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# How do we know the universe is expanding?

Year 10 Earth and space · The big bang

<!-- Speaker note: 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 (Engage and Explore 9 min · Explain 8 min · Evaluate 3 min · Typical adds 2 min, Wider about 1 min). -->

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## Today

**We are learning to** 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.

- 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.

<!-- Speaker note: 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. -->

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## Why it matters

> In 1929 Edwin Hubble found that almost every galaxy is moving away from us. Does that put us at the centre of the universe?

<!-- Speaker note: The big bang model predicts an expanding universe. This lesson tests that prediction. -->

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## Engage: predict

**Almost every galaxy is moving away from the Milky Way. What do you predict that means?**

- Galaxies are scared of us
- The Milky Way is shrinking
- We are at the centre of the universe
- Space is expanding, and every galaxy would see the others moving away

<!-- Speaker note: What happens: Space is expanding, so from any galaxy, the others move away. There is no centre: the same pattern appears wherever you live. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: count how many chose 'we are at the centre'. -->

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## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fearth-10%2Fredshift%2Fstep-2&code=widget%3A+universe%0Amode%3A+expand%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/earth-10/redshift/?full=step-2#step-2)

<!-- Speaker note:  -->

<!-- Speaker note: Teacher: collect a few class points on one graph. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## What happened?

* Space is expanding, so from any galaxy, the others move away. There is no centre: the same pattern appears wherever you live.

<!-- Speaker note: Reveal after running. Ask who predicted it, and what made them think so. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Redshift

**What does a redshift in a galaxy's light tell us?**

- The galaxy is red
- The galaxy is moving away from us: its light is stretched to longer, redder wavelengths
- The galaxy is hot
- The galaxy is moving towards us

* Answer: The galaxy is moving away from us: its light is stretched to longer, redder wavelengths

<!-- Speaker note:  -->

<!-- Speaker note: Teacher: the big bang link is the checkpoint. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## The pattern

**On your graph, how does speed depend on distance?**

- Nearer galaxies move faster
- All galaxies move at the same speed
- There's no pattern
- Farther galaxies move away faster, in proportion: double the distance, double the speed

* Answer: Farther galaxies move away faster, in proportion: double the distance, double the speed

<!-- Speaker note:  -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Back in time

**How does an expanding universe support the big bang theory?**

- It doesn't
- It proves the universe is shrinking
- Run the expansion backwards and all galaxies were once together, in a hot, dense state
- It shows the universe has a centre

* Answer: Run the expansion backwards and all galaxies were once together, in a hot, dense state

<!-- Speaker note:  -->

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<span style="background:#fef3c7; color:#1a1a1a; border:1.5px solid #f59e0b; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✍️ Activity</span>

## Elaborate: no centre

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fearth-10%2Fredshift%2Fstep-4&code=widget%3A+universe%0Amode%3A+expand%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/earth-10/redshift/?full=step-4#step-4)

<!-- Speaker note: What this step is for: Seeing the same pattern from two home galaxies shows expansion has no centre. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Same pattern

**From the second home galaxy, what did the other galaxies do?**

- Only some moved
- They moved towards it
- They all moved away, farther ones faster: the same pattern as before
- Nothing

* Answer: They all moved away, farther ones faster: the same pattern as before

<!-- Speaker note:  -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Elaborate: a blueshift

**The Andromeda galaxy, our nearest big neighbour, is blueshifted. Does this disprove the expanding universe?**

- It means Andromeda is red
- Yes: the universe is shrinking
- No: nearby galaxies' own motions under gravity can beat the expansion; over large distances, the redshift pattern is clear
- Yes: Hubble was wrong

* Answer: No: nearby galaxies' own motions under gravity can beat the expansion; over large distances, the redshift pattern is clear

<!-- Speaker note: What this step is for: A local exception shows the difference between a galaxy's own motion and the expansion of space. -->

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## 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**.

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Exit ticket

**What does redshift show, and how does it support the big bang?**

* Galaxies are moving away, farther ones faster: the universe is expanding. Run backwards, everything was once together in a hot, dense state.

<!-- Speaker note: Students answer on paper or a mini whiteboard before the reveal. Anyone who cannot answer needs the main idea again next lesson. Where this leads: Next: the cosmic microwave background and matter proportions (nodes 4 and 5), then Earth's energy balance (AC9S10U04). -->