# Explain how red-shift provides evidence that the universe is expanding, and describe what this suggests about how the universe began.

> OCR GCSE Physics A: Gateway Science (J249) — P6.3 Astronomy · Explain · 4 marks

> Astronomers studying distant galaxies have observed that the light reaching us from these galaxies has a longer wavelength than expected.

## Mark scheme (4 marks)

1. Light from distant galaxies is red-shifted, meaning its wavelength is observed to be longer (shifted towards the red end of the spectrum).
2. Red-shift occurs because distant galaxies are moving away from Earth, and the further away a galaxy is, the faster it is moving away and the greater the red-shift observed.
3. Red-shift from galaxies in all directions shows that the universe itself is expanding.
4. If the universe is currently expanding, it must have originally started from a single, very small, hot and dense point — consistent with the Big Bang Theory.

## Key terms

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

## Related

- [Revision notes for OCR GCSE Physics A: Gateway Science (J249)](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-red-shift-provides-evidence-that-6958830c) · Published by Druglandscape Ltd.