Autore: ExtremeTech

Hubble photo of the Pinwheel galaxy

Half a mile underground, in a disused mine in Minnesota, researchers have discovered the strongest ever hints of dark matter — a form of matter that is theorized to make up almost all of the matter in the universe, but which has — until now — led a completely hypothetical existence and never been observed.

According to the standard model of Big Bang cosmology — the current best-fit model for explaining the universe’s existence — the universe consists of 4.9% ordinary matter, 26.8% dark matter, and 68.3% dark energy. Ordinary matter is exactly what it sounds like, making up everything that you and I have ever seen or touched in the universe. Dark energy is a hypothesized form of energy that explains the accelerated expansion of the universe, but which has never been observed. Dark matter, on the other hand, is a theorized form of matter that explains the “missing mass” of large bodies — such as galaxies — that seem to have more gravitational oomph than their ordinary matter would imply.

CDMS silicon sensor, for detecting dark matter

So far, detecting dark matter has been impossible because it seems to neither emit or absorb any kind of electromagnetic radiation. The most widely accepted theory of dark matter is that it’s composed of WIMPs — weakly interacting massive particles that only interact with the rest of the universe via the weak force and gravity, making them extraordinarily difficult to detect. So far, most of humanity’s efforts at detecting dark matter involve placing some kind of detector down a deep hole, away from interfering radiation, and hoping that you can build a sensor that can detect some of the universe’s weakest signals.

Which leads us neatly onto Cryogenic Dark Matter Search experiment, situated hundreds of meters underground in the Soudan mine in Minnesota and operated by a collaboration of American universities, including MIT, Stanford, and Fermilab. The CDMS basically consists of a silicon or germanium crystal (pictured above), covered a thin layer of aluminium and tungsten, and then cooled to just 50 millikelvin — 50 thousands of a Kelvin above absolute zero, or -273.1 Celsius (-459.58F). At this temperature, the silicon and germanium crystals are superconductive and incredibly sensitive — so sensitive that, in theory, one particle of dark matter hitting a silicon or germanium atom should create a measurable temperature change in the aluminium-tungsten layer.

Now the CDMS experiment is reporting that one of its silicon crystals has detected three signals that appear to be WIMPs that are consistent with dark matter. These three signals occurred at a mass-energy of around 8.6 GeV (about 10 times the mass of a proton), which is far lower than most particle physicists would expect, but it is still consistent with some theories of dark matter, such as the dark sector. A mass of 8.6 GeV doesn’t correlate with findings from an underground experiment in Italy that pegged dark matter at 50 GeV a couple of years ago, but when dealing with something as flighty as dark matter, it’s not unheard of for experiments to have different results. It does mean that one of the experiments is wrong, though.

Inside the CDMS experiment; the silicon and germanium sensors slot into the hexagonal holes

Inside the CDMS experiment; the silicon and germanium sensors slot into the hexagonal holes

The chance of the three events being caused by something other than WIMPs/dark matter is 0.19%, which is around 3-sigma certainty. For something to be classified as an actual scientific discovery, such as the recently-confirmed Higgs boson, the results must show dark matter with 5-sigma certainty — 99.9999% certainty that it’s dark matter, or more correctly a 0.00001% chance that the detected signals aren’t dark matter. 3-sigma is certainly an exciting result that will warm the hearts of particle physicists the world over, though — along with the Higgs boson, the hunt for dark matter is one of the most important tasks currently being undertaken by particle physicists. For now, though — as always with science — we need more data.

Now read: What is the Higgs boson and why is it important to science?

Research paper: “Dark Matter Search Results Using the Silicon Detectors of CDMS II” [Free PDF]