---
title: "How do we know the universe is expanding?"
subtitle: "Year 10 Earth and space · The big bang · Lesson 2"
---

# How do we know the universe is expanding?

**Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}}

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


:::teacher-note

**In one sentence:** 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).

**Students should be able to say:** 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.

**Width:** the unmarked parts are the floor every student does (Engage, Explore, Explain, Evaluate). Typical and Wider sections add contexts, not difficulty. Students work in the online tutorial alongside this sheet.

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## Engage


:::keep-together

**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

My prediction, and why:

{{lines-3}}

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:::teacher-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.

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## Explore

Try it on the board in the tutorial. Was your prediction right? What was different?

{{lines-2}}

## Explain: redshift and Hubble's law


:::keep-together

### 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

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:::teacher-note

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

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:::keep-together

### 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

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:::teacher-note

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

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:::keep-together

### 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

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:::teacher-note

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

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:::typical

## Elaborate: no centre


:::keep-together

### 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

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:::teacher-note

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

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:::wider

## Elaborate: a blueshift


:::keep-together

**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

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:::teacher-note

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

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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**.

## Exit ticket

:::keep-together

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

{{lines-3}}

:::


:::teacher-note

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

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