# 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.

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

> Scientists have proposed different theories to explain the origin and evolution of the universe. Two of these 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. An expanding universe means it must have originated from a single small, hot, dense point (Big Bang)
3. Cosmic microwave background radiation (CMBR) is detected uniformly in all directions across the universe
4. CMBR is explained as high-energy radiation from the hot early universe that has been stretched / red-shifted to microwave wavelengths as the universe expanded
5. The Steady State Theory is rejected because it cannot account for / explain CMBR, and/or because observations show the universe has evolved (different stars existed at different eras), contradicting the idea that the universe always looks the same

## 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)

## 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)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-big-bang-theory-14d56243) · Published by Druglandscape Ltd.