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Overhead view of a cloud chamber revealing the paths of particles. Video: Zac Kenny.
Why explore dark matter?

Dark matter makes up about 84% of matter in the universe and reveals itself through its gravitational effects. Drift: Art and Dark Matter allowed artists and scientists to come together and consider what dark matter might be, and why it is thought to exist.

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Drift: Art and Dark Matter

Why explore dark matter?

Astronomical observations have revealed that something we don’t fully understand is helping to shape the universe. Gravity holds planets in their orbits, and keeps stars within their galaxiesbut measurements show there is more gravity holding galaxies together than what could be generated by all the matter we can see (stars, planets, dust, etc.). It is dark matter, invisible to human senses, that gives the universe the structures it has. Until we learn more about it, “dark matter” is the label we use for the cause of this extra gravity.

In science, the unexpected occurs when the result of an experiment is different than predicted by existing knowledge.

Think about how you know something—that the sun will rise tomorrow, for example—and how that knowledge sets your expectations.

To find the mysteries in science is to find those moments when assumptions about knowledge are shown to be wrong: this is the scientific process. Scientists must avoid their biases to be able to recognize the unexpected.

That dark matter exists is one of the most important and unexpected discoveries of the last century. The search for dark matter is the scientific endeavour to understand what this unknown matter really is.

A liquid nitrogen tank being used at SNOLAB. Video: Dan Thomson.
How do we know dark matter is there?
Although we haven’t detected it directly, there are many types of evidence that, when put together, all point to dark matter’s existence.
Galaxy rotations
Galaxy rotations
Image: ESA/Hubble & NASA. Acknowledgement: Judy Schmidt (Geckzilla).
Galaxies are made up of gas, dust, and stars, as seen here in the beautiful spiral galaxy NGC 6814. Like planets in our solar system, a star’s speed as it travels around the centre of its galaxy reveals the strength of gravity’s pull. Galaxies are not collapsing inward or flying apart, so the star’s speed must balance the pull of gravity, just as the International Space Station stays in steady orbit around Earth. Yet the speeds of these stars often tell us that there is much more gravity than can be accounted for by normal matter, and that this extra gravity is present out to at least the edge of the galaxy.

Image: ESA/Hubble & NASA. Acknowledgement: Judy Schmidt (Geckzilla).
Galaxy clusters
Galaxy clusters
Image: NASA, ESA, and the Hubble Heritage Team (STScI/AURA). Acknowledgment: D. Carter (Liverpool John Moores University) and the Coma HST ACS Treasury Team.
Many galaxies are not found all alone, but instead belong to large groups. Here we see the Coma Cluster, one of the galaxy clusters closest to us in the universe. Like stars in a galaxy, the galaxies themselves orbit around a central pull of gravity. Galaxies are not falling into or flying away from the cluster, so the galaxy motions must balance gravity. Yet the speeds of these galaxies reveal that there is extra gravity present throughout the cluster, not just where we see the light from normal matter, such as galaxies’ stars.

Image: NASA, ESA, and the Hubble Heritage Team (STScI/AURA). Acknowledgment: D. Carter (Liverpool John Moores University) and the Coma HST ACS Treasury Team.
Gravitational Lensing
Gravitational Lensing
Image: NASA, ESA, and Johan Richard (Caltech, USA). Acknowledgement: Davide de Martin & James Long (ESA/Hubble)
Here we see a galaxy cluster with many galaxies. Some galaxies have shapes that look like long arcs. These arcs are not their real shapes, but the result of light bending as it travels to us. Those galaxies are far behind the cluster, and just as gravity deflects the path of matter, it also deflects the light. The geometry of the arcs reveals the effect of gravity, and so we can determine that the total gravity in the cluster is more than can be caused by normal matter.

