# Explain why red-shift and cosmic microwave background radiation (CMBR) are used as evidence to support the Big Bang Theory rather than the Steady State Theory.

> Edexcel A-Level Physics (9PH0) — 7.2 Capacitors · Explain · 5 marks

> Scientists have proposed two competing theories to explain the origin and nature of the universe: the Big Bang Theory and the Steady State Theory. Only one of these theories is currently accepted by the scientific community.

## Mark scheme (5 marks)

1. Red-shift shows that distant galaxies are moving away from Earth / the universe is expanding
2. The expansion of the universe implies it originated from a single small, hot, dense point (consistent with the Big Bang)
3. CMBR is low-energy microwave radiation detected uniformly throughout the universe
4. CMBR is explained as high-energy radiation from the early hot universe that has been stretched / red-shifted to microwave wavelengths as the universe expanded
5. The Steady State Theory cannot account for CMBR, so it is rejected in favour of the Big Bang Theory

## Key terms

- [red-shift](https://www.gradenine.co.uk/glossary/red-shift)
- [CMBR](https://www.gradenine.co.uk/glossary/cmbr)
- [Big Bang Theory](https://www.gradenine.co.uk/glossary/big-bang-theory)
- [Steady State Theory](https://www.gradenine.co.uk/glossary/steady-state-theory)

## Related

- [Revision notes for Edexcel A-Level Physics (9PH0)](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-why-red-shift-and-cosmic-microwave-eb1a8f8a) · Published by Druglandscape Ltd.