Module 4 – Lesson 9

Lesson 9: Dark Matter

Overview

The detection of something in the Universe that is totally invisible at all wavelengths of light is demonstrated through the detection of magnets hidden between two smooth foam boards. This we call dark matter. Students will explore invisible “detection” after reading a segment about dark matter and our current understanding of its nature based on objects that dark matter is affecting through its gravitational force. 

Learning Outcomes

• Describe why it is called “dark matter.”
• Explain what observational evidence there is for its existence. 
• List those objects we know dark matter is not. 
• Summarize what WIMPs are and how we expect to someday detect these subatomic particles.

Materials

• “Dark matter detection board”:
• Two foam rectangular boards, tape, several smaller magnets, and a stronger magnet. 
• The smaller magnets are taped between the boards, as shown in Figure 4.9.1. 

Figure 4.9.1. The dark matter detection board consists of two rectangles of foam board (about 12 x 14 inches) with 2 – 3 flat magnets sandwiched in between. The foam boards are then taped together. Magnets such as the one shown at the left, can then be moved over the foam board to “detect” dark matter

Pre-assessment Questions and Discussion

Q. How would you go about finding something that appeared to be totally invisible here on Earth? A. See if it bumps into something. See if it leaves footprints. Similar answers.

Q. Have you heard of “dark matter” before?
A. Encourage all answers.

Q. What do you think dark matter might be?
A. Lots of soccer balls? Huge number of black holes? A gazillion Jupiter-like planets?

Text

You may have heard of the mysterious term “dark matter” before. Similar to black holes, dark matter does not emit light. However, unlike black holes, dark matter does not absorb light either. Instead, dark matter lets light pass right through it, so it is impossible to take pictures of it. 

You might wonder how we know dark matter really exists if we can’t take a picture of it. The reason we know it exists is that we can measure its influence on normal matter in space. Recall gravity is a force that exists between any two objects with mass. Because dark matter has mass it exerts a discernible gravitational force on the objects that surround it. 

Dark matter was first proposed by Fritz Zwicky in 1934 to explain why galaxies seemed to move much more quickly around each other than predicted. He suggested that if there were unseen matter tugging on these galaxies, it would cause them to orbit one another at a faster pace. In the mid-1970s, astrophysicist Vera Rubin showed that objects located at the outer limits of spiral galaxies also revolve far too quickly to be explained by just the stars, gas, and dust present in the galaxy alone. In other words, if there were not something with mass that was gravitationally holding on to the galactic material, the stars, gas, dust, and other material would be flying out away from the galaxies. 

Today we understand that most of the matter in the Universe must be this unseen matter; in fact, only 5% of matter in the Universe is the normal matter that we are made of and another 23% is what we refer to as dark matter. There is thought to be something else in the Universe that makes up the remaining 72%, dark energy. We will have to leave our lessons about dark energy for the future as scientists are still trying to figure out what it is. 

Originally, scientists thought that dark matter was just normal matter that was hard to photograph; dark stars and dark planets were proposed as potential candidates for dark matter. However, the results of astronomical surveys like the Optical Gravitational Lensing Experiment or OGLE survey in Poland have convinced scientists that there are not enough dark stars or planets in the Universe to cause the gravitational effects they have measured. 

Most scientists think that dark matter is something much more bizarre than normal matter. It is likely a special type of exotic sub-atomic particle that we refer to as a WIMP. WIMP stands for Weakly Interacting Massive Particle. While an individual WIMP likely has a lot more mass than a neutrino or an electron, it is likely to be extremely small. To have such an effect on other objects, there must be a whole lot of them together in space. Dark matter exists everywhere in our galaxy, from the central bulge to well beyond the edge of the disk.

Experimental physicists are currently trying to create and detect WIMPs in the lab.  The investigation of dark matter is a very active area of research in both astronomy and physics.

Follow-up Questions on the Reading 

1. Why do scientists call dark matter “dark”? (Be specific.)
2. What kind of force does dark matter have on other objects?
3. If we cannot actually see dark matter, how do we know it exists? 
4. What observations have provided evidence that dark matter is present in spiral galaxies? 
5. What kinds of objects have scientists eliminated as possible candidates for dark matter?
6. What are these things called WIMPs? 

Reinforcing Hands-On Activity

We will now do a demonstration to show how this works. Much like holding two magnets near each other exerts an electromagnetic tug between them, dark matter tugs on stars and gas in galaxies through the force of gravity. While we can’t see the dark matter, we are reasonably sure it exists because of how the stars and gas move as a result of its gravitational influence. Imagine you were being twirled around very fast in a circle, so fast that if your partner let go, you’d go flying away. But, as long as your hands are joined, you stay together no matter how fast you are going. Stars and gas at the very edge of the Milky Way we see, are moving way too fast to stay attached to it. Something we cannot see is exerting a gravitational force on them, a force so strong that the stars and gas do not fly away.Dark detection board exploration:

• Have students place the flat foam board on their lap. 
• Ask them to glide their hands along the top of the board to establish that they can see nothing unusual about the surface. Is it perfectly smooth?
• Give them a magnet and ask them to slowly glide it over the foam board surface.

Ask the following questions:

1. Is it easy to glide the magnet over the board?
2. Are there regions of the board that the magnet wants to be in more than others? 
3. Why might the magnet want to be one region more than another? 
4. Do you know for sure that there are magnets underneath the board? 
5. Is it likely there magnets underneath the board? 

Summary and Post-Assessment

1. Why is it called “dark matter.”
2. Discuss how the magnet-board or “Dark-Matter Detection Board” relates to the real dark matter and how scientists detect it even though it is invisible. Is the detection board a good analogy for the observational evidence astronomers rely upon?
3. Explain everything you know about dark matter to someone who has never heard about it. 
4. List those objects we know dark matter is not.
5. What do we think those WIMPs are? How we expect to someday detect them?

Relevant Information and Links

• Dark Energy, Dark Matter — http://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy/
• Dark Matter — https://imagine.gsfc.nasa.gov/science/objects/dark_matter1.html
• What is Dark Matter? A Mystery of the Universe — https://youtu.be/XCwWxrx1SIU