Image: NASA, ESA, and Johan Richard (Caltech, USA). Acknowledgement: Davide de Martin & James Long (ESA/Hubble)
Bullet Cluster
Bullet Cluster
Image: X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; agellan/U.Arizona/D.Clowe et al.; Lensing Map: NASA/ STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.
In this photograph combining images from multiple telescopes we see two clusters of galaxies that have collided with one another. Normal matter gas between the galaxies collided and got stuck between the two clusters and became so hot that it emitted x-rays (pink). But gravity bends the path of light as much as matter, and this bending changes the appearance of a galaxy’s shape, like looking through a drinking glass. The shapes of galaxies reveal where gravity’s pull is strongest (purple). Gravity is not strongest where the normal matter is (the gas and stars); instead, gravity is strongest from something that passed through itself.

Image: X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; agellan/U.Arizona/D.Clowe et al.; Lensing Map: NASA/ STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.
Afterglow from the Big Bang
Afterglow from the Big Bang
Image: ESA and the Planck Collaboration / March 2013 Planck Collaboration Results.
Small differences in the universe’s temperature shortly after the Big Bang, depicted in this image from the Planck microwave and mid-infrared telescope, are explained by differences in the gravitational pull from one place to another. This can be explained by some places in the universe having more matter than other places. But at this early time (over 13 billion years ago), it was too hot for normal matter to clump together. Only weirder stuff that mostly interacts through gravity could group up enough to explain the temperatures astronomers see at this early time.

Image: ESA and the Planck Collaboration / March 2013 Planck Collaboration Results.

What could dark matter be?

Scientists know that something we currently call dark matter exists because of its effects on other objects in the universe, such as galaxies. From observing how galaxies spin and how light sometimes bends, astronomers know that whatever dark matter is, it must have mass. So far, everything else that has mass exists as a particle, so the current leading theory is that dark matter is also a particle. Many dark matter experiments—including those at SNOLAB—are looking for dark matter in the form of a new particle called a WIMP (weakly interacting massive particle).

Arthur B. McDonald discusses dark matter and his Nobel Prize winning research with the Drift: Art and Dark Matter team, including artists Nadia Lichtig and Josèfa Ntjam. Photo: Tim Forbes
Why search for dark matter deep underground?

The signals from dark matter interactions are predicted to be very small, so detectors are designed to be incredibly sensitive. The challenge is that dark matter particles aren’t the only things that can interact with the detectors. Other particles, such as those that make up background radiation on Earth’s surface, can also interact with dark matter detectors. These particles (many of which come from cosmic rays) would overwhelm a signal from dark matter, so we need a way to filter them out. Setting up a dark matter detector on the Earth’s surface would be like trying to hear a pin drop at a rock concert. Two kilometres of rock acts as excellent shielding and filters out most of these particles, reducing unwanted signals by a factor of 50 million. For this reason, labs searching for rare particles are located underground. Some, like SNOLAB and Boulby Laboratory, are in mines, and some, like Gran Sasso, are built inside mountains.

Cloud chambers are a way to visualize this cosmic radiation on Earth’s surface. In the video below, appearing with images of the Drift team making their descent into SNOLAB, you can see beta particles as narrow streaks that often change direction. Their heavier cousin, the muon particle, is seen through narrow streaks that do not change direction. Finally, the larger trails are caused by alpha particles. Each particle that generates a track would create noise in the data of dark matter experiments, making it impossible to see the signal we are looking for. If a cloud chamber were set up in an underground lab, you would almost never see a particle track in it because of how much the background radiation is reduced.

