# Explain how the observed redshift of light from distant galaxies provides evidence that the universe is expanding.

> AQA A-Level Physics (7408) — 3.9.3 Cosmology · Explain · 5 marks

> Astronomers study the light from distant galaxies and observe that the wavelengths of spectral lines are shifted compared to the same lines measured in a laboratory on Earth.

## Mark scheme (5 marks)

1. The light from distant galaxies is shifted towards longer wavelengths (redshift)
2. This redshift occurs because the galaxies are moving away from us
3. The further away a galaxy is, the greater its redshift / the faster it is moving away
4. This means all galaxies are moving away from each other / space itself is stretching
5. This provides evidence that the universe began from a single point / supports the Big Bang theory

## Key terms

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

## Related

- [Revision notes for AQA A-Level Physics (7408)](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-the-observed-redshift-of-2dfb81f5) · Published by Druglandscape Ltd.