Dark Matter: Probing What You Can't See

Astronomers have known for many years that most of the matter in the Universe is invisible. Identifying this "dark matter" is a crucial step in understanding of the Universe. Dark matter is transparent and emits no light. Different theories for the composition of dark matter predict the scales on which it can clump gravitationally. The gravitational attraction of the dark matter drives the development of structure in the Universe. But if we can't see it, how will we ever learn anything about it? How will we ever answer such questions as

Astronomers are coming up with all sorts of inventive ways to approach the unveiling of dark matter. We'll learn more about this research in later sections, as this lesson looks into how we learn about things we cannot see...by using standards-based subjects such as gravity, rotation, torque, equilibrium of forces, and more.


Student Labsheets

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Background Information

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Evidence for Dark Matter

Possibilities for Dark Matter: What in the Universe could it be?

There are two basic possibilities for what sort of matter may make up the "missing mass". First, it may be regular matter that is just not hot enough to be emitting any radiation. Alternatively, it might be some sort of more exotic matter -- the likes of which we don't have naturally here on Earth (but may one day be able to create in our particle physics laboratories). So far, all we seem to know about the matter that makes up most of the Universe is that it behaves just like regular matter as far as gravity is concerned.

More Details About the Possibilities for Dark Matter

The matter that comprises dark matter might be baryonic, non-baryonic -- or perhaps a combination of the two. Below we give details about what each type of matter means, and what the possibilities are for its being the solution to the dark matter mystery.

Revealing the Nature of Dark Matter

Scientists at Bell Laboratories are studying images of faraway galaxies that are "lensed" by intervening concentrations of mass. As the light from a distant galaxy passes by a large mass concentration, its ray path is bent, causing the distant source to appear at an altered place on the sky and resulting in a tell-tale distortion of its shape. This gravitational lensing effect provides the first, and currently only, way to directly "weigh" cosmic mass. Clusters of galaxies contain a considerable amount of dark matter (at least five times as much mass as you can see in the galaxies and the hot gas between them) and make excellent, although complicated lenses. By studying the images that are created by gravitational lensing, scientists are able to determine the mass and location of the dark matter in the cluster.

An Introduction to Moment of Force

One of the fundamental beliefs about Dark Matter is that it behaves like ordinary matter when it comes to gravity. Remember that Newton taught us that gravitational force is defined by the equation:

F=G M1M2/r2

where F is gravitational force between two objects, G is the universal gravitational constant, M1 and M2 are the masses of two objects, and R is the distance between these two objects. In other words, Dark Matter exerts gravitational force on the masses around it. What Newton's Law doesn't say (and what Einstein showed) is that the gravitational force also affects light. Scientists study the effects due to this force to determine how much dark matter there is, and where it is located.

In physics, a common introductory-level subject is forces and how they act on bodies. The effect produced on a body by a force of a given strength and direction depends on the position of the line of action of the force relative to the position of a reference point on the body . So, you have to know how strong the force is and where it is located. The force arm, or moment arm, of a force can be found by giving the perpendicular distance between some reference point (usually an axis of the object, such as a rotation axis) and the line of action. The mathematical product of the force and the force arm is called the moment of force, or the torque, about the axis.

Look at the situation shown in the image below. The force F1 wants to cause the object to rotate in a counterclockwise direction; F2 wants the object to rotate in a clockwise direction. Since these forces act in opposite directions relative to the reference point, they have opposite signs. For the object not to rotate at all, the torque (or turning effect) caused by one force must be exactly equal to the torque caused by the other force. In other words, the sum of all of the torques applied to an object must be equal to zero.

Scale with m1 on one side and m2 on the other.

In the case shown above, F1 is equal to the mass of the object put at location A times the acceleration due to gravity. So, F1 = m1 * g, where g on Earth is equal to 9.8 m/sec2. Similarly, F2 is equal to the mass of the object put at location B times the acceleration due to gravity, m2*g . The torque introduced by each force is then F1*X1 and F2 * X2, where X1 and X2 are the distances measured from points A and B to the reference point (in this case, the suspension point, O). For the meter stick to be in balance, F1*X1 must be equal in value to F2*X2. Solving this equation, we see that for equilibrium, M1X1 = M2X2. Think about what this means: a smaller mass placed farther from the axis will balance a larger mass placed nearer the axis. Try this the next time you get into a revolving door. You have to push much less if you push toward the outside of the door than if you push near the inside!


Teacher Notes

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Additional Resources

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National Standards

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Origin and Evolution of the Universe: In grades 9 - 12 all students should:

"Early in the history of the universe, matter, primarily in the form of hydrogen and helium, clumped together by gravitational attraction to form countless trillions of stars. Billions of galaxies, each of which is a gravitationally bound cluster of billions of stars, now form most of the visible mass of the universe; Stars produce energy from nuclear reactions, primarily the fusion of hydrogen to form helium. These and other processes in stars have led to the formation of all the other elements."


Algebra: In grades 9 - 12 all students should:

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