With a simple experiment, would you like to demonstrate to your class that dark matter, in fact, does exist?
SNOLAB is one of the world’s most foremost particle physics research facilities, and is located in Sudbury, at Vale’s Creighton Mine, two kilometres underground. At SNOLAB, scientists build experiments to detect tiny particles called neutrinos and dark matter. SNOLAB celebrated its official grand opening on May 17th, 2012.
Visiting SNOLAB is an amazing experience. After walking through dirty dark mining drifts deep underground, you suddenly come into one of the cleanest places on Earth. Everyone has to shower and change into clean clothes before entering the lab. The reason for this is that everything, even human hairs and dust particles, contain small amounts of radioactive materials. If any such materials enter the lab, the radiation from these items could interfere with the extremely sensitive experiments.
The lab is a network of tunnels and huge caverns where giant experiments are ongoing and taking shape. The first experiment was called SNO, or the Sudbury Neutrino Observatory, and was built and operated in the 1990s and early 2000s. It consisted of a huge acrylic sphere, 12 metres in diameter, filled with heavy water. The whole sphere was suspended in an even larger cavern filled with water. It was surrounded by thousands of light detectors, looking for the faint glow a neutrino creates when it collides with a heavy water molecule from time to time. Heavy water is water where the hydrogen atoms have a neutron as well as a proton in their core, making it slightly heavier than regular water.
Even though over a billion neutrinos pass through your thumbnail every second, we see no evidence of this in everyday life. The neutrinos go right through us and even through the Earth. That is why a huge tank of water is needed to detect just a few of them. The reason SNOLAB is so deep under the Earth is to get away from the radiation at the surface, such as cosmic rays, that would interfere with these extremely sensitive experiments.
Many neutrinos come from the Sun; they are emitted in the nuclear burning of hydrogen into helium. SNO was one of the first experiments to confirm that we receive the expected amount of neutrinos from the Sun; that we understand the physics of the Sun.
Today, SNOLAB is gearing up to look for even more elusive particles — dark matter. No experiment has ever directly measured dark matter, even though we know that it makes up over 80% of the matter in the universe. Like it’s name suggests, dark matter does not emit light; it is completely dark. It does not, in fact, interact through the electromagnetic force at all, thus making it very hard to detect.
We know that dark matter exists from several pieces of evidence. We can, for example, look at the speed at which stars are orbiting galaxies. What we find is that stars in the outer regions of galaxies orbit much faster than would be expected if the only matter were the stars we can see. Since the galaxies are not flying apart, this means there must be extra matter present that creates an additional gravitational force to keep these stars in their orbits. The suggested activity which follows this article helps to demonstrate this concept further.
Another piece of evidence for dark matter comes from gravitational lensing. When the light of a distant galaxy, for example, passes by a more nearby galaxy cluster, it bends due to the gravity of the cluster. As a consequence, the cluster can act like a lens, and distort the image of the distant galaxy we see. By measuring the amount of distortion, astronomers can determine the mass in the cluster. Again, we find that the luminous matter can only account for about 20% of the total matter.
The prevailing scientific theory is that dark matter is a weakly interacting massive particle, or WIMP. Just like neutrinos, WIMPs only rarely interact with regular matter (other than through gravity). The environment at SNOLAB is therefore ideal to build highly sensitive experiments to try to detect these particles directly for the first time. Several dark matter
experiments are now being built or already taking data at SNOLAB. Sudbury could, in fact, become known around the world as the location where scientists first obtained proof for what kind of matter makes up most of our universe!
Dr. Simon Strasser (Ph.D., B.Ed.) is a Staff Scientist at Science North, in Sudbury, Ontario.