# Explain how astronomers use red-shift to provide evidence that the universe is expanding.

> WJEC A-Level Physics (Wales) — 4.3 Orbits and the wider universe · Explain · 5 marks

> When astronomers observe light from distant galaxies, they notice that the wavelengths of known spectral lines are shifted compared with the same lines measured in a laboratory on Earth.

## Mark scheme (5 marks)

1. Light from distant galaxies is red-shifted (wavelengths are longer / shifted towards the red end of the spectrum)
2. Red-shift means the galaxies are moving away from us
3. The greater the red-shift, the greater the speed at which the galaxy is moving away
4. Galaxies are moving away in all directions, so the universe must be getting bigger / expanding
5. This supports the idea that the universe began from a single point / the Big Bang

## Key terms

- [red-shift](https://www.gradenine.co.uk/glossary/red-shift)
- [universe](https://www.gradenine.co.uk/glossary/universe)

## Related

- [Revision notes for WJEC A-Level Physics (Wales)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-astronomers-use-red-shift-to-4fc4174a) · Published by Druglandscape Ltd.