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# How did the universe begin?

Year 10 Earth and space · The big bang

<!-- Speaker note: Students describe the big bang as the origin of space, time and matter about 13.8 billion years ago and outline the main stages as the universe expanded and cooled (AC9S10U03, node 2, a hinge node). About 22 minutes (Engage and Explore 7 min · Explain 9 min · Evaluate 3 min · Typical adds 1 min, Wider about 2 min). -->

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## Today

**We are learning to** the big bang theory: space, time, matter and energy began about 13.8 billion years ago in an extremely hot, dense state, and the universe has expanded and cooled ever since: from particles to atoms, stars, galaxies and planets.

- I can describe what the big bang theory says.
- I can outline at least three stages from the big bang to galaxies.
- I can explain that the universe has been expanding and cooling.

<!-- Speaker note: Before class: A concept tutorial for introducing or revising the idea. Set the width with `?width=minimum`, `typical` (default) or `wider`. The hinge is a clear model to test evidence against (nodes 3 to 5). The common misconception is an explosion at a point in empty space. Board notes: ten stages, evenly spaced on the timeline (the real times run from about 10⁻⁴³ s to 13.8 billion years). The patch of universe is a picture, not a simulation: its grid widens to show expansion, and its colour follows the temperature. Times and temperatures are rounded textbook values. -->

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## Why it matters

> The hydrogen nuclei in the water you drink were made in the first minutes of the universe. The carbon in you was made inside stars.

<!-- Speaker note: Before weighing the evidence for the big bang, you need a clear picture of what the theory says. -->

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<span style="background:#dbeafe; color:#1a1a1a; border:1.5px solid #3b82f6; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">💬 Discussion</span>

## Engage: predict

**What does the big bang theory say happened?**

- A huge explosion at one point in empty space, throwing matter outwards
- Stars collided to make the universe
- Space, time and matter began together in a hot, dense state, and space itself has been expanding ever since
- The universe has always looked the way it does now

<!-- Speaker note: What happens: Space, time and matter began together, extremely hot and dense, and space itself has been expanding and cooling ever since. There was no empty space for it to explode into, and there is no centre. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: count how many chose the explosion. -->

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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>

## Explore it

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

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

<!-- Speaker note: Press **Later →** to move through time. Watch the temperature and what has formed at each stage. -->

<!-- Speaker note: Teacher: ask what happened to the temperature at every step. -->

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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, time and matter began together, extremely hot and dense, and space itself has been expanding and cooling ever since. There was no empty space for it to explode into, and there is no centre.

<!-- 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>

## Order

**Put the stages of the universe in order.**

```science
The first stars
Protons and neutrons form
The first atoms; light set free
The sun and Earth form
The first nuclei (hydrogen and helium)
The big bang: space and time begin
Galaxies
Inflation
```

<!-- Speaker note: Correct order: The big bang: space and time begin Inflation Protons and neutrons form The first nuclei (hydrogen and helium) The first atoms; light set free The first stars Galaxies The sun and Earth form -->

<!-- Speaker note: Teacher: the atoms question is the checkpoint (cooling is the cause). -->

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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>

## Atoms

**Why could atoms only form after about 380 000 years?**

- Gravity was too weak
- There were no electrons
- Stars hadn't made them yet
- Before then the universe was too hot: electrons moved too fast to stay attached to nuclei

* Answer: Before then the universe was too hot: electrons moved too fast to stay attached to nuclei

<!-- 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>

## Your atoms

**Where did the carbon and oxygen in your body come from?**

- Made on Earth
- Made inside stars and spread by supernovae, long after the big bang
- Made in the first three minutes
- They have always existed

* Answer: Made inside stars and spread by supernovae, long after the big bang

<!-- 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>

## Expanding and cooling

**About how long ago did the big bang happen, and what has the universe been doing since?**

- About 13.8 billion years ago; it has been expanding and heating up
- About 13.8 billion years ago; it expanded at first and has stayed the same size since the galaxies formed
- About 4.6 billion years ago; it has been expanding and cooling ever since
- About 13.8 billion years ago; it has been expanding and cooling ever since

* Answer: About 13.8 billion years ago; it has been expanding and cooling ever since

<!-- 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: stars as element factories

**The very first stars were made of hydrogen and helium only. The sun also contains carbon, oxygen and iron. What does that tell us?**

- The sun is older than the first stars
- Carbon was made in the big bang
- The sun formed from material that earlier stars had made and spread out
- The sun has already made its own carbon, oxygen and iron by fusion

* Answer: The sun formed from material that earlier stars had made and spread out

<!-- Speaker note: What this step is for: Connecting the first stars to heavier elements explains why later stars and planets differ from the first ones. -->

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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: open questions

**Measurements show the universe's expansion is speeding up, and galaxies hold far more mass than we can see. How do scientists treat these findings?**

- They throw out the big bang theory
- They prove the universe is shrinking
- They add new ideas (dark energy, dark matter) to the model and test them with new observations, from telescopes such as ASKAP in Western Australia
- They ignore them

* Answer: They add new ideas (dark energy, dark matter) to the model and test them with new observations, from telescopes such as ASKAP in Western Australia

<!-- Speaker note: What this step is for: Recognising what the model doesn't yet explain shows how scientific theories grow. -->

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## Evaluate: look back

Write one sentence: "At first I thought… Now I think… because…"

**Revision summary**

- The **big bang theory**: space, time, matter and energy began about **13.8 billion years ago**, extremely hot and dense; the universe has been **expanding and cooling** ever since.
- Stages: inflation → protons and neutrons → hydrogen and helium nuclei (3 min) → atoms and free light (380 000 years) → first stars (about 200 million years) → galaxies → the sun and Earth (9.2 billion years).
- Heavier elements were made **in stars**.

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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

**Outline three stages of the universe after the big bang.**

* For example: the first nuclei of hydrogen and helium (3 minutes), atoms forming and light set free (380 000 years), the first stars (about 200 million years), galaxies (about 1 billion years).

<!-- 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: redshift and the expanding universe (node 3). Later: the cosmic microwave background and the proportions of hydrogen and helium as evidence (nodes 4 and 5). -->