The Big Bang

Courtesy of Luis Fernández García via Wikimedia Commons

Image courtesy of Wikimedia Commons

The big bang theory is the most accepted theory for the formation of the universe. The “big bang” is a misnomer because there was no explosion, but rather a rapid expansion. It is not known what existed the moment before the big bang, however the evolution of the universe from the first instant after the big bang and onwards is well understood. There is considerable evidence supporting the big bang theory, including the following observations:

  • There is an abundance of light elements (hydrogen and helium) in the universe. This is as predicted by the big bang theory.
  • The universe cooled as it expanded so that today the Cosmic Microwave Background (CMB) radiation corresponds to a temperature of 2.7 K. This is consistent with scientists’ predictions of that an expanding universe would by now have cooled to a temperature in this range.
  • The universe is expanding and the galaxies appear to be moving away from us at speeds directly proportional to their distance from us.

This balloon activity models the universe: once compact but now expanding. This activity follows the PEOE model.


safety goggles balloonmarker 50 cm length of stringruler

Safety Considerations

Some students may have latex allergies. If this is the case, check the composition of balloons and advise students with latex allergies not to touch the balloons.


Organize your class into groups of 2–4 students. The following instructions are for the students.

  1. Draw 4 dots on and around the un-inflated balloon. Label the dots G1, G2, G3, and G4. The 4 dots represent galaxies.
  2. Predict/Explain

In your group, predict what will happen to the “galaxies” on the balloon when the balloon is blown up. Why might the distance between galaxies increase? Is the increase in distance constant for all galaxies?

  1. Observe

Imagine that you are located in G1. The other dots represent other galaxies. Measure how far apart each galaxy is from the others.

  1. Blow up the balloon to 10 cm in diameter. This represents the universe after 1 billion years.
  2. Do not tie off balloon, but twist the neck and hold it tightly closed. Use a piece of string to measure the distances between the galaxy you are in (G1) and the other three galaxies. You will measure around the curve of the balloon.
  3. Blow up the balloon to a diameter of 20 cm and repeat the measurements.
  4. Blow up the balloon to a diameter of 30 cm and repeat the measurements.
  5. Draw a graph with “Initial distance from G1” on the x axis and “Final distance from G1” on the y Draw a dot on the graph for each of the galaxies G2, G3, and G4.
  6. Explain

Explain how your balloon model represents the universe.


Balloons may be deflated and disposed of in the garbage.

What happens?

The dots (galaxies) move further apart from one another. Also, the galaxies that were initially further apart on the balloon move a greater distance from the home galaxy compared to those that were closer.

How does it work?

When the balloon is inflated, its surface area increases and so the dots on the surface (representing galaxies) move further apart. Students’ graphs should illustrate that the increase in distance is not constant: it increases with distance from G1. This observation supports the big bang theory that the universe is expanding and the galaxies are therefore moving further apart. The galaxies are just “hitched along for the ride” in the expansion of space.

Teaching Suggestions/Hints

  1. Remind students that galaxies exist in a 3 dimensional space…not just on the surface of an imaginary sphere.
  2. Clarify the misconception of the big bang being an explosion instead of it being a rapid expansion.

Next Steps

  • This activity is a great introduction to the formation of the universe and could lead into a discussion/research for the formation and classifications of galaxies.
  • Extension: If students record the amount of time for each interval of inflation, as well as distance measurements, then students can perform several rate-of-change calculations. These calculations will predict that galaxies farther from G1 will recede faster than closer galaxies. This is true of the expansion of the universe as determined by the Hubble constant, from observations of the WMAP (Wilkinson Microwave Anisotropy Probe). (See Additional Resources #4.)

Note: the distances in the equation are at the beginning and end of inflation and the time is the interval between inflations.

  • The following questions may prompt students to research many of the unsolved mysteries of the universe:
    • What existed before the big bang if matter can neither be created nor destroyed?
    • Measurements indicate that, during the initial expansion, matter travelled at speeds greater than the speed of light. How did the initial expansion defy the laws of physics?
    • Observations made using the Hubble telescope indicate that the more distant galaxies appear to be moving away faster than closer ones. This suggests that the Universe is expanding. Why has the speed of expansion increased?
    • What is the fate of the universe?

Additional Resources

  1. How Stuff Works – how the big bang theory explains the formation of the universe:
  1. Australia Outreach and Education – evidence for the expanding universe
  2. NASA – the history of the big bang theory
  3. NASA – the expansion of the universe


Specific Expectations


A1.8 analyse and interpret qualitative and/or quantitative data to determine whether the evidence supports or refutes the initial prediction or hypothesis, identifying possible sources of error, bias, or uncertainty

A1.10 draw conclusions based on inquiry results and research findings, and justify their conclusions

D3.1 describe observational and theoretical evidence relating to the origin and evolution of the universe (e.g., evidence supporting the big bang theory)


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