An experiment called Darkside50 is about to start in a laboratory deep under the Gran Sasso mountain in Italy. The researchers at the Gran Sasso National Laboratory are looking for Weakly Interacting Massive Particles (WIMPs), believed to be responsible for so called dark matter. But what exactly is dark matter, what is the evidence for it and how to the physicists hope to detect the particles?
The history of dark matter
The first evidence for dark matter came from observations by Fritz Zwicky of the Coma cluster of galaxies. In the early 1930s Zwicky noticed that the speed of the galaxies in this cluster were too large to be gravitationally bound in the cluster based on the amount of matter one could see. For the galaxy speeds Zwicky measured, the cluster should fly apart, and yet it was clearly not doing so. He suggested that the cluster was dominated by unseen matter which had a gravitational effect which kept the cluster from flying apart.
Zwicky’s discovery, like many in science, went largely unnoticed for several decades. Then, in the 1970s and 1980s astronomers like Vera Rubin in the USA and Albert Bosma in France started measuring the motion of stars in individual spiral galaxies. Spiral galaxies, like our own Milky Way Galaxy, rotate. Rubin and Bosma used the Doppler effect to measure the speed of stars in the outer parts of spiral galaxies. What they found was quite a surprise. They found the so-called rotation curves of the galaxies they studied to be flat.
Once one is beyond most of the mass in a galaxy (presumed to be in the central bulge), the rotation curve should drop away as shown. This is known as a Keplerian rotation curve, because it follows the same distance-speed relationship as is found for the planets, which is known as Kepler’s third law. The fact that rotation curves of spiral galaxies remain constant suggests that there is unseen mass out as far as one is able to observe stars. The neutral hydrogen in galaxies extends far beyond the stars, and studies of the rotation curves of the neutral hydrogen also show their rotation curves to be flat.
Even the Sun’s motion about the Galactic centre seems to be affected by dark matter. Based on the matter we can see, the Sun should move at about 160 km per second, and yet we find it to be closer to 220 km per second.
Dark matter is used to explain this difference between the measured rotation speed of the Sun and its expected rotation speed.
Other evidence for dark matter
The motions of galaxies in clusters and the flat rotation curves of spiral galaxies are not the only evidence for dark matter. Other evidence comes from details of the cosmic microwave background, and from the gravitational lensing of background galaxies by foreground clusters of galaxies (and even by individual galaxies). Also large scale surveys of the structure of the Universe, by the Sloan Digital Sky Survey and the 2dF Galaxy Redshift Survey have shown that the structure of the Universe is best explained by a model where dark matter dominates normal matter by a factor of about 4, or to put it another way that only about 20% of the matter in the Universe seems to be in the form of normal matter, the other 80% seems to be in the form of dark matter.
What could the dark matter be?
It is believed that dark matter is in the form of weakly interacting massive particles (WIMPs). “Weakly interacting” means that they only interact through the weak nuclear force, the same force that is responsible for radioactive decay. We already know of an elusive particle which only interacts through the weak nuclear force – the neutrino. The neutrino was suggested in 1930 by Wolfgang Pauli to explain some observed details of radioactive beta decay. It was not until 1956 that they were actually detected. Since then we have built large neutrino detectors, for example the Sudbury Neutrino Observatory in Canada and the IceCube Neutrino Observatory in Antarctica. Even though billions of neutrinos pass through us unimpeded every second, we now regularly detect these elusive particles in these massive neutrino detectors. Physicists hope the same will be true for WIMPs.
The experiment to detect WIMPs at the Gran Sasso National Laboratory is called DarkSide50. Details of it can be found in this article. The experiment hopes to detect WIMPs by waiting for direct collisions between a WIMP and the nucleus of an argon atom. The detector will contain 100kg of liquid argon, and should a WIMP collide with an argon nucleus it will cause the argon nucleus to recoil, emitting a flash of light (known as a scintillation) which can be detected by photomultipliers arranged around the spherical walls of the detector.
It may take years to detect any WIMPs. Or, in fact, it might be that WIMPs do not exist and that we need to modify our understanding of gravity. Either result will be tremendously exciting.