Laboratory - The Hubble Constant
Teacher's Notes

Focus question

Why do distant galaxies seem to be moving away from us at greater speeds than galaxies closer to us?

Essential materials

• wide rubber band with a diameter of at least 4 cm or 1.5 inches
• centimeter ruler
• ballpoint pen or fine tip marker
• data table below or other paper to record results
• scissors
• tape (necessary only if you are working without a partner)

Main ideas and background information

• When a rubber band is stretched, its length increases. As the length increases, each point on the rubber band has to move further away from adjacent points along the direction of the stretching.
• Each point on the stretching rubber band is moving away from all other points on the rubber band in two ways. One way is due to its own stretching. The other way is due to the stretching of all other points between it and any reference point.
• In this activity, the Milky Way is the reference point. We stretch the rubber band so that Galaxy A moves one additional centimeter away from the Milky Way. At the same time, each of the other galaxies moves away from its adjacent two galaxies by one additional centimeter. On the centimeter ruler, the distance between any two adjacent galaxies increases from one centimeter to two centimeters.
• Galaxy B moves one additional centimeter away from Galaxy A. However, Galaxy B moves a total of two additional centimeters away from the Milky Way. This is because it moves one additional centimeter away from Galaxy A, and Galaxy A moves one additional centimeter away from the Milky Way.
• Galaxies in an expanding universe behave in a way similar to the points on a rubber band that is being stretched. Galaxies that are distant from any selected reference point appear to move away from that reference point faster than the galaxies that are closer to the reference point.

Procedural tips

• Make sure the rubber band is pliable and not stiff.
• For Step 3, make the first mark about one centimeter from the end of the rubber band. Line up this mark with the one-centimeter line of the ruler. Then make the other marks at the centimeter lines. Be sure not to stretch the rubber band while lining it up along the ruler.
• While students are making their measurements in Step 6, they need to keep the Milky Way mark at the end of the ruler while making sure that the Galaxy A mark is at the two-centimeter line.
• During Step 6, make sure the students notice that the distance between any two galaxies is two centimeters.
• After the measurement in Step 6 is complete, have the students stretch the rubber band several more times while keeping the Milky Way mark at the end of the ruler. During the first stretch, ask them to observe the movement of Galaxy A. During another stretch, ask them to observe another galaxy farther away, such as Galaxy E or F, noting how it moves farther during the same time of stretching. As they hold the Milky Way mark in place and observe how the other galaxies move at different speeds, they will be able to make some dynamic observations to help them understand Hubble’s work.
• If a spreadsheet is available, students may want to use it for recording and graphing their data.

Discussion

1. How did the distances between the galaxy marks increase as the rubber band was stretched? (As the rubber band was stretched, each galaxy mark moved farther away from its neighbors on either side. The distance between adjacent galaxies was about the same at any moment during the stretching.)

2. Did the distances between the galaxy marks increase at the same rate? Explain. (The distances between any two adjacent galaxy marks increased at the same rate. However, the distance between the Milky Way mark (a reference point) and a distant galaxy mark increased more rapidly. This is because the extra distance stretched by each of the intervening galaxies must be taken into account.)

3. What do you think might happen if you placed Galaxy B at the end of the ruler, instead of the Milky Way? (Regardless of which galaxy is the reference point, the distances between galaxies will increase as the rubber band is stretched. The galaxies farthest away from the reference point will appear to be moving away from the reference point at greater speeds than galaxies closer to the reference point.)

4. How might this activity relate to everyday life? (Make sure students discuss this in relation to galaxies in space. As the universe expands, galaxies seem to be moving apart from each other. The galaxies more distant from us appear to move away at greater speeds.)

Note: Students can also feel this effect by lining up shoulder-to-shoulder and then increasing the distance between themselves to an arm’s length apart. If one student is asked to stay in place while everyone else adjusts his or her position, the students farther away will see that they have to move a greater distance than those located nearer the student fixed in position. If the students are required to make this adjustment within a short amount of time, the distant students will definitely have to move faster than those who are nearer the person in the fixed position. If you repeat the activity with a different person fixed in place, the effect will still be the same for the others. Members of a marching band might have experienced something similar. You can make this activity more general by having several lines of students radiating outward in different directions from the fixed center. This would give a two-dimensional picture instead of the one-dimensional approach with the rubber band.

Another place this applies is in the use of extension mechanisms that use a scissors movement. As seen from the base, the far end of the mechanism moves away faster than any link in between.

Assessment

Are students able to describe why distant galaxies seem to be moving away from us at greater speeds than galaxies closer to us? (The universe seems to be expanding, somewhat like a rubber band that is being stretched. The distant galaxies of the universe seem to be moving away just as points on a rubber band appear to move apart as the rubber band expands. Because all galaxies are moving away from each other, the distant galaxies must move a greater distance in the same amount of time. This results in a greater apparent speed for these galaxies.)

Extensions and further investigations

• Have students repeat the experiment using

  • rubber bands of other sizes and shapes.

  • marks at different distances.

  • larger or smaller stretches.

  • faster or slower stretches.

• Before the students carry out each additional experiment, challenge them to predict what they think will happen and why. Then have them compare the results with their predictions and develop possible explanations for any discrepancies.

Career connections

• Astronomy and space-related careers
• Planning for fabric expansion and shrinkage in garments and other coverings
• Architectural styles of building covering that must take into account expansion and shrinking
• Movement of extension arms, especially those with scissors or telescoping movement
• Movement of people in formations such as marching bands when increasing the distance between each person

Correlations with Standards

United States: This activity correlates with portions of NSES Content Standard A, Science as Inquiry, and Content Standard G, History and Nature of Science, Grades 5-8 and 9-12, and with the following additional standards:

Grades 5-8
Standard B - Physical Science: B2

Grades 9-12
Standard B - Physical Science: B4, D4

Britain: This activity correlates with the English National Curriculum standard Sc1, Science Enquiry, and the following additional standards:

KS4, Sc4, 4d

Glossary/vocabulary

galaxy
universe
Hubble Constant

Resource links

The Expanding Universe from "HyperPhysics***** Astrophysics"

The information on this page includes an analogy of raisins in a rising loaf of bread.

Foundations of Modern Cosmology

One of the links shows an expanding sphere that is similar to our rubber band.
See also Expansion: A 2D Spherical Example

Hubble Flow: The Expanding Universe from "Windows to the Universe"

This site presents three different levels of information about the expanding universe. The link given here takes you to the elementary level. To go to higher levels, click on the links to Intermediate or Advanced at the top of the screen.

The Expanding Universe from Mullard Space Science Laboratory

Here you will find an animation showing how galaxies are moving away from each other as the universe expands.

NASA's The Universe

Information is given at two different levels, along with many related areas to explore.