Red-shift observations of distant galaxies provide evidence about the universe. Describe what red-shift is and explain what red-shift observations of distant galaxies tell us about the universe.

WJEC GCSE Physics (Wales) — 2.6 Particle physics and using physics to investigate the universe · Describe and 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 from a galaxy towards longer wavelengths, i.e. towards the red end of the spectrum.

It occurs because the galaxy is moving away from us; the Doppler effect stretches the light waves.

More distant galaxies show greater red‑shift, meaning they are receding faster.

This demonstrates that the universe is expanding.

Examiner tips

  • Use the term ‘red‑shift’ and ‘longer wavelengths’ to gain full marks for the description. Show the Doppler effect as the cause of the shift. Link greater red‑shift to greater recession speed. Finish with the conclusion that the universe is expanding.

Common mistakes

  • Confusing red‑shift with blue‑shift or using ‘shorter wavelengths’. Failing to mention the Doppler effect as the cause. Not stating that greater red‑shift means faster recession. Leaving out the conclusion that the universe is expanding.

Mark scheme (4 marks)

  1. Red-shift is when light from a galaxy is shifted towards longer wavelengths / the red end of the spectrum
  2. Red-shift occurs because the galaxy is moving away from us / the Earth
  3. More distant galaxies show greater red-shift, meaning they are moving away faster
  4. This shows the universe is expanding

Key terms in this question

red-shift · universe

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