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# Quick check: scale, the cosmic glow and the evidence

Year 10 Earth and space · The big bang

<!-- Speaker note: A self-test on the scale of the universe, the CMB and matter proportions, and how the lines of evidence fit together (AC9S10U03, nodes 1, 4 and 5, taught in class; node 5 a hinge). About 16 minutes (Node 1 6 min · Node 4 5 min · Node 5 5 min). -->

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## Today

**We are learning to** a few quick questions on the scale of the universe, the cosmic microwave background and matter proportions, and how the evidence fits together. Taught in class; use this to check yourself.

- I can order the parts of the universe by scale and use light-years and AU.
- I can say what the CMB and the hydrogen-helium mix show.
- I can say why several independent lines of evidence strengthen a theory.

<!-- Speaker note: Before class: A Quick check: a few questions on what students should know for AC9S10U03 nodes 1, 4 and 5, which you teach in class. It has no board, no clip and no new teaching, and it is optional, so it never holds up course completion. Set the width with `?width=minimum`, `typical` (default) or `wider`. Use it as a self-test after your teaching, a starter, or revision. The big bang and redshift lessons are on screen (nodes 2 and 3). A scale activity (a class-sized solar system, or a number line) and a discussion of dark matter work better in the room. Teacher: Node 1: have students place objects on a scale line before the questions. Teacher: Node 5: ask which line of evidence they would show a sceptic first, and why. -->

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## Why it matters

> The light you see from some stars left them before humans existed. How far is that, and what else tells us the universe had a beginning?

<!-- Speaker note: Your teacher covers scale and the extra evidence in class. These questions check the basics. -->

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## Smallest to largest

**Which list is in order from smallest to largest?**

- Galaxy, star, planet, universe, solar system
- Star, planet, galaxy, solar system, universe
- Universe, galaxy, solar system, star, planet
- Planet, star, solar system, galaxy, universe

* Answer: Planet, star, solar system, galaxy, universe

<!-- Speaker note: A planet is part of a solar system around a star; billions of stars make a galaxy; the universe holds billions of galaxies. (A star is larger than a planet.) -->

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## Units

**A light-year is a unit of what?**

- Speed
- Time: one year
- Brightness
- Distance: how far light travels in a year

* Answer: Distance: how far light travels in a year

<!-- Speaker note: Light travels about 9.5 trillion kilometres in a year. The distance, not the time, is the unit. -->

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## Which unit?

**Which unit suits the distance from Earth to the Sun?**

- Kilogram
- Astronomical unit (AU)
- Metre
- Light-year

* Answer: Astronomical unit (AU)

<!-- Speaker note: One AU is the average distance from Earth to the Sun, about 150 million km. Light-years are for distances between stars. -->

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## Scientific notation

**The Sun is about 1.5 × 10^8 km from Earth. Written out in full, that is:**

- 150 000 000 km
- 15 000 000 km
- 1 500 000 000 km
- 1.5 million km

* Answer: 150 000 000 km

<!-- Speaker note: 10^8 is 100 million, so 1.5 × 10^8 is 150 million. -->

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## The glow

**The cosmic microwave background is a faint radiation coming from every direction. What does it show?**

- Stars are cooling
- Earth is surrounded by a hot shell
- Space is full of hot gas
- The early universe was hot and dense, and has been cooling as it expanded

* Answer: The early universe was hot and dense, and has been cooling as it expanded

<!-- Speaker note: This radiation was released when the universe was young and hot. It has cooled and stretched into microwaves. -->

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## What matter is made of

**About how much of the mass of ordinary matter in the universe is hydrogen and helium?**

- Mostly oxygen and carbon
- Mostly heavier elements, made in stars
- Almost all of it: by mass, about three-quarters hydrogen and one-quarter helium
- Equal parts of every element

* Answer: Almost all of it: by mass, about three-quarters hydrogen and one-quarter helium

<!-- Speaker note: The big bang model predicts roughly this mix from the first few minutes, and observations match it. -->

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## Independent evidence

**Is it evidence for the big bang that does not depend on redshift?**

Groups: Independent evidence · Not independent

The cosmic microwave background · The proportions of hydrogen and helium · Distant galaxies moving away from us (redshift)

* Answer: The cosmic microwave background → Independent evidence; The proportions of hydrogen and helium → Independent evidence; Distant galaxies moving away from us (redshift) → Not independent

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## Match the evidence

**Which line of evidence is it?**

Groups: Redshift · CMB · Matter proportions

Distant galaxies are moving away from us · A faint glow reaches us from every direction · About three-quarters of the mass is hydrogen, almost all the rest helium · The universe is expanding · The early universe was very hot and has cooled · Hydrogen and helium nuclei formed in the first few minutes

* Answer: Distant galaxies are moving away from us → Redshift; A faint glow reaches us from every direction → CMB; About three-quarters of the mass is hydrogen, almost all the rest helium → Matter proportions; The universe is expanding → Redshift; The early universe was very hot and has cooled → CMB; Hydrogen and helium nuclei formed in the first few minutes → Matter proportions

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## Why it is strong

**Why does it strengthen the big bang theory that redshift, the CMB and matter proportions all fit?**

- They are all one measurement in three forms
- They disprove each other
- They are independent lines of evidence that agree on the same model
- They prove the theory can never change

* Answer: They are independent lines of evidence that agree on the same model

<!-- Speaker note: When different kinds of evidence from different methods point the same way, the model is hard to explain away. -->

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## Still open

**Which is still an open question in cosmology?**

- What dark matter and dark energy are
- Whether other galaxies exist
- Whether there is a CMB
- Whether the universe is expanding

* Answer: What dark matter and dark energy are

<!-- Speaker note: Observations suggest most of the universe is dark matter and dark energy, but scientists do not yet know what they are. -->

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## Exit ticket

**Name two lines of evidence for the big bang and what each shows.**

* Redshift: the universe is expanding. CMB: it was hot and dense early. (Or hydrogen and helium proportions.)

<!-- 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: Back in class: a model of the expanding universe (dots on a balloon); discussion of what is still unknown. -->