# Explain how red-shift and cosmic microwave background radiation (CMBR) together provide evidence for the Big Bang Theory, and why the Steady State Theory cannot fully account for this evidence.

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

> Astronomers studying distant galaxies observe that the light reaching Earth has a longer wavelength than the light originally emitted. Separately, sensitive radio telescopes detect low-level microwave radiation coming from all directions in the sky.

## 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, tracing back in time means it must have originated from a single, very small, hot and dense point — consistent with the Big Bang Theory
3. CMBR is low-level microwave radiation detected uniformly in all directions across the sky
4. CMBR is the stretched / red-shifted remnant of the high-energy radiation emitted by the very hot, young universe shortly after the Big Bang
5. The Steady State Theory cannot explain CMBR, so it is rejected in favour of the Big Bang Theory

## Key terms

- [red-shift](https://www.gradenine.co.uk/glossary/red-shift)
- [cosmic microwave background radiation (CMBR)](https://www.gradenine.co.uk/glossary/cosmic-microwave-background-radiation-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-how-red-shift-and-cosmic-microwave-df11e989) · Published by Druglandscape Ltd.