The Expanding Universe

Cosmic Background Explorer Data Overview 

This activity is designed to help students gain a deeper understanding of cosmology.  Students develop authentic models and gather evidence supporting the Big Bang theory.  This lesson uses observation, interactive media, and scientific models.

Learner Outcomes

By completing this activity, the learner will: 

  • illustrate the expansion of the Universe with a model.
  • explain and provide examples of Cepheid variables and apply (redshift velocity) = (distance)*( Hubble constant).
  • give supporting evidence for the Big Bang theory.
National Science Education Standards

National Mathematics Education Standards

Materials and Technology

Scientific Background

Teacher Lesson Plans

Activity 1: Explore and define the Hubble Law.

Activity 2: Create a model of the expanding universe.

Activity 3: Analyze and explain what happens when using different measuring devices.

Activity 4: Answer summary questions to better understand the Hubble Law.

Activity 5: Create an electronic report that describes a Hubble Space Telescope cosmological finding and explain how it relates to the balloon activity.

Activity 6: Classroom debriefing.

Hubble's Law diagramTeacher Lesson Plans

Learning Cycle Format


Using the graph at the right, have students explore the various axes and look for clues to what the graph might mean.  In particular, the horizontal axis is distance from our galaxy, the Milky Way, to other galaxies.  The vertical axis is recessional velocity.  Note how there is a clear proportionality between distance and recessional velocity.  This proportionality is known as the Hubble Law.  The slope of this line, with dimensions of velocity over distance, is called the Hubble Constant.  The inverse of the Hubble Constant then has the dimension of time, and can be taken as an estimate of the age of the Universe. 

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Concept Introduction

The Hubble Law states that the recessional velocity of a distant galaxy is proportional to its distance from us.  The recessional velocity of a galaxy is measured by examining the Doppler shift of lines in the spectrum of flight from the galaxy.  The distance to the galaxy is more difficult to measure, but can be estimated from its apparent angular size or by the brightness of objects in it. 

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Instructions on Student Sheet

1. In this activity, you are going to create a model of the expanding Universe.  You will need one balloon, a flexible metric ruler and a paper strip for conducting measurements, a copy of this page, and a marker.

2. Use the markers to make 10 - 15 dots on the balloon and number 10 of them after the balloon is partially inflated.

3. Inflate balloon with 4 medium breaths to about the size of your fist; do not over inflate the balloon!

4. Bend the end of the balloon down and paper clip it so that no air escapes.

5. Record below what happens to the dots.  Be very specific - use complete sentences.

6. Measure and record the distance between dot number one (your "home" dot) and neighboring dots with the METRIC RULERS.  Be careful not to indent the balloon by pressing on it.

7. Now measure and record the distance between dot number one (your "home" dot) and the other 10 dots with the paper strip.  Note any differences in the two measuring techniques.

8. Double the size of the balloon by inflating it slowly; do not over inflate the balloon!  Measure and record the data from the enlarged balloon using both tools.

9. Answer the summary questions below.

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Partially Expanded 

Dot Initial Distance from Dot #1
using the ruler
Initial Distance from Dot #1
using the paper strip
2 2 2 2
3 2 2 22
4 2 2 2
5 2 2 2
6 2 2 2
7 2 2 2
8 2 2 2
9 2 2 2
10 2 2 2
11 2 2 2


Totally Expanded 

Dot Final Distance from Dot #1
using the ruler
Final Distance from Dot #1 
using the paper strip
Difference Change from Before to After
2 2 2 2 2
3 2 2 2 2
4 2 2 2 2
5 2 2 2 2
6 2 2 2 2
7 2 2 2 2
8 2 2 2 2
9 22 2 2 2
10 2 2 2 2
11 2 2 2 2


Summary Questions

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a. If the dots represent galaxies, do they get larger as the balloon expands? 

Why do you think this is or is not so? 

b. What relationship exists between the speed of the galaxies moving apart and their initial distance from one another? 

Name this Law. 

c. Which measuring tool was more accurate? 


d. What is harder for the astronomer to measure: A galaxy's redshift (indicating recessional velocity) or its distance from Earth?  Why? 

Explain your answer. 

Galaxy M100f. Astronomers often use Cepheid variables to determine distance.  What is the period of this Cepheid variable in the galaxy called M101 (animated_gif and digital image)? 

Explain how astronomers use Cepheid variables to determine distance. 

Concept Application

Hubble Space TelescopeThe Space Telescope Science Institute (STSci) in Baltimore maintains current information and press releases regarding findings from the Hubble Space Telescope.  With a partner, investigate one of the many cosmology findings from the HST.  Create an electronic report that describes the HST finding and how it relates to the balloon activity you just completed. 

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Cosmology is the search for origins.  It seems as if everyone wants to know how the Universe began.  The Big Bang theory is the result of several important observations.  In 1927, Edwin Hubble first observed that light from distant galaxies is red shifted and that galaxies are moving farther and farther away from us.  Second, he determined that the farther away a galaxy is from us, the faster it is receding from us.  If the Universe is expanding, then one can assume that the galaxies that compose our Universe were once much closer together than they are now.  By simply measuring how far apart galaxies are and how fast they are moving, we determine the Hubble Constant (estimates range from 50 to 100 km/s per kiloparsec).  It is very easy to determine the recessional velocity of galaxies; on the other hand, their current positions are difficult to measure.  Distances to galaxies are typically measured by finding Cepheid variable stars or supernovae with known brightness. 

If we run the expansion process backward, we get two results.  The first is that it probably took approximately 15 billion years for the Universe to grow to its present size.  Second, the Universe must have begun its expansion in an awesome event that astronomers call the Big Bang. 

There are four fundamental observations and inferences that suggest that a Big Bang of some type did actually occur very long ago. 

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Almost all galaxies are red-shifted. Almost all galaxies are moving away from the Milky Way.
The most distant galaxies exhibit the greatest red-shift. The most distant galaxies are moving away the fastest.
The ratio of recessional velocity to distance is between 50 and 100 km/s per kiloparsec and is called the Hubble Constant. The Universe has been expanding for 8 to 15 billion years.
The Cosmic Background Explorer (COBE) found that the temperature of intergalactic space was not zero. The universe has not yet cooled from the rapid Big Bang expansion.

What are some examples that demonstrate the difference between an observation and an inference? 

FFS - Facts for Students

The origin of the universe remains one of the greatest questions in science. The "big bang" theory places the origin between 10 and 20 billion years ago, when the universe began in a hot dense state;  according to this theory, the universe has been expanding ever since.