A scientist observes that a galaxy 500 million light-years away shows a greater red-shift than a galaxy 100 million light-years away. Explain what this difference in red-shift tells us about the motion of these two galaxies.

AQA GCSE Physics (8463) — 4.8.2 Red-shift (Phys only) · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Both galaxies are moving away from us.
Red‑shift means the wavelength of light from the galaxy has increased.
A greater red‑shift indicates a greater recessional speed, so the more distant galaxy is moving away faster.
This shows that more distant galaxies move away faster, evidence that the universe is expanding.

Examiner tips

  • Use the word ‘red‑shift’ to explain wavelength increase. Show the link between greater red‑shift and higher speed. Mention the Hubble–Lemaître law. Keep answer concise and to the point.

Common mistakes

  • Confusing red‑shift with blue‑shift. Failing to state that both galaxies are receding. Not linking greater red‑shift to higher recessional speed.

Mark scheme (4 marks)

  1. Both galaxies are moving away from us / the Earth / our galaxy
  2. Red-shift means the wavelength of light from the galaxy has increased / been stretched
  3. A greater red-shift indicates a greater recessional speed / the more distant galaxy is moving away faster
  4. More distant galaxies move away faster, showing that the universe is expanding

Key terms in this question

red-shift · galaxy

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