Artists Nadia Lichtig, Josèfa Ntjam and Jol Thoms suit up for their journey through the mine to SNOLAB’s underground facility. Credit: Zac Kenny.
Zac Kenny from the McDonald Institute walks through the drift, a horizontal tunnel in a mine, in this case stretching between the elevator and the clean lab spaces of SNOLAB. Credit: Gerry Kingsley.
The Drift: Art and Dark Matter team, along with SNOLAB staff, prepare to walk through the drift outside of SNOLAB’s clean lab. Credit: Gerry Kingsley.
The Drift: Art and Dark Matter team, along with SNOLAB staff, prepare to enter the clean lab after walking through the drift. Credit: Gerry Kingsley.
The Drift: Art and Dark Matter team, including artists Anne Riley and Jol Thoms, in front of CUTE (the Cryogenic Underground Test Facility) at SNOLAB. Credit: Zac Kenny.
What kind of dark matter experiments happen at SNOLAB?
PICO
PICO
Image: The PICO Collaboration (formed from the merger of two existing groups, PICASSO and COUPP) uses bubble chambers like this one to search for galactic dark matter. Credit: Mathieu Laurin.
The PICO collaboration uses bubble chambers to search for dark matter. PICO-40 is the third detector the collaboration has operated at SNOLAB and is currently commissioning. The detector contains a super-heated fluid, held at high pressure to keep it from boiling. When a particle enters the detector it hits the target material, transferring a small amount of energy and causing a bubble to form as the material boils. The bubble is captured by microphones and cameras; studying the bubble can tell scientists about the particle that caused it.

Diagram of PICO-40, a dark matter experiment using bubble chambers. Credit: PICO Collaboration.
Image (left): The PICO Collaboration (formed from the merger of two existing groups, PICASSO and COUPP) uses bubble chambers like this one to search for galactic dark matter. Credit: Mathieu Laurin.
SuperCDMS
SuperCDMS
Image: This is SuperCDMS (the super-cryogenic dark matter search), a dark matter detector using crystals at extremely low temperatures. Credit: Fermilab.
SuperCDMS—the super-cryogenic dark matter search—is a dark matter detector that uses silicon and germanium crystals. The crystals are stacked together and held at almost absolute zero, which greatly reduces the vibration within the crystal structure. When a dark matter particle enters the detector and hits the crystal molecules, it causes a small energy change in the form of vibration, which is recorded and analyzed.

Diagram illustrating the scale and interior of SuperCDMS. Credit: SuperCDMS Collaboration.
Image (left): This is SuperCDMS (the super-cryogenic dark matter search), a dark matter detector using crystals at extremely low temperatures. Credit: Fermilab.
News-G
News-G
Image: NEWS-G uses a spherical copper vessel, similar to this prototype, filled with noble gas to search for dark matter. Credit: SNOLAB.
NEWS-G (new experiments with spheres – gas) uses a spherical copper vessel filled with a noble gas to search for dark matter. A sensor directly in the middle of the sphere attracts any ions inside it. When a particle (such as dark matter) enters the sphere, it deposits energy and ionizes the gas, creating electrons. These electrons are attracted to the sensor, and the charge they generate can be measured.

Diagram of NEWS-G’s interior. Credit: NEWS-G Collaboration.
Image (left): NEWS-G uses a spherical copper vessel, similar to this prototype, filled with noble gas to search for dark matter. Credit: SNOLAB.
DAMIC
DAMIC
Image: The DAMIC is an experiment at SNOLAB using CCDs (charged coupled devices) to search for dark matter. Credit: DAMIC Collaboration.
DAMIC uses CCDs (charged coupled devices) to look for dark matter interactions. The silicon CCDs are electrical circuits made up of many capacitors—CCDs are also used in digital cameras but those in DAMIC are larger and more sensitive. When a dark matter particle interacts, that energy change can be measured, creating a signal in the data. The CCDs are sensitive to very small changes in energy so they can detect very small interactions.

Diagram illustrating how particles generate signals within DAMIC. Credit: DAMIC Collaboration.
Image (left): The DAMIC is an experiment at SNOLAB using CCDs (charged coupled devices) to search for dark matter. Credit: DAMIC Collaboration.
DEAP-3600
DEAP-3600
DEAP-3600 uses a vessel of liquid argon to look for dark matter. When a particle such as dark matter enters the detector, it excites some of the argon atoms. When argon atoms are excited, they produce ultraviolet light, which is detected by sensors surrounding the vessel. Studying what the signal looks like and which sensors picked it up can tell scientists about the particle that generated it.

Diagram of the DEAP-3600 experiment, which searches for dark matter using liquid argon. Credit: DEAP Collaboration.
A data readout from DEAP-3600, a dark matter experiment located underground at SNOLAB. Video: Zac Kenny.
Footnotes
Image Credits