A scientist observes two distant galaxies, Galaxy P and Galaxy Q. Galaxy P is twice as far from Earth as Galaxy Q. Compare the red-shift of light from Galaxy P with that from Galaxy Q, and explain what this tells us about the motion of the two galaxies.

Pearson Edexcel International GCSE Physics (4PH1) — 8.3 Cosmology · Compare · 4 marks · View as Markdown

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

Model answer (4 marks)

Galaxy P shows a greater red‑shift than Galaxy Q
Both galaxies are moving away from Earth
Galaxy P is moving faster than Galaxy Q
The greater the distance of a galaxy, the faster it moves away – this supports the Big Bang theory that the universe is expanding

Examiner tips

  • Use the word ‘greater’ to compare red‑shifts; mention both galaxies are receding; state the speed difference; link distance to expansion

Common mistakes

  • Confusing red‑shift with blue‑shift; saying galaxies are moving towards Earth; not mentioning the Big Bang/expansion link

Mark scheme (4 marks)

  1. Galaxy P shows a greater red-shift than Galaxy Q
  2. Both galaxies are moving away from Earth / the observer
  3. Galaxy P is moving faster / at a greater speed than Galaxy Q
  4. The greater the distance of a galaxy, the faster it moves away / this supports the Big Bang theory / the universe is expanding

Key terms in this question

red-shift · galaxy

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