Focus question

How do galaxies in the universe move relative to each other?

Essential materials

• round balloon
• masking tape
• tape measure
• pen or pencil
• clothespin (optional)

Optional materials

• Meter stick or measuring tape
• Stopwatch

Main ideas and background information

• When a balloon expands, both its internal volume and its surface area increase. As the volume increases, each point on the surface has to move outward away from the center of the inside of the balloon. As the surface area increases, each point on the surface has to move farther away from the other surface points.
• Galaxies in an expanding universe behave in a way similar to the points on the surface of an expanding balloon. As the universe expands, the galaxies move farther outward and, at the same time, farther apart from each other.

Procedural tips

• A round balloon will give the best results. Pear-shaped and teardrop-shaped ones will work fairly well, but long balloons will not.
• Stretch the balloon before blowing it up. Pull on it from both ends a few times. This often makes it easier to inflate.
• It is usually easier to inflate a larger balloon than a smaller one.
• Make sure students decide specifically what distance they will measure (e.g., the center-to-center distance between galaxy dots) before affixing the bits of tape to the balloon.
• For the intermediate balloon sizes, caution students not to blow too much air into the balloon at first. Small puffs will do. This will save room for expansion for the final measurements.
• If stickers or tape are not available, students may use a permanent marker to mark the balloon directly with small dots. However, the size of the dots themselves will increase as the balloon expands. This expansion may affect their distance measurements,
• It may also reduce strain or pressure on the eardrum or sinus area while blowing into the balloon, especially during the initial inflation.

Discussion

1. How did the distances between the masking tape galaxies change as the balloon expanded? How would you explain this? (The distances between the galaxies increased as the balloon expanded. Points on the surface of the balloon move farther apart because the surface area is increasing as the balloon expands.)
2. What do you think might happen if you initially place the masking tape galaxies farther apart? Closer together? (Regardless of whether the initial distances between masking tape galaxies are small or large, the distances will increase as the balloon is inflated.)
3. How might this activity relate to everyday life? (Make sure students discuss this in relation to galaxies in space. As the universe expands, the distant galaxies seem to be moving apart from each other. They could also think about how the raisins in a loaf of raisin bread move apart as the bread expands during baking. The spreading ripples from a pebble dropped into a pond might also remind them of this idea. What others can your students think of? )

Assessment

Are students able to describe how galaxies in the universe move relative to each other? (The universe seems to be expanding, somewhat like a balloon that is being inflated. Just as points on the surface of the balloon appear to move farther apart as the balloon expands, the distant galaxies appear to be moving apart.)

Extensions and further investigations

• Have students repeat the experiment using
  – balloons of other sizes and shapes.
  – objects other than bits of masking tape.
  – objects placed at different places on the balloon.
  – objects placed in different configurations (i.e., four objects placed in a square formation) on the surface of the balloon.
  – larger or smaller puffs of air.
• 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

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

cosmos
galaxy
universe

Resource links

HyperPhysics***** Astrophysics: Expanding Universe

Includes an analogy of raisins in a rising loaf of bread.

Foundations of Modern Cosmology

Includes a link to a page that shows an expanding sphere very much like our balloon.

Expansion: A 2D Spherical Example
Windows to the Universe: Hubble Flow: The Expanding Universe

Has three different levels of information about the expanding universe. This is the link to the elementary level. To go to higher levels, click on links to Intermediate and Advanced at the top of the screen.

MSSL Astrophysics Group: Expanding Universe

Includes an animation showing how galaxies are moving away from each other as the universe expands.

World Almanac: Cosmology

Includes an excellent overview of cosmology.

NASA: The Universe

Has many related areas to explore.