Explain what is meant by red shift and describe what the red shift of light from distant galaxies tells scientists about how the universe began.

Eduqas GCSE Physics — 10.2 Red shift and the Big Bang · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Red shift is the shift of light towards longer wavelengths when a source moves away from the observer.

Almost all distant galaxies show red‑shifted light, indicating they are receding from us.

The further a galaxy is, the greater its red shift, meaning it is moving away faster.

This pattern supports the Big Bang theory: the universe began as a very small point and has been expanding ever since.

Examiner tips

  • Use the term ‘red shift’ and ‘longer wavelengths’ for full credit. Show the link between distance and red shift. Explain how this supports the Big Bang. Keep each point brief and to the point.

Common mistakes

  • Confusing red shift with blue shift. Failing to mention that red shift indicates recession. Not linking greater red shift to faster recession. Not connecting the evidence to the Big Bang theory.

Mark scheme (4 marks)

  1. Red shift is when light from a source moving away from an observer is shifted towards longer wavelengths / the red end of the spectrum
  2. Almost all distant galaxies show red-shifted light, meaning they are moving away from us
  3. Galaxies that are further away show greater red shift, meaning they are moving away faster
  4. This evidence supports the Big Bang theory — the universe started from a very small point and has been expanding ever since

Key terms in this question

red shift · universe

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