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The Dark Matter Data Bonanza
After years of skepticism over the existence of dark matter, a growing number of experiments are revealing that dark matter was there all along.
The universe is filled with mysterious invisible stuff that refuses to interact with light. It doesn’t reflect, emit or absorb light. But astronomers know it is there because of its gravitational effect on the visible stuff. They call it dark matter.
But there is a problem. If dark matter exists (and on this blog we’ve looked at a number of alternative ideas), there ought to be a lot of it out there. Astronomers estimate that 83 percent of the mass of the universe should take this form. The rest, a mere 17 percent, is visible.
So where is all this stuff? It should permeate the Solar System, the Earth and our environment. And yet when physicists look for it, they find zip.
At least, most physicists find nothing. For the last few years, one group of scientists have been shouting from the rooftops that they can see dark matter.
These guys have placed a giant lump of salt at the bottom of a mine in Italy. This lump is a 250 kg crystal of sodium iodide doped with thallium. The thinking is that a collision between an exotic particle and a nuclei in the crystal would generate a photon that can be picked up by sensitive light detectors nearby.
This experiment is called DAMA/LIBRA and its results are controversial. While particles of dark matter can certainly generate photons in the crystal, any other kind of particle can also generate light too. So the experiment also picks up cosmic radiation, thermal neutrons and background radioactivity. This makes the results extremely noisy.
There is a way to separate the dark matter signal from all this background, however. As the Sun moves through the galaxy, it must also be moving through a sea of dark matter. And as the Earth moves around the Sun, it will plough more quickly into the sea of dark matter at some times of the year and at other times more slowly.
So the dark matter signal ought to have an annual modulation.
This is exactly what the DAMA/LIBRA people say they can see. The dark matter signal peaks in May and then drops away. And this no weak tentative signal–the DAMA/LIBRA people say the statistical evidence is so clear that there is almost no possibility that they are mistaken.
But most astrophysicists have ignored and even ridiculed the DAMA/LIBRA result. The reason is that there are many other dark matter detectors at the bottom of other mines around the world that see nothing. Many of these are thought to be more reliable because they screen ought the background noise from cosmic radiation and so on.
They should only see the dark matter. But they don’t.
Or at least they didn’t. A few weeks ago, a team with a detector called CoGeNT at the bottom of a mine in Minnesota announced that it had gathered very similar evidence to the DAMA/LIBRA experiment. Their evidence of dark matter is not as statistically strong but it is modulated in exactly the same way, peaking in late April or early May.
Today, Dan Hooper at the Fermi National Accelerator Laboratory and Chris Kelso from the University of Chicago review the data from CoGenT and DAMA/LIBRA and say they are compatible with each other. “If the true phase peaks in early May, this would represent a modulation consistent with that reported by the DAMA/LIBRA collaboration,” they say.
That’s quite a statement given the scepticism that many researchers have showed towards the DAMA/LIBRA team.
But the evidence doesn’t stop there. Hooper and Kelso also say that the type of dark matter that these results imply is consistent with other indirect evidence of dark matter that other experiments have seen. Things like the spectrum of gamma rays observed by the Fermi Gamma Ray Space Telescope and the haze seen by the WMAP spacecraft, thought to be generated by electrons near the centre of the galaxy emitting photons.
And there is more to come. Hooper and Kelso say that another experiment is on the verge of publishing detailed results that back up the DAMA/LIBRA-CoGenT claims. “The CRESST collaboration has reported the observation of an excess of events roughly consistent with that anticipated from a CoGeNT-like dark matter particle.”
So the world of dark matter research has been turned on its head in just a few months.After years of negative reports, we suddenly have an avalanche of positive ones.
That makes it an interesting topic not just for physicists but also for psychologists studying group dynamics too. The process by which scientific ideas become scientific facts has always been murky and strange.
But the truth is that it is as a susceptible to human foibles as any other field of endeavour and so just as likely to experience fads and fashions and sudden changes in opinion. It’ll be interesting to see what historians of science make of this episode.
Ref: http://arxiv.org/abs/1106.1066: Implications of CoGeNT’s New Results For Dark Matter