit consists of a thin gold sheet with many strands of single-strand DNA hanging from it, like bead curtains or a hanging forest. Each strand of DNA is identical except for a label at the free hanging end, which identifies where on the gold sheet it sits.
The idea is that a dark matter particle smashes into a heavy gold nucleus in the sheet, sending it careering out of the gold foil and through the DNA forest. The gold nucleus then severs DNA strands as it travels, cutting a swathe through the forest.
These strands fall onto a collecting tray below, which is removed every hour or so. The segments can then be copied many times using a polymerase chain reaction, thereby amplifying the signal a billion times over.
Since the sequence and location of each strand is known, it is straightforward to work out where it was cut, which allows the passage of the gold particle to be reconstructed with nanometre precision.
The entire detector consists of hundreds or thousands of these sheets sandwiched between mylar sheets, like pages in a book. In total, a detector the size of a tea chest would require about a kilogram of gold and about 100 grams of single-strand DNA.
The advantage of this design is manifold. First, the DNA sequence determines the vertical position of the cut to within the size of a nucleotide. That kind of nanometre resolution is many orders of magnitude better than is possible today.
Second, this detector works at room temperature, unlike other designs which have to be cooled to measure the energy that dark matter collisions produce.
And finally, the mylar sheets make the detector directional. Each sheet should absorb the gold nucleus of this energy after it has passed through the DNA forest. Any higher energy nuclei, from background radiation or cosmic rays for example, should pass through several ‘pages’, which allows them to be spotted and excluded.
With the device facing in one direction, a dark matter particle strikes a gold nucleus, propelling it into the DNA forest. But in the other, the gold nucleus is propelled into mylar sheet where it is absorbed. That’s what makes it directional–the detector should only record events coming from one direction.
This should allow the device to spot the change in dark matter signal each day, which in turn should make the detection much less statistically demanding.
That’s a fascinating idea that’s likely to generate much interest. However, it’s not without some challenges of its own.
First up, nobody really knowns how rapidly-moving, highly-ionised gold nuclei will interact with single strands of DNA or indeed with forests of them. This is something the team plans to study in some detail before a detector can be built.
Then there is the challenge of making DNA strands that are long enough to present a reasonable ‘forest’ for gold nuclei to pass through. Church, Freese and co say they’d like strands consisting of 10,000 bases to create a forest that entirely absorbs the energy of a gold nucleus passing through it.
By contrast, off-the shelf arrays offer DNA strands with only 250 bases or so. These guys say they’ll probably have to settle for strands of about 1000 bases.
The DNA strands also have to hang straight down, rather than curled up. That’s a tall order over the area of a square metre or so that the detector will cover. At this scale, electric and magnetic fields trump gravity and these are likely to be a nuisance, particularly when it comes to collecting the severed DNA.
So the team will have to devise some kind DNA ‘comb’ that straightens the hair. One idea is attaching a tiny magnet to the free end of each strand, allowing it to be pulled downward.
The DNA strands will also have to be made from carbon-12 and 13, since carbon-14 is naturally radioactive and would otherwise produce an unwanted hiss of background noise. Using only very old carbon, in which all the carbon-14 has decayed, should do the trick.
Finally, there is the significant engineering challenge in making metre square DNA arrays, collecting trays that catch the severed DNA strands and fitting them altogether into a working detector.
There are more than a few unknowns in this approach which makes it high risk. But there is also high potential pay off because other designs for directional dark matter detectors are huge, complex and potentially vastly more expensive to build and run. That makes this approach exciting.
The discoverers of dark matter are a shoe-in for a Nobel prize. Given these stakes, we might see some investment in this idea sooner rather than later.
But there are also reasons to be cautious. A small but vocal minority of physicists say dark matter doesn’t exist, that other ideas better explain the structure of galaxies.
If they’re right, we’ll one day look back on these efforts in the same way we think about the search for phlogiston or the debate about the spontaneous emergence of lower life forms: as a mildly amusing cul de sac of 21st century physics.
Ref: arxiv.org/abs/1206.6809: New Dark Matter Detectors using DNA for Nanometer Tracking
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