# Explain why the Big Bang Theory is the currently accepted model for the origin of the universe, whilst the Steady State Theory has been rejected. Refer to red-shift and cosmic microwave background radiation (CMBR) in your answer.

> Edexcel A-Level Physics (9PH0) — 7.3 Magnetic fields and electromagnetic induction · Explain · 5 marks

> Scientists have proposed different theories to explain the origin and evolution of the universe. Two such theories are the Big Bang Theory and the Steady State Theory.

## Mark scheme (5 marks)

1. Red-shift shows that distant galaxies are moving away from Earth / the universe is expanding
2. If the universe is currently expanding, it must originally have started from a single small, hot, dense point (consistent with the Big Bang Theory)
3. CMBR is low-level microwave radiation detected in all directions / throughout the universe
4. CMBR is explained by the Big Bang Theory 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 explain CMBR / is rejected because it predicts the universe looks the same at all times, but evidence shows the universe has evolved, for example different types of stars existed at different eras

## Key terms

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

## 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-the-big-bang-theory-90576777) · Published by Druglandscape Ltd.