Light from distant galaxies is redshifted: shifted to longer wavelengths, so they are moving away. Farther galaxies move away faster (Hubble's law), exactly what an expanding universe predicts, from any galaxy. Run backwards, everything was once together: evidence for the big bang.
In 1929 Edwin Hubble found that almost every galaxy is moving away from us. Does that put us at the centre of the universe?
The big bang model predicts an expanding universe. This lesson tests that prediction.
Engage: Commit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Explore: Try it on the board and compare what happens with your prediction.
Explain: Use the particle model to explain what you saw.
Elaborate: Use the same idea in a new context.
Evaluate: Look back at your first ideas and what changed.
WidthMore contexts, same idea.
Make your prediction first. Help opens once you've locked it in.
EngageCommit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Engage: the centre?
ExploreTry it on the board and compare what happens with your prediction.
Explore: stretch space
Spread a galaxy's light out and you get a spectrum, a rainbow from violet (short wavelengths) to red (long wavelengths). Hydrogen gas leaves dark lines in it at fixed wavelengths, a fingerprint you can compare with the same lines measured in a lab.
Press Let space expand. Then click galaxies near and far: compare their distances, speeds and light.
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Let space expand, then measure at least three galaxies at different distances.
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: redshift and Hubble's law
In your book: sketch your distance–speed graph and write two sentences linking it to the big bang.
Elaborate TypicalUse the same idea in a new context.
Elaborate: no centre
Try it yourselfYour edits are saved in this browser.
Live in two different galaxies and let space expand from each.
Use the board. This ticks when you get there.
Elaborate WiderUse the same idea in a new context.
Elaborate: a blueshift
EvaluateLook back at your first ideas and what changed.
Evaluate: look back
Write one sentence: "At first I thought… Now I think… because…"
Revision summary
Redshift: light from distant galaxies is shifted to longer wavelengths: they are moving away.
Hubble's law: farther galaxies move away faster, in proportion to distance.
This is what an expanding universe looks like from any galaxy (no centre). Run backwards, everything was once together: evidence for the big bang.
For teachers
Students explain redshift as evidence that galaxies are moving away, describe Hubble's distance–speed pattern, and connect an expanding universe to the big bang model (AC9S10U03, node 3). About 23 minutes.
I can explain what redshift tells us about a galaxy's motion.
I can describe how speed depends on distance (Hubble's law).
I can explain how an expanding universe supports the big bang theory.
Before class
A concept tutorial for introducing or revising the idea. Set the width with ?width=minimum, typical (default) or wider.
Assumed: light as a wave with a wavelength, and a spectrum as light spread out by wavelength (Year 9 waves). No tutorial teaches these, so Explore opens with a short "Reading a spectrum" recap; if the class has not met spectra, show one through a diffraction grating or a prism first.
The misconception to watch is galaxies flying away from a centre (and us being at it). Changing the home galaxy shows the same pattern from everywhere.
Board notes: a 2D field of galaxies; "Let space expand" stretches every distance by 1.6. Distances are in millions of light-years (one grid square is 100 million) and speeds use a Hubble constant of about 70 km/s per megaparsec (21.5 km/s for every million light-years). Spectra show four hydrogen lines, with the shift drawn 5 times bigger. Strictly, cosmological redshift is space stretching the light, not a Doppler effect from motion through space; the Doppler explanation is the curriculum's model.
Running the lesson
Engage: count how many chose 'we are at the centre'.
Explore: collect a few class points on one graph.
Explain: the big bang link is the checkpoint.
The hard part (on purpose)
Seeing that every galaxy sees the others moving away, so there is no centre. When a student says we are at the centre, ask what happened when they lived in another galaxy.
When students get stuck
A student says galaxies are moving away from us because we're special. Ask: What happened when you lived in another galaxy? Fix: The same pattern: every galaxy sees the others moving away.
Stretch and support
Stretch: Use the graph's slope to estimate how long ago the galaxies were together (distance ÷ speed).
Support: Give a two-column table (distance, speed) to fill from the board.
Where it leads
Next: the cosmic microwave background and matter proportions (nodes 4 and 5), then Earth's energy balance (AC9S10U04).
What each step is for
Engage: the centre?: Students who picture galaxies flying away from a centre conclude we are at the centre; changing the home galaxy in the board removes it.
Explore: stretch space: Stretching space and measuring several galaxies shows speed growing with distance, the pattern Hubble found.
Explain: redshift and Hubble's law: Linking redshift to motion away, and the distance–speed pattern to expansion, is the evidence chain.
Elaborate: no centre: Seeing the same pattern from two home galaxies shows expansion has no centre.
Elaborate: a blueshift: A local exception shows the difference between a galaxy's own motion and the expansion of space.
Now you can
Explain what redshift tells us about a galaxy's motion.
Describe how speed depends on distance (Hubble's law).
Explain how an expanding universe supports the big bang theory.